Some miles up the harbour, in a line of cliffs formed of slightly

metamorphosed tufaceous and porphyritic claystone beds, I observed

three vertical dikes, so closely resembling in general appearance

ordinary volcanic dikes, that I did not doubt, until closely examining

their composition, that they had been injected from below. The first is

straight, with parallel sides, and about four feet wide; it consists of

whitish, indurated tufaceous matter, precisely like some of the beds

intersected by it. The second dike is more remarkable; it is slightly

tortuous, about eighteen inches thick, and can be traced for a

considerable distance along the beach; it is of a purplish-red or brown

colour, and is formed chiefly of ROUNDED grains of quartz, with broken

crystals of earthy feldspar, scales of black mica, and minute fragments

of claystone porphyry, all firmly united together in a hard sparing

base. The structure of this dike shows obviously that it is of

mechanical and sedimentary origin; yet it thinned out upwards, and did

not cut through the uppermost strata in the cliffs. This fact at first

appears to indicate that the matter could not have been washed in from

above (Upfilled fissures are known to occur both in volcanic and in

ordinary sedimentary formations. At the Galapagos Archipelago “Volcanic

Islands” etc., there are some striking examples of pseudo-dikes

composed of hard tuff.); but if we reflect on the suction which would

result from a deep-seated fissure being formed, we may admit that if

the fissure were in any part open to the surface, mud and water might

well be drawn into it along its whole course. The third dike consisted

of a hard, rough, white rock, almost composed of broken crystals of

glassy feldspar, with numerous scales of black mica, cemented in a

scanty base; there was little in the appearance of this rock, to

preclude the idea of its having been a true injected feldspathic dike.

The matter composing these three pseudo-dikes, especially the second

one, appears to have suffered, like the surrounding strata, a certain

degree of metamorphic action; and this has much aided the deceptive

appearance. At Bahia, in Brazil, we have seen that a true injected

hornblendic dike, not only has suffered metamorphosis, but has been

dislocated and even diffused in the surrounding gneiss, under the form

of separate crystals and of fragments.

FALKLAND ISLANDS.

I have described these islands in a paper published in the third volume

of the “Geological Journal.” The mountain-ridges consist of quartz, and

the lower country of clay-slate and sandstone, the latter containing

Palaeozoic fossils. These fossils have been separately described by

Messrs. Morris and Sharpe: some of them resemble Silurian, and others

Devonian forms. In the eastern part of the group the several parallel

ridges of quartz extend in a west and east line; but further westward

the line becomes W.N.W. and E.S.E., and even still more northerly. The

cleavage-planes of the clay- slate are highly inclined, generally at an

angle of above 50 degrees, and often vertical; they strike almost

invariably in the same direction with the quartz ranges. The outline of

the indented shores of the two main islands, and the relative positions

of the smaller islets, accord with the strike both of the main axes of

elevation and of the cleavage of the clay- slate.

TIERRA DEL FUEGO.

My notes on the geology of this country are copious, but as they are

unimportant, and as fossils were found only in one district, a brief

sketch will be here sufficient. The east coast from the S. of Magellan

(where the boulder formation is largely developed) to St. Polycarp’s

Bay is formed of horizontal tertiary strata, bounded some way towards

the interior by a broad mountainous band of clay-slate. This great

clay-slate formation extends from St. Le Maire westward for 140 miles,

along both sides of the Beagle Channel to near its bifurcation. South

of this channel, it forms all Navarin Island, and the eastern half of

Hoste Island and of Hardy Peninsula; north of the Beagle Channel it

extends in a north-west line on both sides of Admiralty Sound to

Brunswick Peninsula in the St. of Magellan, and I have reason to

believe, stretches far up the eastern side of the Cordillera. The

western and broken side of Tierra del Fuego towards the Pacific is

formed of metamorphic schists, granite and various trappean rocks: the

line of separation between the crystalline and clay-slate formations

can generally be distinguished, as remarked by Captain King, by the

parallelism in the clay-slate districts of the shores and channels,

ranging in a line between [W. 20 degrees to 40 degrees N.] and [E. 20

degrees to 40 degrees S.]. (“Geographical Journal” volume 1 page 155.)

The clay-slate is generally fissile, sometimes siliceous or

ferruginous, with veins of quartz and calcareous spar; it often

assumes, especially on the loftier mountains, an altered feldspathic

character, passing into feldspathic porphyry: occasionally it is

associated with breccia and grauwacke. At Good Success Bay, there is a

little intercalated black crystalline limestone. At Port Famine much of

the clay-slate is calcareous, and passes either into a mudstone or into

grauwacke, including odd-shaped concretions of dark argillaceous

limestone. Here alone, on the shore a few miles north of Port Famine,

and on the summit of Mount Tarn (2,600 feet high), I found organic

remains; they consist of:—

1. Ancyloceras simplex, d’Orbigny “Pal Franc” Mount Tarn. 2. Fusus (in

imperfect state), d’Orbigny “Pal Franc” Mount Tarn. 3. Natica,

d’Orbigny “Pal Franc” Mount Tarn. 4. Pentacrimus, d’Orbigny “Pal Franc”

Mount Tarn. 5. Lucina excentrica, G.B. Sowerby, Port Famine. 6. Venus

(in imperfect state), G.B. Sowerby, Port Famine. 7. Turbinolia (?),

G.B. Sowerby, Port Famine. 8. Hamites elatior, G.B. Sowerby, Port

Famine.

M. d’Orbigny states that MM. Hombron and Grange found in this

neighbourhood an Ancyloceras, perhaps A. simplex, an Ammonite, a

Plicatula and Modiola. (“Voyage” Part Geolog. page 242.) M. d’Orbigny

believes from the general character of these fossils, and from the

Ancyloceras being identical (as far as its imperfect condition allows

of comparison) with the A. simplex of Europe, that the formation

belongs to an early stage of the Cretaceous system. Professor E.

Forbes, judging only from my specimens, concurs in the probability of

this conclusion. The Hamites elatior of the above list, of which a

description has been given by Mr. Sowerby, and which is remarkable from

its large size, has not been seen either by M. d’Orbigny or Professor

E. Forbes, as, since my return to England, the specimens have been

lost. The great clay-slate formation of Tierra del Fuego being

cretaceous, is certainly a very interesting fact,—whether we consider

the appearance of the country, which, without the evidence afforded by

the fossils, would form the analogy of most known districts, probably

have been considered as belonging to the Palaeozoic series,—or whether

we view it as showing that the age of this terminal portion of the

great axis of South America, is the same (as will hereafter be seen)

with the Cordillera of Chile and Peru.

The clay-slate in many parts of Tierra del Fuego, is broken by dikes

and by great masses of greenstone, often highly hornblendic (In a

greenstone-dike in the Magdalen Channel, the feldspar cleaved with the

angle of albite. This dike was crossed, as well as the surrounding

slate, by a large vein of quartz, a circumstance of unusual

occurrence.): almost all the small islets within the clay-slate

districts are thus composed. The slate near the dikes generally becomes

paler-coloured, harder, less fissile, of a feldspathic nature, and

passes into a porphyry or greenstone: in one case, however, it became

more fissile, of a red colour, and contained minute scales of mica,

which were absent in the unaltered rock. On the east side of Ponsonby

Sound some dikes composed of a pale sonorous feldspathic rock,

porphyritic with a little feldspar, were remarkable from their

number,—there being within the space of a mile at least one

hundred,—from their nearly equalling in bulk the intermediate

slate,—and more especially from the excessive fineness (like the finest

inlaid carpentry) and perfect parallelism of their junctions with the

almost vertical laminae of clay-slate. I was unable to persuade myself

that these great parallel masses had been injected, until I found one

dike which abruptly thinned out to half its thickness, and had one of

its walls jagged, with fragments of the slate embedded in it.

In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary,

becomes much altered and feldspathic. Thus on Wollaston Island slate

and grauwacke can be distinctly traced passing into feldspathic rocks

and greenstones, including iron pyrites and epidote, but still

retaining traces of cleavage with the usual strike and dip. One such

metamorphosed mass was traversed by large vein-like masses of a

beautiful mixture (as ascertained by Professor Miller) of green

epidote, garnets, and white calcareous spar. On the northern point of

this same island, there were various ancient submarine volcanic rocks,

consisting of amygdaloids with dark bole and agate,—of basalt with

decomposed olivine—of compact lava with glassy feldspar,—and of a

coarse conglomerate of red scoriae, parts being amygdaloidal with

carbonate of lime. The southern part of Wollaston Island and the whole

of Hermite and Horn Islands, seem formed of cones of greenstone; the

outlying islets of Il Defenso and D. Raminez are said to consist of

porphyritic lava. (Determined by Professor Jameson. Weddell’s “Voyage”

page 169.) In crossing Hardy Peninsula, the slate still retaining

traces of its usual cleavage, passes into columnar feldspathic rocks,

which are succeeded by an irregular tract of trappean and basaltic

rocks, containing glassy feldspar and much iron pyrites: there is,

also, some harsh red claystone porphyry, and an almost true trachyte,

with needles of hornblende, and in one spot a curious slaty rock

divided into quadrangular columns, having a base almost like trachyte,

with drusy cavities lined by crystals, too imperfect, according to

Professor Miller, to be measured, but resembling Zeagonite. (See Mr.

Brooke’s Paper in the “London Philosophical Magazine” volume 10. This

mineral occurs in an ancient volcanic rock near Rome.) In the midst of

these singular rocks, no doubt of ancient submarine volcanic origin, a

high hill of feldspathic clay-slate projected, retaining its usual

cleavage. Near this point, there was a small hillock, having the aspect

of granite, but formed of white albite, brilliant crystals of

hornblende (both ascertained by the reflecting goniometer) and mica;

but with no quartz. No recent volcanic district has been observed in

any part of Tierra del Fuego.

Five miles west of the bifurcation of the Beagle Channel, the slate-

formation, instead of becoming, as in the more southern parts of Tierra

del Fuego, feldspathic, and associated with trappean or old volcanic

rocks, passes by alternations into a great underlying mass of fine

gneiss and glossy clay-slate, which at no great distance is succeeded

by a grand formation of mica-slate containing garnets. The folia of

these metamorphic schists strike parallel to the cleavage-planes of the

clay-slate, which have a very uniform direction over the whole of this

part of the country: the folia, however, are undulatory and tortuous,

whilst the cleavage- laminae of the slate are straight. These schists

compose the chief mountain-chain of Southern Tierra del Fuego, ranging

along the north side of the northern arm of the Beagle Channel, in a

short W.N.W. and E.S.E. line, with two points (Mounts Sarmiento and

Darwin) rising to heights of 6,800 and 6,900 feet. On the south-western

side of this northern arm of the Beagle Channel, the clay-slate is seen

with its STRATA dipping from the great chain, so that the metamorphic

schists here form a ridge bordered on each side by clay-slate. Further

north, however, to the west of this great range, there is no

clay-slate, but only gneiss, mica, and hornblendic slates, resting on

great barren hills of true granite, and forming a tract about sixty

miles in width. Again, westward of these rocks, the outermost islands

are of trappean formation, which, from information obtained during the

voyages of the “Adventure” and “Beagle,” seem, together with granite,

chiefly to prevail along the western coast as far north as the entrance

of the St. of Magellan (See the Paper by Captain King in the

“Geographical Journal”; also a Letter to Dr. Fitton in “Geological

Proceedings” volume 1 page 29; also some observations by Captain

Fitzroy “Voyages” volume 1 page 375. I am indebted also to Mr. Lyell

for a series of specimens collected by Lieutenant Graves.): a little

more inland, on the eastern side of Clarence Island and S. Desolation,

granite, greenstone, mica-slate, and gneiss appear to predominate. I am

tempted to believe, that where the clay-slate has been metamorphosed at

great depths beneath the surface, gneiss, mica- slate, and other allied

rocks have been formed, but where the action has taken place nearer the

surface, feldspathic porphyries, greenstones, etc., have resulted,

often accompanied by submarine volcanic eruptions.

Only one other rock, met with in both arms of the Beagle Channel,

deserves any notice, namely a granulo-crystalline mixture of white

albite, black hornblende (ascertained by measurement of the crystals,

and confirmed by Professor Miller), and more or less of brown mica, but

without any quartz. This rock occurs in large masses, closely

resembling in external form granite or syenite: in the southern arm of

the Channel, one such mass underlies the mica-slate, on which

clay-slate was superimposed: this peculiar plutonic rock which, as we

have seen, occurs also in Hardy Peninsula, is interesting, from its

perfect similarity with that (hereafter often to be referred to under

the name of andesite) forming the great injected axes of the Cordillera

of Chile.

The stratification of the clay-slate is generally very obscure, whereas

the cleavage is remarkably well defined: to begin with the extreme

eastern parts of Tierra del Fuego; the cleavage-planes near the St. of

Le Maire strike either W. and E. or W.S.W. and E.N.E., and are highly

inclined; the form of the land, including Staten Island, indicates that

the axes of elevation have run in this same line, though I was unable

to distinguish the planes of stratification. Proceeding westward, I

accurately examined the cleavage of the clay-slate on the northern,

eastern, and western sides (thirty-five miles apart) of Navarin Island,

and everywhere found the laminae ranging with extreme regularity,

W.N.W. and E.S.E., seldom varying more than one point of the compass

from this direction. (The clay-slate in this island was in many places

crossed by parallel smooth joints. Out of five cases, the angle of

intersection between the strike of these joints and that of the

cleavage-laminae was in two cases 45 degrees and in two others 79

degrees.) Both on the east and west coasts, I crossed at right angles

the cleavage-planes for a space of about eight miles, and found them

dipping at an angle of between 45 degrees and 90 degrees, generally to

S.S.W., sometimes to N.N.E., and often quite vertically. The S.S.W. dip

was occasionally succeeded abruptly by a N.N.E. dip, and this by a

vertical cleavage, or again by the S.S.W. dip; as in a lofty cliff on

the eastern end of the island the laminae of slate were seen to be

folded into very large steep curves, ranging in the usual W.N.W. line,

I suspect that the varying and opposite dips may possibly be accounted

for by the cleavage- laminae, though to the eye appearing straight,

being parts of large abrupt curves, with their summits cut off and worn

down.

In several places I was particularly struck with the fact, that the

fine laminae of the clay-slate, where cutting straight through the

bands of stratification, and therefore indisputably true

cleavage-planes, differed slightly in their greyish and greenish tints

of colour, in compactness, and in some of the laminae having a rather

more jaspery appearance than others. I have not seen this fact

recorded, and it appears to me important, for it shows that the same

cause which has produced the highly fissile structure, has altered in a

slight degree the mineralogical character of the rock in the same

planes. The bands of stratification, just alluded to, can be

distinguished in many places, especially in Navarin Island, but only on

the weathered surfaces of the slate; they consist of slightly

undulatory zones of different shades of colour and of thicknesses, and

resemble the marks (more closely than anything else to which I can

compare them) left on the inside of a vessel by the draining away of

some dirty slightly agitated liquid: no difference in composition,

corresponding with these zones, could be seen in freshly fractured

surfaces. In the more level parts of Navarin Island, these bands of

stratification were nearly horizontal; but on the flanks of the

mountains they were inclined from them, but in no instance that I saw

at a very high angle. There can, I think, be no doubt that these zones,

which appear only on the weathered surfaces, are the last vestiges of

the original planes of stratification, now almost obliterated by the

highly fissile and altered structure which the mass has assumed.

The clay-slate cleaves in the same W.N.W. and E.S.E. direction, as on

Navarin Island, on both sides of the Beagle Channel, on the eastern

side of Hoste Island, on the N.E. side of Hardy Peninsula, and on the

northern point of Wollaston Island; although in these two latter

localities the cleavage has been much obscured by the metamorphosed and

feldspathic condition of the slate. Within the area of these several

islands, including Navarin Island, the direction of the stratification

and of the mountain- chains is very obscure; though the mountains in

several places appeared to range in the same W.N.W. line with the

cleavage: the outline of the coast, however, does not correspond with

this line. Near the bifurcation of the Beagle Channel, where the

underlying metamorphic schists are first seen, they are foliated (with

some irregularities), in this same W.N.W. line, and parallel, as before

stated, to the main mountain-axis of this part of the country. Westward

of this main range, the metamorphic schists are foliated, though less

plainly, in the same direction, which is likewise common to the zone of

old erupted trappean rocks, forming the outermost islets. Hence the

area, over which the cleavage of the slate and the foliation of the

metamorphic schists extends with an average W.N.W. and E.S.E. strike,

is about forty miles in a north and south line, and ninety miles in an

east and west line.

Further northward, near Port Famine, the stratification of the

clay-slate and of the associated rocks, is well defined, and there

alone the cleavage and strata-planes are parallel. A little north of

this port there is an anticlinal axis ranging N.W. (or a little more

westerly) and S.E.: south of the port, as far as Admiralty Sound and

Gabriel Channel, the outline of the land clearly indicates the

existence of several lines of elevation in this same N.W. direction,

which, I may add, is so uniform in the western half of the St. of

Magellan, that, as Captain King has remarked, “a parallel ruler placed

on the map upon the projecting points of the south shore, and extended

across the strait, will also touch the headlands on the opposite

coast.” (“Geographical Journal” volume 1 page 170.) It would appear,

from Captain King’s observations, that over all this area the cleavage

extends in the same line. Deep-water channels, however, in all parts of

Tierra del Fuego have burst through the trammels both of stratification

and cleavage; most of them may have been formed during the elevation of

the land by long- continued erosion, but others, for instance the

Beagle Channel, which stretches like a narrow canal for 120 miles

obliquely through the mountains, can hardly have thus originated.

Finally, we have seen that in the extreme eastern point of Tierra del

Fuego, the cleavage and coast-lines extend W. and E. and even W.S.W.

and E.N.E.: over a large area westward, the cleavage, the main range of

mountains, and some subordinate ranges, but not the outlines of the

coast, strike W.N.W., and E.S.E.: in the central and western parts of

the St. of Magellan, the stratification, the mountain-ranges, the

outlines of the coast, and the cleavage all strike nearly N.W. and S.E.

North of the strait, the outline of the coast, and the mountains on the

mainland, run nearly north and south. Hence we see, at this southern

point of the continent, how gradually the Cordillera bend, from their

north and south course of so many thousand miles in length, into an E.

and even E.N.E. direction.

WEST COAST, FROM THE SOUTHERN CHONOS ISLANDS TO NORTHERN CHILE.

The first place at which we landed north of the St. of Magellan was

near Cape Tres Montes, in latitude 47 degrees S. Between this point and

the Northern Chonos Islands, a distance of 200 miles, the “Beagle”

visited several points, and specimens were collected for me from the

intermediate spaces by Lieutenant Stokes. The predominant rock is

mica-slate, with thick folia of quartz, very frequently alternating

with and passing into a chloritic, or into a black, glossy, often

striated, slightly anthracitic schist, which soils paper, and becomes

white under a great heat, and then fuses. Thin layers of feldspar,

swelling at intervals into well crystallised kernels, are sometimes

included in these black schists; and I observed one mass of the

ordinary black variety insensibly lose its fissile structure, and pass

into a singular mixture of chlorite, epidote, feldspar, and mica. Great

veins of quartz are numerous in the mica-schists; wherever these occur

the folia are much convoluted. In the southern part of the Peninsula of

Tres Montes, a compact altered feldspathic rock with crystals of

feldspar and grains of quartz is the commonest variety; this rock

exhibits occasionally traces of an original brecciated structure, and

often presents (like the altered state of Tierra del Fuego) traces of

cleavage- planes, which strike in the same direction with the folia of

mica-schist further northward. (The peculiar, abruptly conical form of

the hills in this neighbourhood, would have led any one at first to

have supposed that they had been formed of injected or intrusive rocks.

At Inchemo Island, a similar rock gradually becomes granulo-crystalline

and acquires scales of mica; and this variety at S. Estevan becomes

highly laminated, and though still exhibiting some rounded grains of

quartz, passes into the black, glossy, slightly anthracitic schist,

which, as we have seen, repeatedly alternates with and passes into the

micaceous and chloritic schists. Hence all the rocks on this line of

coast belong to one series, and insensibly vary from an altered

feldspathic clay-slate into largely foliated, true mica-schist.

The cleavage of the homogeneous schists, the foliation of those

composed of more or less distinct minerals in layers, and the planes of

alternation of the different varieties or so-called stratification, are

all parallel, and preserve over this 200 miles of coast a remarkable

degree of uniformity in direction. At the northern end of the group, at

Low’s Harbour, the well- defined folia of mica-schist everywhere ranged

within eight degrees (or less than one point of the compass) of N. 19

degrees W. and S. 19 degrees E.; and even the point of dip varied very

little, being always directed to the west and generally at an angle of

forty degrees; I should mention that I had here good opportunities of

observation, for I followed the naked rock on the beach, transversely

to the strike, for a distance of four miles and a half, and all the way

attended to the dip. Along the outer islands for 100 miles south of

Low’s Harbour, Lieutenant Stokes, during his boat- survey, kindly

observed for me the strike of the foliation, and he assures me that it

was invariably northerly, and the dip with one single exception to the

west. Further south at Vallenar Bay, the strike was almost universally

N. 25 degrees W. and the dip, generally at an angle of about 40 degrees

to W. 25 degrees S., but in some places almost vertical. Still farther

south, in the neighbourhood of the harbours of Anna Pink, S. Estevan

and S. Andres, and (judging from a distance) along the southern part of

Tres Montes, the foliation and cleavage extended in a line between [N.

11 degrees to 22 degrees W.] and [S. 11 degrees to 22 degrees E.]; and

the planes dipped generally westerly, but often easterly, at angles

varying from a gentle inclination to vertical. At A. Pink’s Harbour,

where the schists generally dipped easterly, wherever the angle became

very high, the strike changed from N. 11 degrees W. to even as much as

N. 45 degrees W.: in an analogous manner at Vallenar Bay, where the dip

was westerly (viz. on an average directed to W. 25 degrees S.), as soon

as the angle became very high, the planes struck in a line more than 25

degrees west of north. The average result from all the observations on

this 200 miles of coast, is a strike of N. 19 degrees W. and S. 19

degrees E.: considering that in each specified place my examination

extended over an area of several miles, and that Lieutenant Stokes’

observations apply to a length of 100 miles, I think this remarkable

uniformity is pretty well established. The prevalence, throughout the

northern half of this line of coast, of a dip in one direction, that is

to the west, instead of being sometimes west and sometimes east, is,

judging from what I have elsewhere seen, an unusual circumstance. In

Brazil, La Plata, the Falkland Islands, and Tierra del Fuego, there is

generally an obvious relation between the axis of elevation, the

outline of the coast, and the strike of the cleavage or foliation: in

the Chonos Archipelago, however, neither the minor details of the

coast-line, nor the chain of the Cordillera, nor the subordinate

transverse mountain-axes, accord with the strike of the foliation and

cleavage: the seaward face of the numerous islands composing this

Archipelago, and apparently the line of the Cordillera, range N. 11

degrees E., whereas, as we have just seen, the average strike of the

foliation is N. 19 degrees W.

There is one interesting exception to the uniformity in the strike of

the foliation. At the northern point of Tres Montes (latitude 45

degrees 52 minutes) a bold chain of granite, between two and three

thousand feet in height, runs from the coast far into the interior, in

an E.S.E. line, or more strictly E. 28 degrees S. and W. 28 degrees N.

(In the distance, other mountains could be seen apparently ranging

N.N.E. and S.S.W. at right angles to this one. I may add, that not far

from Vallenar Bay there is a fine range, apparently of granite, which

has burst through the mica-slate in a N.E. by E. and S.W. by S. line.)

In a bay, at the northern foot of this range, there are a few islets of

mica-slate, with the folia in some parts horizontal, but mostly

inclined at an average angle of 20 degrees to the north. On the

northern steep flank of the range, there are a few patches (some quite

isolated, and not larger than half a-crown!) of the mica-schist,

foliated with the same northerly dip. On the broad summit, as far as

the southern crest, there is much mica-slate, in some places even 400

feet in thickness, with the folia all dipping north, at angles varying

from 5 degrees to 20 degrees, but sometimes mounting up to 30 degrees.

The southern flank consists of bare granite. The mica-slate is

penetrated by small veins of granite, branching from the main body.

(The granite within these veins, as well as generally at the junction

with the mica-slate, is more quartzose than elsewhere. The granite, I

may add, is traversed by dikes running for a very great length in the

line of the mountains; they are composed of a somewhat laminated

eurite, containing crystals of feldspar, hornblende, and octagons of

quartz.) Leaving out of view the prevalent strike of the folia in other

parts of this Archipelago, it might have been expected that they would

have dipped N. 28 degrees E., that is directly from the ridge, and,

considering its abruptness, at a high inclination; but the real dip, as

we have just seen, both at the foot and on the northern flank, and over

the entire summit, is at a small angle, and directed nearly due north.

From these considerations it occurred to me, that perhaps we here had

the novel and curious case of already inclined laminae obliquely tilted

at a subsequent period by the granitic axis. Mr. Hopkins, so well known

from his mathematical investigations, has most kindly calculated the

problem: the proposition sent was,—Take a district composed of laminae,

dipping at an angle of 40 degrees to W. 19 degrees S., and let an axis

of elevation traverse it in an E. 28 degrees S. line, what will the

position of the laminae be on the northern flank after a tilt, we will

first suppose, of 45 degrees? Mr. Hopkins informs me, that the angle of

the dip will be 28 degrees 31 minutes, and its direction to north 30

degrees 33 minutes west. (On the south side of the axis (where,

however, I did not see any mica-slate) the dip of the folia would be at

an angle of 77 degrees 55 minutes, directed to west 35 degrees 33

minutes south. Hence the two points of dip on the opposite sides of the

range, instead of being as in ordinary cases directly opposed to each

other at an angle of 180 degrees, would here be only 86 degrees 50

minutes apart.) By varying the supposed angle of the tilt, our

previously inclined folia can be thrown into any angle between 26

degrees, which is the least possible angle, and 90 degrees; but if a

small inclination be thus given to them, their point of dip will depart

far from the north, and therefore not accord with the actual position

of the folia of mica-schist on our granitic range. Hence it appears

very difficult, without varying considerably the elements of the

problem, thus to explain the anomalous strike and dip of the foliated

mica- schist, especially in those parts, namely, at the base of the

range, where the folia are almost horizontal. Mr. Hopkins, however,

adds, that great irregularities and lateral thrusts might be expected

in every great line of elevation, and that these would account for

considerable deviations from the calculated results: considering that

the granitic axis, as shown by the veins, has indisputably been

injected after the perfect formation of the mica-slate, and considering

the uniformity of the strike of the folia throughout the rest of the

Archipelago, I cannot but still think that their anomalous position at

this one point is someway directly and mechanically related to the

intrusion of this W.N.W. and E.S.E. mountain-chain of granite.

Dikes are frequent in the metamorphic schists of the Chonos Islands,

and seem feebly to represent that great band of trappean and ancient

volcanic rocks on the south-western coast of Tierra del Fuego. At S.

Andres I observed in the space of half-a-mile, seven broad, parallel

dikes, composed of three varieties of trap, running in a N.W. and S.E.

line, parallel to the neighbouring mountain-ranges of altered

clay-slate; but they must be of long subsequent origin to these

mountains; for they intersected the volcanic formation described in the

last chapter. North of Tres Montes, I noticed three dikes differing

from each other in composition, one of them having a euritic base

including large octagons of quartz; these dikes, as well as several of

porphyritic greenstone at Vallenar Bay, extended N.E. and S.W., nearly

at right angles to the foliation of the schists, but in the line of

their joints. At Low’s Harbour, however, a set of great parallel dikes,

one ninety yards and another sixty yards in width, have been guided by

the foliation of the mica-schist, and hence are inclined westward at an

angle of 45 degrees: these dikes are formed of various porphyritic

traps, some of which are remarkable from containing numerous rounded

grains of quartz. A porphyritic trap of this latter kind, passed in one

of the dikes into a most curious hornstone, perfectly white, with a

waxy fracture and pellucid edges, fusible, and containing many grains

of quartz and specks of iron pyrites. In the ninety-yard dike several

large, apparently now quite isolated, fragments of mica-slate were

embedded: but as their foliation was exactly parallel to that of the

surrounding solid rock, no doubt these new separate fragments

originally formed wedge-shaped depending portions of a continuous vault

or crust, once extending over the dike, but since worn down and

denuded.

CHILOE, VALDIVIA, CONCEPCION.

In Chiloe, a great formation of mica-schist strikingly resembles that

of the Chonos Islands. For a space of eleven miles on the S.E. coast,

the folia were very distinct, though slightly convoluted, and ranged

within a point of N.N.W. and S.S.E., dipping either E.N.E. or more

commonly W.S.W., at an average angle of 22 degrees (in one spot,

however, at 60 degrees), and therefore decidedly at a lesser

inclination than amongst the Chonos Islands. On the west and

north-western shores, the foliation was often obscure, though, where

best defined, it ranged within a point of N. by W. and S. by E.,

dipping either easterly or westerly, at varying and generally very

small angles. Hence, from the southern part of Tres Montes to the

northern end of Chiloe, a distance of 300 miles, we have closely allied

rocks with their folia striking on an average in the same direction,

namely between N. 11 degrees and 22 degrees W. Again, at Valdivia, we

meet with the same mica-schist, exhibiting nearly the same

mineralogical passages as in the Chonos Archipelago, often, however,

becoming more ferruginous, and containing so much feldspar as to pass

into gneiss. The folia were generally well defined; but nowhere else in

South America did I see them varying so much in direction: this seemed

chiefly caused by their forming parts, as I could sometimes distinctly

trace, of large flat curves: nevertheless, both near the settlement and

towards the interior, a N.W. and S.E. strike seemed more frequent than

any other direction; the angle of the dip was generally small. At

Concepcion, a highly glossy clay-slate had its cleavage often slightly

curvilinear, and inclined, seldom at a high angle, towards various

points of the compass: but here, as at Valdivia, a N.W. and S.E. strike

seemed to be the most frequent one. ((FIGURE 23.) I observed in some

parts that the tops of the laminae of the clay-slate (b in Figure 23)

under the superficial detritus and soil (a) were bent, sometimes

without being broken, as represented in Figure 23, which is copied from

one given by Sir H. De la Beche (page 42 “Geological Manual”) of an

exactly similar phenomenon in Devonshire. Mr. R.A.C. Austen, also, in

his excellent paper on S.E. Devon (“Geological Transactions” volume 6

page 437), has described this phenomenon; he attributes it to the

action of frosts, but at the same time doubts whether the frosts of the

present day penetrate to a sufficient depth. As it is known that

earthquakes particularly affect the surface of the ground, it occurred

to me that this appearance might perhaps be due, at least at

Concepcion, to their frequent occurrence; the superficial layers of

detritus being either jerked in one direction, or, where the surface

was inclined, pushed a little downwards during each strong vibration.

In North Wales I have seen a somewhat analogous but less regular

appearance, though on a greater scale (“London Philosophical Magazine”

volume 21 page 184), and produced by a quite different cause, namely,

by the stranding of great icebergs; this latter appearance has also

been observed in N. America.)

In certain spots large quartz veins were numerous, and near them, the

cleavage, as was the case with the foliation of the schists in the

Chonos Archipelago, became extremely tortuous.

At the northern end of Quiriquina Island, in the Bay of Concepcion, at

least eight rudely parallel dikes, which have been guided to a certain

extent by the cleavage of the slate, occur within the space of a

quarter of a mile. They vary much in composition, resembling in many

respects the dikes at Low’s Harbour: the greater number consist of

feldspathic porphyries, sometimes containing grains of quartz: one,

however, was black and brilliant, like an augitic rock, but really

formed of feldspar; others of a feldspathic nature were perfectly

white, with either an earthy or crystalline fracture, and including

grains and regular octagons of quartz; these white varieties passed

into ordinary greenstones. Although, both here and at Low’s Harbour,

the nature of the rock varied considerably in the same dike, yet I

cannot but think that at these two places and in other parts of the

Chonos group, where the dikes, though close to each other and running

parallel, are of different composition, that they must have been formed

at different periods. In the case of Quiriquina this is a rather

interesting conclusion, for these eight parallel dikes cut through the

metamorphic schists in a N.W. and S.E. line, and since their injection

the overlying cretaceous or tertiary strata have been tilted (whilst

still under the sea) from a N.W. by N. and S.E. by S. line; and again,

during the great earthquake of February 1835, the ground in this

neighbourhood was fissured in N.W. and S.E. lines; and from the manner

in which buildings were thrown down, it was evident that the surface

undulated in this same direction. (“Geological Transactions” volume 6

pages 602 and 617. “Journal of Researches” 2nd edition page 307.)

CENTRAL AND NORTHERN CHILE.

Northward of Concepcion, as far as Copiapo, the shores of the Pacific

consist, with the exception of some small tertiary basins, of gneiss,

mica- schist, altered clay-slate, granite, greenstone and syenite:

hence the coast from Tres Montes to Copiapo, a distance of 1,200 miles,

and I have reason to believe for a much greater space, is almost

similarly constituted.

Near Valparaiso the prevailing rock is gneiss, generally including much

hornblende: concretionary balls formed of feldspar, hornblende and

mica, from two or three feet in diameter, are in very many places

conformably enfolded by the foliated gneiss: veins of quartz and

feldspar, including black schorl and well-crystallised epidote, are

numerous. Epidote likewise occurs in the gneiss in thin layers,

parallel to the foliation of the mass. One large vein of a coarse

granitic character was remarkable from in one part quite changing its

character, and insensibly passing into a blackish porphyry, including

acicular crystals of glassy feldspar and of hornblende: I have never

seen any other such case. (Humboldt “Personal Narrative” volume 4 page

60, has described with much surprise, concretionary balls, with

concentric divisions, composed of partially vitreous feldspar,

hornblende, and garnets, included within great veins of gneiss, which

cut across the mica-slate near Venezuela.)

I shall in the few following remarks on the rocks of Chile allude

exclusively to their foliation and cleavage. In the gneiss round

Valparaiso the strike of the foliation is very variable, but I think

about N. by W. and S. by E. is the commonest direction; this likewise

holds good with the cleavage of the altered feldspathic clay-slates,

occasionally met with on the coast for ninety miles north of

Valparaiso. Some feldspathic slate, alternating with strata of

claystone porphyry in the Bell of Quillota and at Jajuel, and

therefore, perhaps, belonging to a later period than the metamorphic

schists on the coast, cleaved in this same direction. In the Eastern

Cordillera, in the Portillo Pass, there is a grand mass of mica- slate,

foliated in a north and south line, and with a high westerly dip: in

the Uspallata range, clay-slate and grauwacke have a highly inclined,

nearly north and south cleavage, though in some parts the strike is

irregular: in the main or Cumbre range, the direction of the cleavage

in the feldspathic clay-slate is N.W. and S.E.

Between Coquimbo and Guasco there are two considerable formations of

mica- slate, in one of which the rock passed sometimes into common

clay-slate and sometimes into a glossy black variety, very like that in

the Chonos Archipelago. The folia and cleavage of these rocks ranged

between [N. and N.W. by N.] and [S. and S.W. by S.]. Near the Port of

Guasco several varieties of altered clay-slate have a quite irregular

cleavage. Between Guasco and Copiapo, there are some siliceous and

talcaceous slates cleaving in a north and south line, with an easterly

dip of between 60 and 70 degrees: high up, also, the main valley of

Copiapo, there is mica-slate with a high easterly dip. In the whole

space between Valparaiso and Copiapo an easterly dip is much more

common than an opposite or westerly one.

CONCLUDING REMARKS ON CLEAVAGE AND FOLIATION.

In this southern part of the southern hemisphere, we have seen that the

cleavage-laminae range over wide areas with remarkable uniformity,

cutting straight through the planes of stratification, but yet being

parallel in strike to the main axes of elevation, and generally to the

outlines of the coast. (In my paper on the Falkland Islands “Geological

Journal” volume 3 page 267, I have given a curious case on the

authority of Captain Sulivan, R.N., of much folded beds of clay-slate,

in some of which the cleavage is perpendicular to the horizon, and in

others it is perpendicular to each curvature or fold of the bed: this

appears a new case.) The dip, however, is as variable, both in angle

and in direction (that is, sometimes being inclined to the one side and

sometimes to the directly opposite side), as the strike is uniform. In

all these respects there is a close agreement with the facts given by

Professor Sedgwick in his celebrated memoir in the “Geological

Transactions,” and by Sir R.I. Murchison in his various excellent

discussions on this subject. The Falkland Islands, and more especially

Tierra del Fuego, offer striking instances of the lines of cleavage,

the principle axes of elevation, and the outlines of the coast,

gradually changing together their courses. The direction which prevails

throughout Tierra del Fuego and the Falkland Islands, namely, from west

with some northing to east with some southing, is also common to the

several ridges in Northern Patagonia and in the western parts of Banda

Oriental: in this latter province, in the Sierra Tapalguen, and in the

Western Falkland Island, the W. by N., or W.N.W. and E.S.E., ridges,

are crossed at right angles by others ranging N.N.E. and S.S.W.

The fact of the cleavage-laminae in the clay-slate of Tierra del Fuego,

where seen cutting straight through the planes of stratification, and

where consequently there could be no doubt about their nature,

differing slightly in colour, texture, and hardness, appears to me very

interesting. In a thick mass of laminated, feldspathic and altered

clay-slate, interposed between two great strata of porphyritic

conglomerate in Central Chile, and where there could be but little

doubt about the bedding, I observed similar slight differences in

composition, and likewise some distinct thin layers of epidote,

parallel to the highly inclined cleavage of the mass. Again, I

incidentally noticed in North Wales, where glaciers had passed over the

truncated edges of the highly inclined laminae of clay-slate, that the

surface, though smooth, was worn into small parallel undulations,

caused by the competent laminae being of slightly different degrees of

hardness. (“London Philosophical Magazine” volume 21 page 182.) With

reference to the slates of North Wales, Professor Sedgwick describes

the planes of cleavage, as “coated over with chlorite and

semi-crystalline matter, which not only merely define the planes in

question, but strike in parallel flakes through the whole mass of the

rock.” (“Geological Transactions” volume 3 page 471.) In some of those

glossy and hard varieties of clay-slate, which may often be seen

passing into mica-schist, it has appeared to me that the cleavage-

planes were formed of excessively thin, generally slighted convoluted,

folia, composed of microscopically minute scales of mica. From these

several facts, and more especially from the case of the clay-slate in

Tierra del Fuego, it must, I think, be concluded, that the same power

which has impressed on the slate its fissile structure or cleavage has

tended to modify its mineralogical character in parallel planes.

Let us now turn to the foliation of the metamorphic schists, a subject

which has been much less attended to. As in the case of

cleavage-laminae, the folia preserve over very large areas a uniform

strike: thus Humboldt found for a distance of 300 miles in Venezuela,

and indeed over a much larger space, gneiss, granite, mica, and

clay-slate, striking very uniformly N.E. and S.W., and dipping at an

angle of between 60 and 70 degrees to N.W. (“Personal Narrative” volume

6 page 59 et seq.); it would even appear from the facts given in this

chapter, that the metamorphic rocks throughout the north-eastern part

of South America are generally foliated within two points of N.E. and

S.W. Over the eastern parts of Banda Oriental, the foliation strikes

with a high inclination, very uniformly N.N.E. to S.S.W., and over the

western parts, in a W. by N. and E. by S. line. For a space of 300

miles on the shores of the Chonos and Chiloe Islands, we have seen that

the foliation seldom deviates more than a point of the compass from a

N. 19 degrees W. and S. 19 degrees E. strike. As in the case of

cleavage, the angle of the dip in foliated rocks is generally high but

variable, and alternates from one side of the line of strike to the

other side, sometimes being vertical: in the Northern Chonos Islands,

however, the folia are inclined almost always to the west; in nearly

the same manner, the cleavage-laminae in Southern Tierra del Fuego

certainly dip much more frequently to S.S.W. than to the opposite

point. In Eastern Banda Oriental, in parts of Brazil, and in some other

districts, the foliation runs in the same direction with the

mountain-ranges and adjoining coast-lines: amongst the Chonos Islands,

however, this coincidence fails, and I have given my reasons for

suspecting that one granitic axis has burst through and tilted the

already inclined folia of mica-schist: in the case of cleavage, the

coincidence between its strike and that of the main stratification

seems sometimes to fail. (Cases are given by Mr. Jukes in his “Geology

of Newfoundland” page 130.) Foliation and cleavage resemble each other

in the planes winding round concretions, and in becoming tortuous where

veins of quartz abound. (I have seen in Brazil and Chile concretions

thus enfolded by foliated gneiss; and Macculloch “Highlands” volume 1

page 64, has described a similar case. For analogous cases in

clay-slate, see Professor Henslow’s Memoir in “Cambridge Philosophical

Transactions” volume 1 page 379, and Macculloch’s “Classification of

Rocks” page 351. With respect to both foliation and cleavage becoming

tortuous where quartz-veins abound, I have seen instances near Monte

Video, at Concepcion, and in the Chonos Islands. See also Mr.

Greenough’s “Critical Examination” page 78.) On the flanks of the

mountains both in Tierra del Fuego and in other countries, I have

observed that the cleavage-planes frequently dip at a high angle

inwards; and this was long ago observed by Von Buch to be the case in

Norway: this fact is perhaps analogous to the folded, fan-like or

radiating structure in the metamorphic schists of the Alps, in which

the folia in the central crests are vertical and on the two flanks

inclined inwards. (Studer in “Edinburgh New Philosophical Journal”

volume 23 page 144.) Where masses of fissile and foliated rocks

alternate together, the cleavage and foliation, in all cases which I

have seen, are parallel. Where in one district the rocks are fissile,

and in another adjoining district they are foliated, the planes of

cleavage and foliation are likewise generally parallel: this is the

case with the feldspathic homogeneous slates in the southern part of

the Chonos group, compared with the fine foliated mica-schists of the

northern part; so again the clay- slate of the whole eastern side of

Tierra del Fuego cleaves in exactly the same line with the foliated

gneiss and mica-slate of the western coast; other analogous instances

might have been adduced. (I have given a case in Australia. See my

“Volcanic Islands.”)

With respect to the origin of the folia of quartz, mica, feldspar, and

other minerals composing the metamorphic schists, Professor Sedgwick,

Mr. Lyell, and most authors believe, that the constituent parts of each

layer were separately deposited as sediment, and then metamorphosed.

This view, in the majority of cases, I believe to be quite untenable.

In those not uncommon instances, where a mass of clay-slate, in

approaching granite, gradually passes into gneiss, we clearly see that

folia of distinct minerals can originate through the metamorphosis of a

homogeneous fissile rock. (I have described in “Volcanic Islands” a

good instance of such a passage at the Cape of Good Hope.) The

deposition, it may be remarked, of numberless alternations of pure

quartz, and of the elements of mica or feldspar does not appear a

probable event. (See some excellent remarks on this subject, in

D’Aubuisson’s “Traite de Geog.” tome 1 page 297. Also some remarks by

Mr. Dana in “Silliman’s American Journal” volume 45 page 108.) In those

districts in which the metamorphic schists are foliated in planes

parallel to the cleavage of the rocks in an adjoining district, are we

to believe that the folia are due to sedimentary layers, whilst the

cleavage- laminae, though parallel, have no relation whatever to such

planes of deposition? On this view, how can we reconcile the vastness

of the areas over which the strike of the foliation is uniform, with

what we see in disturbed districts composed of true strata: and

especially, how can we understand the high and even vertical dip

throughout many wide districts, which are not mountainous, and

throughout some, as in Western Banda Oriental, which are not even

hilly? Are we to admit that in the northern part of the Chonos

Archipelago, mica-slate was first accumulated in parallel horizontal

folia to a thickness of about four geographical miles, and then

upturned at an angle of forty degrees; whilst, in the southern part of

this same Archipelago, the cleavage-laminae of closely allied rocks,

which none would imagine had ever been horizontal, dip at nearly the

same angle, to nearly the same point?

Seeing, then, that foliated schists indisputably are sometimes produced

by the metamorphosis of homogeneous fissile rocks; seeing that

foliation and cleavage are so closely analogous in the several

above-enumerated respects; seeing that some fissile and almost

homogeneous rocks show incipient mineralogical changes along the planes

of their cleavage, and that other rocks with a fissile structure

alternate with, and pass into varieties with a foliated structure, I

cannot doubt that in most cases foliation and cleavage are parts of the

same process: in cleavage there being only an incipient separation of

the constituent minerals; in foliation a much more complete separation

and crystallisation.

The fact often referred to in this chapter, of the foliation and the

so- called strata in the metamorphic series,—that is, the alternating

masses of different varieties of gneiss, mica-schist, and

hornblende-slate, etc.,—being parallel to each other, at first appears

quite opposed to the view, that the folia have no relation to the

planes of original deposition. Where the so-called beds are not very

thick and of widely different mineralogical composition from each

other, I do not think that there is any difficulty in supposing that

they have originated in an analogous manner with the separate folia. We

should bear in mind what thick strata, in ordinary sedimentary masses,

have obviously been formed by a concretionary process. In a pile of

volcanic rocks on the Island of Ascension, there are strata, differing

quite as much in appearance as the ordinary varieties of the

metamorphic schists, which undoubtedly have been produced, not by

successive flowings of lava, but by internal molecular changes. Near

Monte Video, where the stratification, as it would be called, of the

metamorphic series is, in most parts, particularly well developed,

being as usual, parallel to the foliation, we have seen that a mass of

chloritic schist, netted with quartz-veins, is entangled in gneiss, in

such a manner as to show that it had certainly originated in some

process of segregation: again, in another spot, the gneiss tended to

pass into hornblendic schist by alternating with layers of quartz; but

these layers of quartz almost certainly had never been separately

deposited, for they were absolutely continuous with the numerous

intersecting veins of quartz. I have never had an opportunity of

tracing for any distance, along the line both of strike and of dip, the

so-called beds in the metamorphic schists, but I strongly suspect that

they would not be found to extend with the same character, very far in

the line either of their dip or strike. Hence I am led to believe, that

most of the so-called beds are of the nature of complex folia, and have

not been separately deposited. Of course, this view cannot be extended

to THICK masses included in the metamorphic series, which are of

totally different composition from the adjoining schists, and which are

far extended, as is sometimes the case with quartz and marble; these

must generally be of the nature of true strata. (Macculloch

“Classification of Rocks” page 364, states that primary limestones are

often found in irregular masses or great nodules, “which can scarcely

be said to possess a stratified shape!”) Such strata, however, will

almost always strike in the same direction with the folia, owing to the

axes of elevation being in most countries parallel to the strike of the

foliation; but they will generally dip at a different angle from that

of the foliation; and the angle of the foliation in itself almost

always varies much: hence, in crossing a metamorphosed schistose

district, it would require especial attention to discriminate between

true strata of deposition and complex foliated masses. The mere

presence of true strata in the midst of a set of metamorphic schists,

is no argument that the foliation is of sedimentary origin, without it

be further shown in each case, that the folia not only strike, but dip

throughout in parallel planes with those of the true stratification.

As in some cases it appears that where a fissile rock has been exposed

to partial metamorphic action, for instance from the irruption of

granite, the foliation has supervened on the already existing

cleavage-planes; so perhaps in some instances, the foliation of a rock

may have been determined by the original planes of deposition or of

oblique current-laminae: I have, however, myself, never seen such a

case, and I must maintain that in most extensive metamorphic areas, the

foliation is the extreme result of that process, of which cleavage is

the first effect. That foliation may arise without any previous

structural arrangement in the mass, we may infer from injected, and

therefore once liquified, rocks, both of volcanic and plutonic origin,

sometimes having a “grain” (as expressed by Professor Sedgwick), and

sometimes being composed of distinct folia or laminae of different

compositions. In my work on “Volcanic Islands,” I have given several

instances of this structure in volcanic rocks, and it is not uncommonly

seen in plutonic masses—thus, in the Cordillera of Chile, there are

gigantic mountain-like masses of red granite, which have been injected

whilst liquified, and which, nevertheless, display in parts a decidedly

laminar structure. (As remarked in a former part of this chapter, I

suspect that the boldly conical mountains of gneiss-granite, near Rio

de Janeiro, in which the constituent minerals are arranged in parallel

planes, are of intrusive origin. We must not, however, forget the

lesson of caution taught by the curious claystone porphyries of Port

Desire, in which we have seen that the breaking up and aggregation of a

thinly stratified tufaceous mass, has yielded a rock semi-porphyritic

with crystals of feldspar, arranged in the planes of original

deposition.)

Finally, we have seen that the planes of cleavage and of foliation,

that is, of the incipient process and of the final result, generally

strike parallel to the principal axes of elevation, and to the outline

of the land: the strike of the axes of elevation (that is, of the lines

of fissures with the strata on their edges upturned), according to the

reasoning of Mr. Hopkins, is determined by the form of the area

undergoing changes of level, and the consequent direction of the lines

of tension and fissure. Now, in that remarkable pile of volcanic rocks

at Ascension, which has several times been alluded to (and in some

other cases), I have endeavoured to show, that the lamination of the

several varieties, and their alternations, have been caused by the

moving mass, just before its final consolidation, having been subjected

(as in a glacier) to planes of different tension; this difference in

the tension affecting the crystalline and concretionary processes. (In

“Volcanic Islands.”) One of the varieties of rock thus produced at

Ascension, at first sight, singularly resembles a fine-grained gneiss;

it consists of quite straight and parallel zones of excessive tenuity,

of more or less coloured crystallised feldspar, of distinct crystals of

quartz, diopside, and oxide of iron. These considerations,

notwithstanding the experiments made by Mr. Fox, showing the influence

of electrical currents in producing a structure like that of cleavage,

and notwithstanding the apparently inexplicable variation, both in the

inclination of the cleavage-laminae and in their dipping first to one

side and then to the other side of the line of strike, lead me to

suspect that the planes of cleavage and foliation are intimately

connected with the planes of different tension, to which the area was

long subjected, AFTER the main fissures or axes of upheavement had been

formed, but BEFORE the final consolidation of the mass and the total

cessation of all molecular movement.

CHAPTER VII.

CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.

Central Chile.—Basal formations of the Cordillera.—Origin of the

porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section

of the Cordillera by the Peuquenes are Portillo Pass.—Great gypseous

formation.—Peuquenes line; thickness of strata, fossils of.—Portillo

line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks

of.—Concluding remarks on the denudation and elevation of the Portillo

line.—Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous

strata.—Section near the Puente del Inca; fossils of.—Great

subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the

Uspallata chain.—Structure and nature of the strata.—Silicified

vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding

remarks on the Uspallata range; origin subsequent to that of the main

Cordillera; two periods of subsidence; comparison with the Portillo

chain.—

The district between the Cordillera and the Pacific, on a rude average,

is from about eighty to one hundred miles in width. It is crossed by

many chains of mountains, of which the principal ones, in the latitude

of Valparaiso and southward of it, range nearly north and south; but in

the more northern parts of the province, they run in almost every

possible direction. Near the Pacific, the mountain-ranges are generally

formed of syenite or granite, and or of an allied euritic porphyry; in

the low country, besides these granitic rocks and greenstone, and much

gneiss, there are, especially northward of Valparaiso, some

considerable districts of true clay-slate with quartz veins, passing

into a feldspathic and porphyritic slate; there is also some grauwacke

and quartzose and jaspery rocks, the latter occasionally assuming the

character of the basis of claystone porphyry: trap-dikes are numerous.

Nearer the Cordillera the ranges (such as those of S. Fernando, the

Prado (Meyen “Reise um Erde” th. 1 s. 235.), and Aconcagua) are formed

partly of granitic rocks, and partly of purple porphyritic

conglomerates, claystone porphyry, greenstone porphyry, and other

rocks, such as we shall immediately see, form the basal strata of the

main Cordillera. In the more northern parts of Chile, this porphyritic

series extends over large tracts of country far from the Cordillera;

and even in Central Chile such occasionally occur in outlying

positions.

I will describe the Campana of Quillota, which stands only fifteen

miles from the Pacific, as an instance of one of these outlying masses.

This hill is conspicuous from rising to the height of 6,400 feet: its

summit shows a nucleus, uncovered for a height of 800 feet, of fine

greenstone, including epidote and octahedral magnetic iron ore; its

flanks are formed of great strata of porphyritic claystone conglomerate

associated with various true porphyries and amygdaloids, alternating

with thick masses of a highly feldspathic, sometimes porphyritic,

pale-coloured slaty rock, with its cleavage-laminae dipping inwards at

a high angle. At the base of the hill there are syenites, a granular

mixture of quartz and feldspar, and harsh quartzose rocks, all

belonging to the basal metamorphic series. I may observe that at the

foot of several hills of this class, where the porphyries are first

seen (as near S. Fernando, the Prado, Las Vacas, etc.), similar harsh

quartzose rocks and granular mixtures of quartz and feldspar occur, as

if the more fusible constituent parts of the granitic series had been

drawn off to form the overlying porphyries.

In Central Chile, the flanks of the main Cordillera, into which I

penetrated by four different valleys, generally consist of distinctly

stratified rocks. The strata are inclined at angles varying from

sometimes even under ten, to twenty degrees, very rarely exceeding

forty degrees: in some, however, of the quite small, exterior,

spur-like ridges, the inclination was not unfrequently greater. The dip

of the strata in the main outer lines was usually outwards or from the

Cordillera, but in Northern Chile frequently inwards,—that is, their

basset-edges fronted the Pacific. Dikes occur in extraordinary numbers.

In the great, central, loftiest ridges, the strata, as we shall

presently see, are almost always highly inclined and often vertical.

Before giving a detailed account of my two sections across the

Cordillera, it will, I think, be convenient to describe the basal

strata as seen, often to a thickness of four or five thousand feet, on

the flanks of the outer lines.

BASAL STRATA OF THE CORDILLERA.

The prevailing rock is a purplish or greenish, porphyritic claystone

conglomerate. The embedded fragments vary in size from mere particles

to blocks as much as six or eight inches (rarely more) in diameter; in

many places, where the fragments were minute, the signs of aqueous

deposition were unequivocally distinct; where they were large, such

evidence could rarely be detected. The basis is generally porphyritic

with perfect crystals of feldspar, and resembles that of a true

injected claystone porphyry: often, however, it has a mechanical or

sedimentary aspect, and sometimes (as at Jajuel) is jaspery. The

included fragments are either angular, or partially or quite rounded

(Some of the rounded fragments in the porphyritic conglomerate near the

Baths of Cauquenes, were marked with radii and concentric zones of

different shades of colour: any one who did not know that pebbles, for

instance flint pebbles from the chalk, are sometimes zoned

concentrically with their worn and rounded surfaces, might have been

led to infer, that these balls of porphyry were not true pebbles, but

had originated in concretionary action.); in some parts the rounded, in

others the angular fragments prevail, and usually both kinds are mixed

together: hence the word BRECCIA ought strictly to be appended to the

term PORPHYRITIC CONGLOMERATE. The fragments consist of many varieties

of claystone porphyry, usually of nearly the same colour with the

surrounding basis, namely, purplish-reddish, brownish, mottled or

bright green; occasionally fragments of a laminated, pale-coloured,

feldspathic rock, like altered clay-slate are included; as are

sometimes grains of quartz, but only in one instance in Central Chile

(namely, at the mines of Jajuel) a few pebbles of quartz. I nowhere

observed mica in this formation, and rarely hornblende; where the

latter mineral did occur, I was generally in doubt whether the mass

really belonged to this formation, or was of intrusive origin.

Calcareous spar occasionally occurs in small cavities; and nests and

layers of epidote are common. In some few places in the finer-grained

varieties (for instance, at Quillota), there were short, interrupted

layers of earthy feldspar, which could be traced, exactly as at Port

Desire, passing into large crystals of feldspar: I doubt, however,

whether in this instance the layers had ever been separately deposited

as tufaceous sediment.

All the varieties of porphyritic conglomerates and breccias pass into

each other, and by innumerable gradations into porphyries no longer

retaining the least trace of mechanical origin: the transition appears

to have been effected much more easily in the finer-grained, than in

the coarser-grained varieties. In one instance, near Cauquenes, I

noticed that a porphyritic conglomerate assumed a spheroidal structure,

and tended to become columnar. Besides the porphyritic conglomerates

and the perfectly characterised porphyries, of metamorphic origin,

there are other porphyries, which, though differing not at all or only

slightly in composition, certainly have had a different origin: these

consist of pink or purple claystone porphyries, sometimes including

grains of quartz,—of greenstone porphyry, and of other dusky rocks, all

generally porphyritic with fine, large, tabular, opaque crystals, often

placed crosswise, of feldspar cleaving like albite (judging from

several measurements), and often amygdaloidal with silex, agate,

carbonate of lime, green and brown bole. (This bole is a very common

mineral in the amygdaloidal rocks; it is generally of a greenish- brown

colour, with a radiating structure; externally it is black with an

almost metallic lustre, but often coated by a bright green film. It is

soft and can be scratched by a quill; under the blowpipe swells greatly

and becomes scaly, then fuses easily into a black magnetic bead. This

substance is evidently similar to that which often occurs in submarine

volcanic rocks. An examination of some very curious specimens of a fine

porphyry (from Jajuel) leads me to suspect that some of these

amygdaloidal balls, instead of having been deposited in pre-existing

air-vesicles, are of concretionary origin; for in these specimens, some

of the pea-shaped little masses (often externally marked with minute

pits) are formed of a mixture of green earth with stony matter, like

the basis of the porphyry, including minute imperfect crystals of

feldspar; and these pea-shaped little masses are themselves

amygdaloidal with minute spheres of the green earth, each enveloped by

a film of white, apparently feldspathic, earthy matter: so that the

porphyry is doubly amygdaloidal. It should not, however, be overlooked,

that all the strata here have undergone metamorphic action, which may

have caused crystals of feldspar to appear, and other changes to be

effected, in the originally simple amygdaloidal balls. Mr. J.D. Dana,

in an excellent paper on Trap-rocks “Edinburgh New Philosophical

Journal” volume 41 page 198, has argued with great force, that all

amygdaloidal minerals have been deposited by aqueous infiltration. I

may take this opportunity of alluding to a curious case, described in

my work on “Volcanic Islands,” of an amygdaloid with many of its cells

only half filled up with a mesotypic mineral. M. Rose has described an

amygdaloid, brought by Dr. Meyen “Reise um Erde” Th. 1. s. 316, from

Chile, as consisting of crystallised quartz, with crystals of stilbite

within, and lined externally by green earth.) These several porphyritic

and amygdaloidal varieties never show any signs of passing into masses

of sedimentary origin: they occur both in great and small intrusive

masses, and likewise in strata alternating with those of the

porphyritic conglomerate, and with the planes of junction often quite

distinct, yet not seldom blended together. In some of these intrusive

masses, the porphyries exhibit, more or less plainly, a brecciated

structure, like that often seen in volcanic masses. These brecciated

porphyries could generally be distinguished at once from the

metamorphosed, porphyritic breccia- conglomerates, by all the fragments

being angular and being formed of the same variety, and by the absence

of every trace of aqueous deposition. One of the porphyries above

specified, namely, the greenstone porphyry with large tabular crystals

of albite, is particularly abundant, and in some parts of the

Cordillera (as near St. Jago) seemed more common even than the purplish

porphyritic conglomerate. Numerous dikes likewise consist of this

greenstone porphyry; others are formed of various fine-grained trappean

rocks; but very few of claystone porphyry: I saw no true basaltic

dikes.

In several places in the lower part of the series, but not everywhere,

thick masses of a highly feldspathic, often porphyritic, slaty rock

occur interstratified with the porphyritic conglomerate; I believe in

one or two cases blackish limestone has been found in a similar

position. The feldspathic rock is of a pale grey or greenish colour; it

is easily fusible; where porphyritic, the crystals of feldspar are

generally small and vitreous: it is distinctly laminated, and sometimes

includes parallel layers of epidote (This mineral is extremely common

in all the formations of Chile; in the gneiss near Valparaiso and in

the granitic veins crossing it, in the injected greenstone crowning the

C. of Quillota, in some granitic porphyries, in the porphyritic

conglomerate, and in the feldspathic clay-slates.); the lamination

appears to be distinct from stratification. Occasionally this rock is

somewhat curious; and at one spot, namely, at the C. of Quillota, it

had a brecciated structure. Near the mines of Jajuel, in a thick

stratum of this feldspathic, porphyritic slate, there was a layer of

hard, blackish, siliceous, infusible, compact clay-slate, such as I saw

nowhere else; at the same place I was able to follow for a considerable

distance the junction between the slate and the conformably underlying

porphyritic conglomerate, and they certainly passed gradually into each

other. Wherever these slaty feldspathic rocks abound, greenstone seems

common; at the C. of Quillota a bed of well-crystallised greenstone lay

conformably in the midst of the feldspathic slate, with the upper and

lower junctions passing insensibly into it. From this point, and from

the frequently porphyritic condition of the slate, I should perhaps

have considered this rock as an erupted one (like certain laminated

feldspathic lavas in the trachytic series), had I not seen in Tierra

del Fuego how readily true clay-slate becomes feldspathic and

porphyritic, and had I not seen at Jajuel the included layer of black,

siliceous clay-slate, which no one could have thought of igneous

origin. The gentle passage of the feldspathic slate, at Jajuel, into

the porphyritic conglomerate, which is certainly of aqueous origin,

should also be taken in account.

The alternating strata of porphyries and porphyritic conglomerate, and

with the occasionally included beds of feldspathic slate, together make

a grand formation; in several places within the Cordillera, I estimated

its thickness at from six to seven thousand feet. It extends for many

hundred miles, forming the western flank of the Chilean Cordillera; and

even at Iquique in Peru, 850 miles north of the southernmost point

examined by me in Chile, the coast-escarpment which rises to a height

of between two and three thousand feet is thus composed. In several

parts of Northern Chile this formation extends much further towards the

Pacific, over the granitic and metamorphic lower rocks, than it does in

Central Chile; but the main Cordillera may be considered as its central

line, and its breadth in an east and west direction is never great. At

first the origin of this thick, massive, long but narrow formation,

appeared to me very anomalous: whence were derived, and how were

dispersed the innumerable fragments, often of large size, sometimes

angular and sometimes rounded, and almost invariably composed of

porphyritic rocks? Seeing that the interstratified porphyries are never

vesicular and often not even amygdaloidal, we must conclude that the

pile was formed in deep water; how then came so many fragments to be

well rounded and so many to remain angular, sometimes the two kinds

being equally mingled, sometimes one and sometimes the other

preponderating? That the claystone, greenstone, and other porphyries

and amygdaloids, which lie CONFORMABLY between the beds of

conglomerate, are ancient submarine lavas, I think there can be no

doubt; and I believe we must look to the craters whence these streams

were erupted, as the source of the breccia- conglomerate; after the

great explosion, we may fairly imagine that the water in the heated and

scarcely quiescent crater would remain for a considerable time

sufficiently agitated to triturate and round the loose fragments, few

or many in number, would be shot forth at the next eruption, associated

with few or many angular fragments, according to the strength of the

explosion. (This certainly seems to have taken place in some recent

volcanic archipelagos, as at the Galapagos, where numerous craters are

exclusively formed of tuff and fragments of lava.) The porphyritic

conglomerate being purple or reddish, even when alternating with dusty-

coloured or bright green porphyries and amygdaloids, is probably an

analogous circumstance to the scoriae of the blackish basalts being

often bright red. The ancient submarine orifices whence the porphyries

and their fragments were ejected having been arranged in a band, like

most still active volcanoes, accounts for the thickness, the

narrowness, and linear extension of this formation.

This whole great pile of rock has suffered much metamorphic action, as

is very obvious in the gradual formation and appearance of the crystals

of albitic feldspar and of epidote—in the bending together of the

fragments— in the appearance of a laminated structure in the

feldspathic slate—and, lastly, in the disappearance of the planes of

stratification, which could sometimes be seen on the same mountain

quite distinct in the upper part, less and less plain on the flanks,

and quite obliterated at the base. Partly owing to this metamorphic

action, and partly to the close relationship in origin, I have seen

fragments of porphyries—taken from a metamorphosed conglomerate—from a

neighbouring stream of lava—from the nucleus or centre (as it appeared

to me) of the whole submarine volcano— and lastly from an intrusive

mass of quite subsequent origin, all of which were absolutely

undistinguishable in external characters.

One other rock, of plutonic origin, and highly important in the history

of the Cordillera, from having been injected in most of the great axes

of elevation, and from having apparently been instrumental in

metamorphosing the superincumbent strata, may be conveniently described

in this preliminary discussion. It has been called by some authors

ANDESITE: it mainly consists of well-crystallised white albite (as

determined with the goniometer in numerous specimens both by Professor

Miller and myself), of less perfectly crystallised green hornblende,

often associated with much mica, with chlorite and epidote, and

occasionally with a few grains of quartz: in one instance in Northern

Chile, I found crystals of orthitic or potash feldspar, mingled with

those of albite. (I here, and elsewhere, call by this name, those

feldspathic minerals which cleave like albite: but it now appears

(“Edinburgh New Philosophical Journal” volume 24 page 181) that Abich

has analysed a mineral from the Cordillera, associated with hornblende

and quartz (probably the same rock with that here under discussion),

which cleaves like albite, but which is a new and distinct kind, called

by him ANDESINE. It is allied to leucite, with the greater proportion

of its potash replaced by lime and soda. This mineral seems scarcely

distinguishable from albite, except by analysis.) Where the mica and

quartz are abundant, the rock cannot be distinguished from granite; and

it may be called andesitic granite. Where these two minerals are quite

absent, and when, as often then happens, the crystals of albite are

imperfect and blend together, the rock may be called andesitic

porphyry, which bears nearly the same relation to andesitic granite

that euritic porphyry does to common granite. These andesitic rocks

form mountain masses of a white colour, which, in their general outline

and appearance—in their joints—in their occasionally including

dark-coloured, angular fragments, apparently of some pre-existing

rock—and in the great dikes branching from them into the superincumbent

strata, manifest a close and striking resemblance to masses of common

granite and syenite: I never, however, saw in these andesitic rocks,

those granitic veins of segregation which are so common in true

granites. We have seen that andesite occurs in three places in Tierra

del Fuego; in Chile, from S. Fernando to Copiapo, a distance of 450

miles, I found it under most of the axes of elevation; in a collection

of specimens from the Cordillera of Lima in Peru, I immediately

recognised it; and Erman states that it occurs in Eastern Kamtschatka.

(“Geographical Journal” volume 9 page 510.) From its wide range, and

from the important part it has played in the history of the Cordillera,

I think this rock has well deserved its distinct name of Andesite.

The few still active volcanoes in Chile are confined to the central and

loftiest ranges of the Cordillera; and volcanic matter, such as appears

to have been of subaerial eruption, is everywhere rare. According to

Meyen, there is a hill of pumice high up the valley of the Maypu, and

likewise a trachytic formation at Colina, a village situated north of

St. Jago. (“Reise um Erde” Th. 1 ss. 338 and 362.) Close to this latter

city, there are two hills formed of a pale feldspathic porphyry,

remarkable from being doubly columnar, great cylindrical columns being

subdivided into smaller four- or five-sided ones; and a third hillock

(Cerro Blanco) is formed of a fragmentary mass of rock, which I

believed to be of volcanic origin, intermediate in character between

the above feldspathic porphyry and common trachyte, and containing

needles of hornblende and granular oxide of iron. Near the Baths of

Cauquenes, between two short parallel lines of elevation, where they

are intersected by the valley, there is a small, though distinct

volcanic district; the rock is a dark grey (andesitic) trachyte, which

fuses into a greenish-grey bead, and is formed of long crystals of

fractured glassy albite (judging from one measurement) mingled with

well- formed crystals, often twin, of augite. The whole mass is

vesicular, but the surface is darker coloured and much more vesicular

than any other part. This trachyte forms a cliff-bounded, horizontal,

narrow strip on the steep southern side of the valley, at the height of

four or five hundred feet above the river-bed; judging from an

apparently corresponding line of cliff on the northern side, the valley

must once have been filled up to this height by a field of lava. On the

summit of a lofty mountain some leagues higher up this same valley of

the Cachapual, I found columnar pitchstone porphyritic with feldspar; I

do not suppose this rock to be of volcanic origin, and only mention it

here, from its being intersected by masses and dikes of a VESICULAR

rock, approaching in character to trachyte; in no other part of Chile

did I observe vesicular or amygdaloidal dikes, though these are so

common in ordinary volcanic districts.

PASSAGE OF THE ANDES BY THE PORTILLO OR PEQUENES PASS.

Although I crossed the Cordillera only once by this pass, and only once

by that of the Cumbre or Uspallata (presently to be described), riding

slowly and halting occasionally to ascend the mountains, there are many

circumstances favourable to obtaining a more faithful sketch of their

structure than would at first be thought possible from so short an

examination. The mountains are steep and absolutely bare of vegetation;

the atmosphere is resplendently clear; the stratification distinct; and

the rocks brightly and variously coloured: some of the natural sections

might be truly compared for distinctness to those coloured ones in

geological works. Considering how little is known of the structure of

this gigantic range, to which I particularly attended, most travellers

having collected only specimens of the rocks, I think my

sketch-sections, though necessarily imperfect, possess some interest.

Section 1/1 in Plate 1 which I will now describe in detail, is on a

horizontal scale of a third of an inch to a nautical mile, and on a

vertical scale of one inch to a mile (or 6,000 feet). The width of the

range (excluding a few outlying hillocks), from the plain on which St.

Jago the capital of Chile stands, to the Pampas, is sixty miles, as far

as I can judge from the maps, which differ from each other and are all

EXCEEDINGLY imperfect. The St. Jago plain at the mouth of the Maypu, I

estimate from adjoining known points at 2,300 feet, and the Pampas at

3,500 feet, both above the level of the sea. The height of the Pequenes

line, according to Dr. Gillies, is 13,210 feet (“Journal of Natural and

Geographical Science” August 1830.); and that of the Portillo line

(both in the gaps where the road crosses them) is 14,345 feet; the

lowest part of the intermediate valley of Tenuyan is 7,530 feet—all

above the level of the sea.

The Cordillera here, and indeed I believe throughout Chile, consist of

several parallel, anticlinal and uniclinal mountain-lines, ranging

north, or north with a little westing, and south. Some exterior and

much lower ridges often vary considerably from this course, projecting

like oblique spurs from the main ranges: in the district towards the

Pacific, the mountains, as before remarked, extend in various

directions, even east and west. In the main exterior lines, the strata,

as also before remarked, are seldom inclined at a high angle; but in

the central lofty ridges they are almost always highly inclined, broken

by many great faults, and often vertical. As far as I could judge, few

of the ranges are of great length: and in the central parts of the

Cordillera, I was frequently able to follow with my eye a ridge

gradually becoming higher and higher, as the stratification increased

in inclination, from one end where its height was trifling and its

strata gently inclined to the other end where vertical strata formed

snow-clad pinnacles. Even outside the main Cordillera, near the baths

of Cauquenes, I observed one such case, where a north and south ridge

had its strata in the valley inclined at 37 degrees, and less than a

mile south of it at 67 degrees: another parallel and similarly inclined

ridge rose at the distance of about five miles, into a lofty mountain

with absolutely vertical strata. Within the Cordillera, the height of

the ridges and the inclination of the strata often became doubled and

trebled in much shorter distances than five miles; this peculiar form

of upheaval probably indicates that the stratified crust was thin, and

hence yielded to the underlying intrusive masses unequally, at certain

points on the lines of fissure.

The valleys, by which the Cordillera are drained, follow the anticlinal

or rarely synclinal troughs, which deviate most from the usual north

and south course; or still more commonly those lines of faults or of

unequal curvature (that is, lines with the strata on both hands dipping

in the same direction, but at a somewhat different angle) which deviate

most from a northerly course. Occasionally the torrents run for some

distance in the north and south valleys, and then recover their eastern

or western course by bursting through the ranges at those points where

the strata have been least inclined and the height consequently is

less. Hence the valleys, along which the roads run, are generally

zigzag; and, in drawing an east and west section, it is necessary to

contract greatly that which is actually seen on the road.

Commencing at the western end of Section 1/1 where the R. Maypu

debouches on the plain of St. Jago, we immediately enter on the

porphyritic conglomerate formation, and in the midst of it find some

hummocks [A] of granite and syenite, which probably (for I neglected to

collect specimens) belong to the andesitic class. These are succeeded

by some rugged hills [B] of dark-green, crystalline, feldspathic and in

some parts slaty rocks, which I believe belong to the altered

clay-slate formation. From this point, great mountains of purplish and

greenish, generally thinly stratified, highly porphyritic

conglomerates, including many strata of amygdaloidal and greenstone

porphyries, extend up the valley to the junction of the rivers Yeso and

Volcan. As the valley here runs in a very southerly course, the width

of the porphyritic conglomerate formation is quite conjectural; and

from the same cause, I was unable to make out much about the

stratification. In most of the exterior mountains the dip was gentle

and directed inwards; and at only one spot I observed an inclination as

high as 50 degrees. Near the junction of the R. Colorado with the main

stream, there is a hill of whitish, brecciated, partially decomposed

feldspathic porphyry, having a volcanic aspect but not being really of

that nature: at Tolla, however, in this valley, Dr. Meyen met with a

hill of pumice containing mica. (“Reise um Erde” Th.1 ss. 338, 341.) At

the junction of the Yeso and Volcan [D] there is an extensive mass, in

white conical hillocks, of andesite, containing some mica, and passing

either into andesitic granite, or into a spotted, semi-granular mixture

of albitic (?) feldspar and hornblende: in the midst of this formation

Dr. Meyen found true trachyte. The andesite is covered by strata of

dark-coloured, crystalline, obscurely porphyritic rocks, and above them

by the ordinary porphyritic conglomerates,—the strata all dipping away

at a small angle from the underlying mass. The surrounding lofty

mountains appear to be entirely composed of the porphyritic

conglomerate, and I estimated its thickness here at between six and

seven thousand feet. Beyond the junction of the Yeso and Volcan, the

porphyritic strata appear to dip towards the hillocks of andesite at an

angle of 40 degrees; but at some distant points on the same ridge they

are bent up and vertical. Following the valley of the Yeso, trending

N.E. (and therefore still unfavourable for our transverse section), the

same porphyritic conglomerate formation is prolonged to near the

Cuestadel Indio, situated at the western end of the basin (like a

drained lake) of Yeso. Some way before arriving at this point, distant

lofty pinnacles capped by coloured strata belonging to the great

gypseous formation could first be seen. From the summit of the Cuesta,

looking southward, there is a magnificent sectional view of a

mountain-mass, at least 2,000 feet in thickness [E], of fine andesite

granite (containing much black mica, a little chlorite and quartz),

which sends great white dikes far into the superincumbent,

dark-coloured, porphyritic conglomerates. At the line of junction the

two formations are wonderfully interlaced together: in the lower part

of the porphyritic conglomerate, the stratification has been quite

obliterated, whilst in the upper part it is very distinct, the beds

composing the crests of the surrounding mountains being inclined at

angles of between 70 and 80 degrees, and some being even vertical. On

the northern side of the valley, there is a great corresponding mass of

andesitic granite, which is encased by porphyritic conglomerate,

dipping both on the western and eastern sides, at about 80 degrees to

west, but on the eastern side with the tips of the strata bent in such

a manner, as to render it probable that the whole mass has been on that

side thrown over and inverted.

In the valley basin of the Yeso, which I estimated at 7,000 feet above

the level of the sea, we first reach at [F] the gypseous formation. Its

thickness is very great. It consists in most parts of snow-white, hard,

compact gypsum, which breaks with a saccharine fracture, having

translucent edges; under the blowpipe gives out much vapour; it

frequently includes nests and exceedingly thin layers of crystallised,

blackish carbonate of lime. Large, irregularly shaped concretions

(externally still exhibiting lines of aqueous deposition) of

blackish-grey, but sometimes white, coarsely and brilliantly

crystallised, hard anhydrite, abound within the common gypsum.

Hillocks, formed of the hardest and purest varieties of the white

gypsum, stand up above the surrounding parts, and have their surfaces

cracked and marked, just like newly baked bread. There is much pale

brown, soft argillaceous gypsum; and there were some intercalated green

beds which I had not time to reach. I saw only one fragment of selenite

or transparent gypsum, and that perhaps may have come from some

subsequently formed vein. From the mineralogical characters here given,

it is probable that these gypseous beds have undergone some metamorphic

action. The strata are much hidden by detritus, but they appeared in

most parts to be highly inclined; and in an adjoining lofty pinnacle

they could be distinctly seen bending up, and becoming vertical,

conformably with the underlying porphyritic conglomerate. In very many

parts of the great mountain-face [F], composed of thin gypseous beds,

there were innumerable masses, irregularly shaped and not like dikes,

yet with well-defined edges, of an imperfectly granular, pale greenish,

or yellowish-white rock, essentially composed of feldspar, with a

little chlorite or hornblende, epidote, iron-pyrites, and ferruginous

powder: I believe that these curious trappean masses have been injected

from the not far distant mountain-mass [E] of andesite whilst still

fluid, and that owing to the softness of the gypseous strata they have

not acquired the ordinary forms of dikes. Subsequently to the injection

of these feldspathic rocks, a great dislocation has taken place; and

the much shattered gypseous strata here overlie a hillock [G], composed

of vertical strata of impure limestone and of black highly calcareous

shale including threads of gypsum: these rocks, as we shall presently

see, belong to the upper parts of the gypseous series, and hence must

here have been thrown down by a vast fault.

Proceeding up the valley-basin of the Yeso, and taking our section

sometimes on one hand and sometimes on the other, we come to a great

hill of stratified porphyritic conglomerate [H] dipping at 45 degrees

to the west; and a few hundred yards farther on, we have a bed between

three or four hundred feet thick of gypsum [I] dipping eastward at a

very high angle: here then we have a fault and anticlinal axis. On the

opposite side of the valley, a vertical mass of red conglomerate,

conformably underlying the gypsum, appears gradually to lose its

stratification and passes into a mountain of porphyry. The gypsum [I]

is covered by a bed [K], at least 1,000 feet in thickness, of a

purplish-red, compact, heavy, fine-grained sandstone or mudstone, which

fuses easily into a white enamel, and is seen under a lens to contain

triturated crystals. This is succeeded by a bed [L], 1,000 feet thick

(I believe I understate the thickness) of gypsum, exactly like the beds

before described; and this again is capped by another great bed [M] of

purplish-red sandstone. All these strata dip eastward; but the

inclination becomes less and less, as we leave the first and almost

vertical bed [I] of gypsum.

Leaving the basin-plain of Yeso, the road rapidly ascends, passing by

mountains composed of the gypseous and associated beds, with their

stratification greatly disturbed and therefore not easily intelligible:

hence this part of the section has been left uncoloured. Shortly before

reaching the great Pequenes ridge, the lowest stratum visible [N] is a

red sandstone or mudstone, capped by a vast thickness of black,

compact, calcareous, shaly rock [O], which has been thrown into four

lofty, though small ridges: looking northward, the strata in these

ridges are seen gradually to rise in inclination, becoming in some

distant pinnacles absolutely vertical.

The ridge of Pequenes, which divides the waters flowing into the

Pacific and Atlantic Oceans, extends in a nearly N.N.W. and S.S.E.

line; its strata dip eastward at an angle of between 30 and 45 degrees,

but in the higher peaks bending up and becoming almost vertical. Where

the road crosses this range, the height is 13,210 feet above the

sea-level, and I estimated the neighbouring pinnacles at from fourteen

to fifteen thousand feet. The lowest stratum visible in this ridge is a

red stratified sandstone [P]; on it are superimposed two great masses

[Q and S] of black, hard, compact, even having a conchoidal fracture,

calcareous, more or less laminated shale, passing into limestone: this

rock contains organic remains, presently to be enumerated. The

compacter varieties fuse easily in a white glass; and this I may add is

a very general character with all the sedimentary beds in the

Cordillera: although this rock when broken is generally quite black, it

everywhere weathers into an ash-grey tint. Between these two great

masses [Q and S], a bed [R] of gypsum is interposed, about three

hundred feet in thickness, and having the same characters as heretofore

described. I estimated the total thickness of these three beds [Q, R,

S] at nearly three thousand feet; and to this must be added, as will be

immediately seen, a great overlying mass of red sandstone.

In descending the eastern slope of this great central range, the

strata, which in the upper part dip eastward at about an angle of 40

degrees, become more and more curved, till they are nearly vertical;

and a little further onwards there is seen on the further side of a

ravine, a thick mass of strata of bright red sandstone [T], with their

upper extremities slightly curved, showing that they were once

conformably prolonged over the beds [S]: on the southern and opposite

side of the road, this red sandstone and the underlying black shaly

rocks stand vertical, and in actual juxtaposition. Continuing to

descend, we come to a synclinal valley filled with rubbish, beyond

which we have the red sandstone [T2] corresponding with [T], and now

dipping, as is seen both north and south of the road, at 45 degrees to

the west; and under it, the beds [S2, R2, Q2, and I believe P2] in

corresponding order and of similar composition, with those on the

western flank of the Pequenes range, but dipping westward. Close to the

synclinal valley the dip of these strata is 45 degrees, but at the

eastern or farther end of the series it increases to 60 degrees. Here

the great gypseous formation abruptly terminates, and is succeeded

eastward by a pile of more modern strata. Considering how violently

these central ranges have been dislocated, and how very numerous dikes

are in the exterior and lower parts of the Cordillera, it is remarkable

that I did not here notice a single dike. The prevailing rock in this

neighbourhood is the black, calcareous, compact shale, whilst in the

valley-basin of the Yeso the purplish red sandstone or mudstone

predominates,—both being associated with gypseous strata of exactly the

same nature. It would be very difficult to ascertain the relative

superposition of these several masses, for we shall afterwards see in

the Cumbre Pass that the gypseous and intercalated beds are

lens-shaped, and that they thin out, even where very thick, and

disappear in short horizontal distances: it is quite possible that the

black shales and red sandstones may be contemporaneous, but it is more

probable that the former compose the uppermost parts of the series.

The fossils above alluded to in the black calcareous shales are few in

number, and are in an imperfect condition; they consist, as named for

me by M. d’Orbigny, of:—

1. Ammonite, indeterminable, near to A. recticostatus, d’Orbigny, “Pal.

Franc.” (Neocomian formation). 2. Gryphaea, near to G. Couloni

(Neocomian formations of France and Neufchatel). 3. Natica,

indeterminable. 4. Cyprina rostrata, d’Orbigny, “Pal. Franc.”

(Neocomian formation). 5. Rostellaria angulosa (?), d’Orbigny, “Pal. de

l’Amer. Mer.” 6. Terebratula (?).

Some of the fragments of Ammonites were as thick as a man’s arm: the

Gryphaea is much the most abundant shell. These fossils M. d’Orbigny

considers as belonging to the Neocomian stage of the Cretaceous system.

Dr. Meyen, who ascended the valley of the Rio Volcan, a branch of the

Yeso, found a nearly similar, but apparently more calcareous formation,

with much gypsum, and no doubt the equivalent of that here described

(“Reise um Erde” etc. Th. 1 s. 355.): the beds were vertical, and were

prolonged up to the limits of perpetual snow; at the height of 9,000

feet above the sea, they abounded with fossils, consisting, according

to Von Buch (“Descript. Phys. des Iles Canaries” page 471.), of:—

1. Exogyra (Gryphaea) Couloni, absolutely identical with specimens from

the Jura and South of France. 2. Trigonia costata, identical with those

found in the upper Jurassic beds at Hildesheim. 3. Pecten striatus,

identical with those found in the upper Jurassic beds at Hildesheim. 4.

Cucullaea, corresponding in form to C. longirostris, so frequent in the

upper Jurassic beds of Westphalia. 5. Ammonites resembling A. biplex.

Von Buch concludes that this formation is intermediate between the

limestone of the Jura and the chalk, and that it is analogous with the

uppermost Jurassic beds forming the plains of Switzerland. Hence M.

D’Orbigny and Von Buch, under different terms, compare these fossils to

those from the same late stage in the secondary formations of Europe.

Some of the fossils which I collected were found a good way down the

western slope of the main ridge, and hence must originally have been

covered up by a great thickness of the black shaly rock, independently

of the now denuded, thick, overlying masses of red sandstone. I

neglected at the time to estimate how many hundred or rather thousand

feet thick the superincumbent strata must have been: and I will not now

attempt to do so. This, however, would have been a highly interesting

point, as indicative of a great amount of subsidence, of which we shall

hereafter find in other parts of the Cordillera analogous evidence

during this same period. The altitude of the Peuquenes Range,

considering its not great antiquity, is very remarkable; many of the

fossils were embedded at the height of 13,210 feet, and the same beds

are prolonged up to at least from fourteen to fifteen thousand feet

above the level of the sea.

THE PORTILLO OR EASTERN CHAIN.

The valley of Tenuyan, separating the Peuquenes and Portillo lines, is,

as estimated by Dr. Gillies and myself, about twenty miles in width;

the lowest part, where the road crosses the river, being 7,500 feet

above the sea-level. The pass on the Portillo line is 14,365 feet high

(1,100 feet higher than that on the Peuquenes), and the neighbouring

pinnacles must, I conceive, rise to nearly 16,000 feet above the sea.

The river draining the intermediate valley of Tenuyan, passes through

the Portillo line. To return to our section:—shortly after leaving the

lower beds [P2] of the gypseous formation, we come to grand masses of a

coarse, red conglomerate [V], totally unlike any strata hitherto seen

in the Cordillera. This conglomerate is distinctly stratified, some of

the beds being well defined by the greater size of the pebbles: the

cement is calcareous and sometimes crystalline, though the mass shows

no signs of having been metamorphosed. The included pebbles are either

perfectly or only partially rounded: they consist of purplish

sandstones, of various porphyries, of brownish limestone, of black

calcareous, compact shale precisely like that in situ in the Peuquenes

range, and CONTAINING SOME OF THE SAME FOSSIL SHELLS; also very many

pebbles of quartz, some of micaceous schist, and numerous, broken,

rounded crystals of a reddish orthitic or potash feldspar (as

determined by Professor Miller), and these from their size must have

been derived from a coarse-grained rock, probably granite. From this

feldspar being orthitic, and even from its external appearance, I

venture positively to affirm that it has not been derived from the

rocks of the western ranges; but, on the other hand, it may well have

come, together with the quartz and metamorphic schists, from the

eastern or Portillo line, for this line mainly consists of coarse

orthitic granite. The pebbles of the fossiliferous slate and of the

purple sandstone, certainly have been derived from the Peuquenes or

western ranges.

The road crosses the valley of Tenuyan in a nearly east and west line,

and for several miles we have on both hands the conglomerate,

everywhere dipping west and forming separate great mountains. The

strata, where first met with, after leaving the gypseous formation, are

inclined westward at an angle of only 20 degrees, which further on

increases to about 45 degrees. The gypseous strata, as we have seen,

are also inclined westward: hence, when looking from the eastern side

of the valley towards the Peuquenes range, a most deceptive appearance

is presented, as if the newer beds of conglomerate dipped directly

under the much older beds of the gypseous formation. In the middle of

the valley, a bold mountain of unstratified lilac-coloured porphyry

(with crystals of hornblende) projects; and further on, a little south

of the road, there is another mountain, with its strata inclined at a

small angle eastwards, which in its general aspect and colour,

resembles the porphyritic conglomerate formation, so rare on this side

of the Peuquenes line and so grandly developed throughout the western

ranges.

The conglomerate is of great thickness: I do not suppose that the

strata forming the separate mountain-masses [V,V,V] have ever been

prolonged over each other, but that one mass has been broken up by

several, distinct, parallel, uniclinal lines of elevation. Judging

therefore of the thickness of the conglomerate, as seen in the separate

mountain-masses, I estimated it at least from one thousand five hundred

to two thousand feet. The lower beds rest conformably on some

singularly coloured, soft strata [W], which I could not reach to

examine; and these again rest conformably on a thick mass of micaceous,

thinly laminated, siliceous sandstone [X], associated with a little

black clay-slate. These lower beds are traversed by several dikes of

decomposing porphyry. The laminated sandstone is directly superimposed

on the vast masses of granite [Y,Y] which mainly compose the Portillo

range. The line of junction between this latter rock, which is of a

bright red colour, and the whitish sandstone was beautifully distinct;

the sandstone being penetrated by numerous, great, tortuous dikes

branching from the granite, and having been converted into a granular

quartz rock (singularly like that of the Falkland Islands), containing

specks of an ochrey powder, and black crystalline atoms, apparently of

imperfect mica. The quartzose strata in one spot were folded into a

regular dome.

The granite which composes the magnificent bare pinnacles and the steep

western flank of the Portillo chain, is of a brick-red colour, coarsely

crystallised, and composed of orthitic or potash feldspar, quartz, and

imperfect mica in small quantity, sometimes passing into chlorite.

These minerals occasionally assume a laminar or foliated arrangement.

The fact of the feldspar being orthitic in this range, is very

remarkable, considering how rare, or rather, as I believe, entirely

absent, this mineral is throughout the western ranges, in which

soda-feldspar, or at least a variety cleaving like albite, is so

extremely abundant. In one spot on the western flank, and on the

eastern flank near Los Manantiales and near the crest, I noticed some

great masses of a whitish granite, parts of it fine- grained, and parts

containing large crystals of feldspar; I neglected to collect

specimens, so I do not know whether this feldspar is also orthitic,

though I am inclined to think so from its general appearance. I saw

also some syenite and one mass which resembled andesite, but of which I

likewise neglected to collect specimens. From the manner in which the

whitish granites formed separate mountain-masses in the midst of the

brick-red variety, and from one such mass near the crest being

traversed by numerous veins of flesh-coloured and greenish eurite (into

which I occasionally observed the brick-red granite insensibly

passing), I conclude that the white granites probably belong to an

older formation, almost overwhelmed and penetrated by the red granite.

On the crest I saw also, at a short distance, some coloured stratified

beds, apparently like those [W] at the western base, but was prevented

examining them by a snowstorm: Mr. Caldcleugh, however, collected here

specimens of ribboned jasper, magnesian limestone, and other minerals.

(“Travels” etc. volume 1 page 308.) A little way down the eastern slope

a few fragments of quartz and mica-slate are met with; but the great

formation of this latter rock [Z], which covers up much of the eastern

flank and base of the Portillo range, cannot be conveniently examined

until much lower down at a place called Mal Paso. The mica-schist here

consists of thick layers of quartz, with intervening folia of

finely-scaly mica, often passing into a substance like black glossy

clay-slate: in one spot, the layers of the quartz having disappeared,

the whole mass became converted into glossy clay-slate. Where the folia

were best defined, they were inclined at a high angle westward, that

is, towards the range. The line of junction between the dark mica-slate

and the coarse red granite was most clearly distinguishable from a vast

distance: the granite sent many small veins into the mica-slate, and

included some angular fragments of it. As the sandstone on the western

base has been converted by the red granite into a granular quartz-rock,

so this great formation of mica-schist may possibly have been

metamorphosed at the same time and by the same means; but I think it

more probable, considering its more perfect metamorphic character and

its well-pronounced foliation, that it belongs to an anterior epoch,

connected with the white granites: I am the more inclined to this view,

from having found at the foot of the range the mica-schist surrounding

a hummock [Y2], exclusively composed of white granite. Near Los

Arenales, the mountains on all sides are composed of the mica-slate;

and looking backwards from this point up to the bare gigantic peaks

above, the view was eminently interesting. The colours of the red

granite and the black mica-slate are so distinct, that with a bright

light these rocks could be readily distinguished even from the Pampas,

at a level of at least 9,000 feet below. The red granite, from being

divided by parallel joints, has weathered into sharp pinnacles, on some

of which, even on some of the loftiest, little caps of mica-schist

could be clearly seen: here and there isolated patches of this rock

adhered to the mountain-flanks, and these often corresponded in height

and position on the opposite sides of the immense valleys. Lower down

the schist prevailed more and more, with only a few quite small points

of granite projecting through. Looking at the entire eastern face of

the Portillo range, the red colour far exceeds in area the black; yet

it was scarcely possible to doubt that the granite had once been almost

wholly encased by the mica-schist.

At Los Arenales, low down on the eastern flank, the mica-slate is

traversed by several closely adjoining, broad dikes, parallel to each

other and to the foliation of the schist. The dikes are formed of three

different varieties of rock, of which a pale brown feldspathic porphyry

with grains of quartz was much the most abundant. These dikes with

their granules of quartz, as well as the mica-schist itself, strikingly

resemble the rocks of the Chonos Archipelago. At a height of about

twelve hundred feet above the dikes, and perhaps connected with them,

there is a range of cliffs formed of successive lava-streams [AA],

between three and four hundred feet in thickness, and in places finely

columnar. The lava consists of dark- greyish, harsh rocks, intermediate

in character between trachyte and basalt, containing glassy feldspar,

olivine, and a little mica, and sometimes amygdaloidal with zeolite:

the basis is either quite compact, or crenulated with air-vesicles

arranged in laminae. The streams are separated from each other by beds

of fragmentary brown scoriae, firmly cemented together, and including a

few well-rounded pebbles of lava. From their general appearance, I

suspect that these lava-streams flowed at an ancient period under the

pressure of the sea, when the Atlantic covered the Pampas and washed

the eastern foot of the Cordillera. (This conclusion might, perhaps,

even have been anticipated, from the general rarity of volcanic action,

except near the sea or large bodies of water. Conformably with this

rule, at the present day, there are no active volcanoes on this eastern

side of the Cordillera; nor are severe earthquakes experienced here.)

On the opposite and northern side of the valley there is another line

of lava- cliffs at a corresponding height; the valley between being of

considerable breadth, and as nearly as I could estimate 1,500 feet in

depth. This field of lava is confined on both sides by the mountains of

mica-schist, and slopes down rapidly but irregularly to the edge of the

Pampas, where, having a thickness of about two hundred feet, it

terminates against a little range of claystone porphyry. The valley in

this lower part expands into a bay-like, gentle slope, bordered by the

cliffs of lava, which must certainly once have extended across this

wide expanse. The inclination of the streams from Los Arenales to the

mouth of the valley is so great, that at the time (though ignorant of

M. Elie de Beaumont’s researches on the extremely small slope over

which lava can flow, and yet retain a compact structure and

considerable thickness) I concluded that they must subsequently to

their flowing have been upheaved and tilted from the mountains; of this

conclusion I can now entertain not the smallest doubt.

At the mouth of the valley, within the cliffs of the above lava-field,

there are remnants, in the form of separate small hillocks and of lines

of low cliffs, of a considerable deposit of compact white tuff

(quarried for filtering-stones), composed of broken pumice, volcanic

crystals, scales of mica, and fragments of lava. This mass has suffered

much denudation; and the hard mica-schist has been deeply worn, since

the period of its deposition; and this period must have been subsequent

to the denudation of the basaltic lava-streams, as attested by their

encircling cliffs standing at a higher level. At the present day, under

the existing arid climate, ages might roll past without a square yard

of rock of any kind being denuded, except perhaps in the rarely

moistened drainage-channel of the valley. Must we then look back to

that ancient period, when the waves of the sea beat against the eastern

foot of the Cordillera, for a power sufficient to denude extensively,

though superficially, this tufaceous deposit, soft although it be?

There remains only to mention some little water-worn hillocks [BB], a

few hundred feet in height, and mere mole-hills compared with the

gigantic mountains behind them, which rise out of the sloping,

shingle-covered margin of the Pampas. The first little range is

composed of a brecciated purple porphyritic claystone, with obscurely

marked strata dipping at 70 degrees to the S.W.; the other ranges

consist of—a pale-coloured feldspathic porphyry,—a purple claystone

porphyry with grains of quartz,— and a rock almost exclusively composed

of brick-red crystals of feldspar. These outermost small lines of

elevation extend in a N.W. by W. and S.E. by S. direction.

CONCLUDING REMARKS ON THE PORTILLO RANGE.

When on the Pampas and looking southward, and whilst travelling

northward, I could see for very many leagues the red granite and dark

mica-schist forming the crest and eastern flank of the Portillo line.

This great range, according to Dr. Gillies, can be traced with little

interruption for 140 miles southward to the R. Diamante, where it

unites with the western ranges: northward, according to this same

author, it terminates where the R. Mendoza debouches from the

mountains; but a little further north in the eastern part of the Cumbre

section, there are, as we shall hereafter see, some mountain-masses of

a brick-red porphyry, the last injected amidst many other porphyries,

and having so close an analogy with the coarse red granite of the

Portillo line, that I am tempted to believe that they belong to the

same axis of injection; if so, the Portillo line is at least 200 miles

in length. Its height, even in the lowest gap in the road, is 14,365

feet, and some of the pinnacles apparently attain an elevation of about

16,000 feet above the sea. The geological history of this grand chain

appears to me eminently interesting. We may safely conclude, that at a

former period the valley of Tenuyan existed as an arm of the sea, about

twenty-miles in width, bordered on one hand by a ridge or chain of

islets of the black calcareous shales and purple sandstones of the

gypseous formation; and on the other hand, by a ridge or chain of

islets composed of mica-slate, white granite, and perhaps to a partial

extent of red granite. These two chains, whilst thus bordering the old

sea-channel, must have been exposed for a vast lapse of time to

alluvial and littoral action, during which the rocks were shattered,

the fragments rounded, and the strata of conglomerate accumulated to a

thickness of at least fifteen hundred or two thousand feet. The red

orthitic granite now forms, as we have seen, the main part of the

Portillo chain: it is injected in dikes not only into the mica-schist

and white granites, but into the laminated sandstone, which it has

metamorphosed, and which it has thrown off, together with the

conformably overlying coloured beds and stratified conglomerate, at an

angle of forty-five degrees. To have thrown off so vast a pile of

strata at this angle, is a proof that the main part of the red granite

(whether or not portions, as perhaps is probable, previously existed)

was injected in a liquified state after the accumulation both of the

laminated sandstone and of the conglomerate; this conglomerate, we

know, was accumulated, not only after the deposition of the

fossiliferous strata of the Peuquenes line, but after their elevation

and long-continued denudation: and these fossiliferous strata belong to

the early part of the Cretaceous system. Late, therefore, in a

geological sense, as must be the age of the main part of the red

granite, I can conceive nothing more impressive than the eastern view

of this great range, as forcing the mind to grapple with the idea of

the thousands of thousands of years requisite for the denudation of the

strata which originally encased it,—for that the fluidified granite was

once encased, its mineralogical composition and structure, and the bold

conical shape of the mountain-masses, yield sufficient evidence. Of the

encasing strata we see the last vestiges in the coloured beds on the

crest, in the little caps of mica-schist on some of the loftiest

pinnacles, and in the isolated patches of this same rock at

corresponding heights on the now bare and steep flanks.

The lava-streams at the eastern foot of the Portillo are interesting,

not so much from the great denudation which they have suffered at a

comparatively late period as from the evidence they afford by their

inclination taken conjointly with their thickness and compactness, that

after the great range had assumed its present general outline, it

continued to rise as an axis of elevation. The plains extending from

the base of the Cordillera to the Atlantic show that the continent has

been upraised in mass to a height of 3,500 feet, and probably to a much

greater height, for the smooth shingle-covered margin of the Pampas is

prolonged in a gentle unbroken slope far up many of the great valleys.

Nor let it be assumed that the Peuquenes and Portillo ranges have

undergone only movements of elevation; for we shall hereafter see, that

the bottom of the sea subsided several thousand feet during the

deposition of strata, occupying the same relative place in the

Cordillera, with those of the Peuquenes ridge; moreover, we shall see

from the unequivocal evidence of buried upright trees, that at a

somewhat later period, during the formation of the Uspallata chain,

which corresponds geographically with that of the Portillo, there was

another subsidence of many thousand feet: here, indeed, in the valley

of Tenuyan, the accumulation of the coarse stratified conglomerate to a

thickness of fifteen hundred or two thousand feet, offers strong

presumptive evidence of subsidence; for all existing analogies lead to

the belief that large pebbles can be transported only in shallow water,

liable to be affected by currents and movements of undulation—and if

so, the shallow bed of the sea on which the pebbles were first

deposited must necessarily have sunk to allow of the accumulation of

the superincumbent strata. What a history of changes of level, and of

wear and tear, all since the age of the latter secondary formations of

Europe, does the structure of this one great mountain-chain reveal!

PASSAGE OF THE ANDES BY THE CUMBRE OR USPALLATA PASS.

This Pass crosses the Andes about sixty miles north of that just

described: the section given in Plate 1, Section 1/2, is on the same

scale as before, namely, at one-third of an inch to a mile in distance,

and one inch to a mile (or 6,000 feet) in height. Like the last

section, it is a mere sketch, and cannot pretend to accuracy, though

made under favourable circumstances. We will commence as before, with

the western half, of which the main range bears the name of the Cumbre

(that is the Ridge), and corresponds to the Peuquenes line in the

former section; as does the Uspallata range, though on a much smaller

scale, to that of the Portillo. Near the point where the river

Aconcagua debouches on the basin plain of the same name, at a height of

about two thousand three hundred feet above the sea, we meet with the

usual purple and greenish porphyritic claystone conglomerate. Beds of

this nature, alternating with numerous compact and amygdaloidal

porphyries, which have flowed as submarine lavas, and associated with

great mountain- masses of various, injected, non-stratified porphyries,

are prolonged the whole distance up to the Cumbre or central ridge. One

of the commonest stratified porphyries is of a green colour, highly

amygdaloidal with the various minerals described in the preliminary

discussion, and including fine tabular crystals of albite. The

mountain-range north (often with a little westing) and south. The

stratification, wherever I could clearly distinguish it, was inclined

westward or towards the Pacific, and, except near the Cumbre, seldom at

angles above 25 degrees. Only at one spot on this western side, on a

lofty pinnacle not far from the Cumbre, I saw strata apparently

belonging to the gypseous formation, and conformably capping a pile of

stratified porphyries. Hence, both in composition and in

stratification, the structure of the mountains on this western side of

the divortium aquarum, is far more simple than in the corresponding

part of the Peuquenes section. In the porphyritic claystone

conglomerate, the mechanical structure and the planes of stratification

have generally been much obscured and even quite obliterated towards

the base of the series, whilst in the upper parts, near the summits of

the mountains, both are distinctly displayed. In these upper portions

the porphyries are generally lighter coloured. In three places [X, Y,

Z] masses of andesite are exposed: at [Y], this rock contained some

quartz, but the greater part consisted of andesitic porphyry, with only

a few well-developed crystals of albite, and forming a great white

mass, having the external aspect of granite, capped by much dark

unstratified porphyry. In many parts of the mountains, there are dikes

of a green colour, and other white ones, which latter probably spring

from underlying masses of andesite.

The Cumbre, where the road crosses it, is, according to Mr. Pentland,

12,454 feet above the sea; and the neighbouring peaks, composed of dark

purple and whitish porphyries, some obscurely stratified with a

westerly dip, and others without a trace of stratification, must exceed

13,000 feet in height. Descending the eastern slope of the Cumbre, the

structure becomes very complicated, and generally differs on the two

sides of the east and west line of road and section. First we come to a

great mass [A] of nearly vertical, singularly contorted strata,

composed of highly compact red sandstones, and of often calcareous

conglomerates, and penetrated by green, yellow, and reddish dikes; but

I shall presently have an opportunity of describing in some detail an

analogous pile of strata. These vertical beds are abruptly succeeded by

others [B], of apparently nearly the same nature but more

metamorphosed, alternating with porphyries and limestones; these dip

for a short space westward, but there has been here an extraordinary

dislocation, which, on the north side of the road, appears to have

determined the excavation of the north and south valley of the R. de

las Cuevas. On this northern side of the road, the strata [B] are

prolonged till they come in close contact with a jagged lofty mountain

[D] of dark- coloured, unstratified, intrusive porphyry, where the beds

have been more highly inclined and still more metamorphosed. This

mountain of porphyry seems to form a short axis of elevation, for south

of the road in its line there is a hill [C] of porphyritic conglomerate

with absolutely vertical strata.

We now come to the gypseous formation: I will first describe the

structure of the several mountains, and then give in one section a

detailed account of the nature of the rocks. On the north side of the

road, which here runs in an east and west valley, the mountain of

porphyry [D] is succeeded by a hill [E] formed of the upper gypseous

strata tilted, at an angle of between 70 and 80 degrees to the west, by

a uniclinal axis of elevation which does not run parallel to the other

neighbouring ranges, and which is of short length; for on the south

side of the valley its prolongation is marked only by a small flexure

in a pile of strata inclined by a quite separate axis. A little further

on the north and south valley of Horcones enters at right angles our

line of section; its western side is bounded by a hill of gypseous

strata [F] dipping westward at about 45 degrees, and its eastern side

by a mountain of similar strata [G] inclined westward at 70 degrees,

and superimposed by an oblique fault on another mass of the same strata

[H], also inclined westward, but at an angle of about 30 degrees: the

complicated relation of these three masses [F, G, H] is explained by

the structure of a great mountain-range lying some way to the north, in

which a regular anticlinal axis (represented in the section by dotted

lines) is seen, with the strata on its eastern side again bending up

and forming a distinct uniclinal axis, of which the beds marked [H]

form the lower part. This great uniclinal line is intersected, near the

Puente del Inca, by the valley along which the road runs, and the

strata composing it will be immediately described. On the south side of

the road, in the space corresponding with the mountains [E, F, and G],

the strata everywhere dip westward generally at an angle of 30 degrees,

occasionally mounting up to 45 degrees, but not in an unbroken line,

for there are several vertical faults, forming separate uniclinal

masses, all dipping in the same direction,—a form of elevation common

in the Cordillera. We thus see that within a narrow space, the gypseous

strata have been upheaved and crushed together by a great uniclinal,

anticlinal, and one lesser uniclinal line [E] of elevation; and that

between these three lines and the Cumbre, in the sandstones,

conglomerates and porphyritic formation, there have been at least two

or three other great elevatory axes.