Chapter VI PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.

Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of

foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in,

decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S.

Ventana.—Claystone porphyry formation of Patagonia; singular

metamorphic rocks; pseudo-dikes.—Falkland Islands, Palæozoic fossils

of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;

cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,

foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,

successive formation of.—Central and Northern Chile.—Concluding remarks

on cleavage and foliation.—Their close analogy and similar origin.

—Stratification of metamorphic schists.—Foliation of intrusive

rocks.—Relation of cleavage and foliation to the lines of tension

during metamorphosis.

The metamorphic and plutonic formations of the several districts

visited by the _Beagle_ will be here chiefly treated of, but only such

cases as appear to me new, or of some special interest, will be

described in detail; at the end of the chapter I will sum up all the

facts on cleavage and foliation,—to which I particularly attended.

_Bahia, Brazil: lat. 13° south._—The prevailing rock is gneiss, often

passing, by the disappearance of the quartz and mica, and by the

feldspar losing its red colour, into a brilliantly grey primitive

greenstone. Not unfrequently quartz and hornblende are arranged in

layers in almost amorphous feldspar. There is some fine-grained

syenitic granite, orbicularly marked by ferruginous lines, and

weathering into vertical, cylindrical holes, almost touching each

other. In the gneiss, concretions of granular feldspar and others of

garnets with mica occur. The gneiss is traversed by numerous dikes

composed of black, finely crystallised, hornblendic rock, containing a

little glassy feldspar and sometimes mica, and varying in thickness

from mere threads to ten feet: these threads, which are often

curvilinear, could sometimes be traced running into the larger dikes.

One of these dikes was remarkable from having been in two or three

places laterally disjointed, with unbroken gneiss interposed between

the broken ends, and in one part with a portion of the gneiss driven,

apparently whilst in a softened state, into its side or wall. In

several neighbouring places, the gneiss included angular, well-defined,

sometimes bent, masses of hornblende rock, quite like, except in being

more perfectly crystallised, that forming the dikes, and, at least in

one instance, containing (as determined by Professor Miller) augite as

well as hornblende. In one or two cases these angular masses, though

now quite separate from each other by the solid gneiss, had, from their

exact correspondence in size and shape, evidently once been united;

hence I cannot doubt that most or all of the fragments have been

derived from the breaking up of the dikes, of which we see the first

stage in the above-mentioned laterally disjointed one. The gneiss close

to the fragments generally contained many large crystals of hornblende,

which are entirely absent or rare in other parts: its folia or laminæ

were gently bent round the fragments, in the same manner as they

sometimes are round concretions. Hence the gneiss has certainly been

softened, its composition modified, and its folia arranged,

subsequently to the breaking up of the dikes,[1] these latter also

having been at the same time bent and softened.

[1] Professor Hitchcock (“Geology of Massachusetts,” vol. ii, p. 673,

gives a closely similar case of a greenstone dike in syenite.

I must here take the opportunity of premising, that by the term

_cleavage_ I imply those planes of division which render a rock,

appearing to the eye quite or nearly homogeneous, fissile. By the term

_foliation_, I refer to the layers or plates of different mineralogical

nature of which most metamorphic schists are composed; there are, also,

often included in such masses, alternating, homogeneous, fissile layers

or folia, and in this case the rock is both foliated and has a

cleavage. By _ stratification_, as applied to these formations, I mean

those alternate, parallel, large masses of different composition, which

are themselves frequently either foliated or fissile,—such as the

alternating so-called strata of mica-slate, gneiss, glossy clay-slate,

and marble.

The folia of the gneiss within a few miles round Bahia generally

strike irregularly, and are often curvilinear, dipping in all

directions at various angles: but where best defined, they extended

most frequently in a N.E. by N. (or East 50° N.) and S.W. by S. line,

corresponding nearly with the coast-line northwards of the bay. I may

add that Mr. Gardner[2] found in several parts of the province of

Ceara, which lies between four and five hundred miles north of Bahia,

gneiss with the folia extending E. 45° N.; and in Guyana according to

Sir R. Schomburgk, the same rock strikes E. 57° N. Again, Humboldt

describes the gneiss-granite over an immense area in Venezuela and even

in Colombia, as striking E. 50° N., and dipping to the N.W. at an angle

of fifty degrees. Hence all the observations hitherto made tend to show

that the gneissic rocks over the whole of this part of the continent

have their folia extending generally within almost a point of the

compass of the same direction.[3]

[2] “Geological Section of the Brit. Assoc.,” 1840. For Sir R.

Schomburgk’s observations see _Geograph. Journal,_ 1842, p. 190. See

also Humboldt’s discussion on Loxodrism in the “Personal Narrative.”

[3] I landed at only one place north of Bahia, namely, at Pernambuco.

I found there only soft, horizontally stratified matter, formed from

disintegrated granitic rocks, and some yellowish impure limestone,

probably of a tertiary epoch. I have described a most singular natural

bar of hard sandstone, which protects the harbour, in the 19th vol.

(1841) p. 258 of the _ London and Edin. Phil. Magazine._

ABROLHOS ISLETS, _lat. 18° S. off the coast of Brazil._—Although

not strictly in place, I do not know where I can more conveniently

describe this little group of small islands. The lowest bed is a

sandstone with ferruginous veins; it weathers into an extraordinary

honeycombed mass; above it there is a dark-coloured argillaceous

shale; above this a coarser sandstone—making a total thickness of

about sixty feet; and lastly, above these sedimentary beds, there

is a fine conformable mass of greenstone, in some parts having a

columnar structure. All the strata, as well as the surface of the

land, dip at an angle of about 12° to N. by W. Some of the islets

are composed entirely of the sedimentary, others of the trappean

rocks, generally, however, with the sandstone, cropping out on the

southern shores.

_Rio de Janeiro._—This whole district is almost exclusively formed of

gneiss, abounding with garnets, and porphyritic with large crystals,

even three and four inches in length, of orthoclase feldspar: in these

crystals mica and garnets are often enclosed. At the western base of

the Corcovado, there is some ferruginous carious quartz-rock; and in

the Tijeuka range, much fine-grained granite. I observed boulders of

greenstone in several places; and on the islet of Villegagnon, and

likewise on the coast some miles northward, two large trappean dikes.

The porphyritic gneiss, or gneiss-granite as it has been called by

Humboldt, is only so far foliated that the constituent minerals are

arranged with a certain degree of regularity, and may be said to have a

“_grain_,” but they are not separated into distinct folia or laminæ.

There are, however, several other varieties of gneiss regularly

foliated, and alternating with each other in so-called strata. The

stratification and foliation of the ordinary gneisses, and the

foliation or “grain” of the gneiss-granite, are parallel to each other,

and generally strike within

a point of N.E. and S.W. dipping at a high angle (between 50° and 60°)

generally to S.E.: so that here again we meet with the strike so

prevalent over the more northern parts of this continent. The mountains

of gneiss-granite are to a remarkable degree abruptly conical, which

seems caused by the rock tending to exfoliate in thick, conically

concentric layers: these peaks resemble in shape those of phonolite and

other injected rocks on volcanic islands; nor is the grain or foliation

(as we shall afterwards see) any difficulty on the idea of the

gneiss-granite having been an intrusive rather than a metamorphic

formation. The lines of mountains, but not always each separate hill,

range nearly in the same direction with the foliation and so-called

stratification, but rather more easterly.

No. 22

Fragment of gneiss embedded in another variety of the same rock.

[Illustration: Fragment of gneiss embedded in another variety of the

same rock.]

On a bare gently inclined surface of the porphyritic gneiss in Botofogo

Bay, I observed the appearance here represented.

A fragment seven yards long and two in width, with angular and

distinctly defined edges, composed of a peculiar variety of gneiss with

dark layers of mica and garnets, is surrounded on all sides by the

ordinary gneiss-granite; both having been dislocated by a granitic

vein. The folia in the fragment and in the surrounding rock strike in

the same N.N.E. and S.S.W. line; but in the fragment they are vertical,

whereas in the gneiss-granite they dip at a small angle, as shown by

the arrows, to S.S.E. This fragment, considering its great size, its

solitary position, and its foliated structure parallel to that of the

surrounding rock, is, as far as I know, a unique case: and I will not

attempt any explanation of its origin.

The numerous travellers[4] in this country, have all been greatly

surprised at the depth to which the gneiss and other granitic rocks, as

well as the talcose slates of the interior, have been decomposed. Near

Rio, every mineral except the quartz has been completely softened, in

some places to a depth little less than one hundred feet.[5] The

minerals retain their positions in folia ranging in the usual

direction; and fractured quartz veins may be traced from the solid

rock, running for some distance into the softened, mottled, highly

coloured, argillaceous mass. It is said that these decomposed rocks

abound with gems of various kinds, often in a fractured state, owing,

as some have supposed, to the collapse of geodes, and that they contain

gold and diamonds. At Rio, it appeared to me that the gneiss had been

softened before the excavation (no doubt by the sea) of the existing,

broad, flat-bottomed valleys; for the depth of decomposition did not

appear at all conformable with the present undulations of the surface.

The porphyritic gneiss, where now exposed to the air, seems to

withstand decomposition remarkably well; and I could see no signs of

any tendency to the production of argillaceous masses like those here

described. I was also struck with the fact, that where a bare surface

of this rock sloped into one of the quiet bays, there were no marks of

erosion at the level of the water, and the parts both beneath and above

it preserved a uniform curve. At Bahia, the gneiss rocks are similarly

decomposed, with the upper parts insensibly losing their foliation, and

passing, without any distinct line of separation, into a bright red

argillaceous earth, including partially rounded fragments of quartz and

granite. From this circumstance, and from the rocks appearing to have

suffered decomposition before the excavation of the valleys, I suspect

that here, as at Rio, the decomposition took place under the sea. The

subject appeared to me a curious one, and would probably well repay

careful examination by an able mineralogist.

[4] Spix and Martius have collected in an Appendix to their “Travels,”

the largest body of facts on this subject. See also some remarks by M.

Lund in his communications to the Academy at Copenhagen; and others by

M. Gaudichaud in Freycinet’s “Voyage.”

[5] Dr. Benza describes granitic rock (_Madras Journal of Lit.,_ etc.,

Oct. 183? p. 246), in the Neelgherries, decomposed to a depth of forty

feet.

_The Northern Provinces of La Plata._—According to some observations

communicated to me by Mr. Fox, the coast from Rio de Janeiro to the

mouth of the Plata seems everywhere to be granitic, with a few trappean

dikes. At Port Alegre, near the boundary of Brazil, there are

porphyries and diorites.[6] At the mouth of the Plata, I examined the

country for twenty-five miles west, and for about seventy miles north

of Maldonado: near this town, there is some common gneiss, and much, in

all parts of the country, of a coarse-grained mixture of quartz and

reddish feldspar, often, however, assuming a little dark-green

imperfect hornblende, and then immediately becoming foliated. The

abrupt hillocks thus composed, as well as the highly inclined folia of

the

common varieties of gneiss, strike N.N.E. or a little more easterly,

and S.S.W. Clay-slate is occasionally met with, and near the L. del

Potrero, there is white marble, rendered fissile from the presence of

hornblende, mica, and asbestus; the cleavage of these rocks and their

stratification, that is the alternating masses thus composed, strike

N.N.E. and S.S.W. like the foliated gneisses, and have an almost

vertical dip. The Sierra Larga, a low range five miles west of

Maldonado, consists of quartzite, often ferruginous, having an

arenaceous feel, and divided into excessively thin, almost vertical

laminæ or folia by microscopically minute scales, apparently of mica,

and striking in the usual N.N.E. and S.S.W. direction. The range itself

is formed of one principal line with some subordinate ones; and it

extends with remarkable uniformity far northward (it is said even to

the confines of Brazil), in the same line with the vertically ribboned

quartz rock of which it is composed. The S. de Las Animas is the

highest range in the country; I estimated it at 1,000 feet; it runs

north and south, and is formed of feldspathic porphyry; near its base

there is a N.N.W. and S.S.E. ridge of a conglomerate in a highly

porphyritic basis.

[6] M. Isabelle, “Voyage à Buenos Ayres,” p. 479.

Northward of Maldonado, and south of Las Minas, there is an E. and W.

hilly band of country, some miles in width, formed of siliceous

clay-slate, with some quartz, rock, and limestone, having a tortuous

irregular cleavage, generally ranging east and west. E. and S.E. of Las

Minas there is a confused district of imperfect gneiss and laminated

quartz, with the hills ranging in various directions, but with each

separate hill generally running in the same line with the folia of the

rocks of which it is composed: this confusion appears to have been

caused by the intersection of the [E. and W.] and [N.N.E. and S.S.W.]

strikes. Northward of Las Minas, the more regular northerly ranges

predominate: from this place to near Polanco, we meet with the

coarse-grained mixture of quartz and feldspar, often with the imperfect

hornblende, and then becoming foliated in a N. and S. line—with

imperfect clay-slate, including laminæ of red crystallised

feldspar—with white or black marble, sometimes containing asbestus and

crystals of gypsum—with quartz-rock—with syenite—and lastly, with much

granite. The marble and granite alternate repeatedly in apparently

vertical masses: some miles northward of the Polanco, a wide district

is said to be entirely composed of marble. It is remarkable, how rare

mica is in the whole range of country north and westward of Maldonado.

Throughout this district, the cleavage of the clay-slate and marble—the

foliation of the gneiss and the quartz—the stratification or

alternating masses of these several rocks—and the range of the hills,

all coincide in direction; and although the country is only hilly, the

planes of division are almost everywhere very highly inclined or

vertical.

Some ancient submarine volcanic rocks are worth mentioning, from their

rarity on this eastern side of the continent. In the valley of the

Tapas (fifty or sixty miles N. of Maldonado) there is a tract three or

four miles in length, composed of various trappean rocks with glassy

feldspar—of apparently metamorphosed grit-stones—of purplish

amygdaloids with large kernels of carbonate of lime[7]—and much of a

harshish rock with glassy feldspar intermediate in character between

claystone porphyry and trachyte. This latter rock was in one spot

remarkable from being full of drusy cavities, lined with quartz

crystals, and arranged in planes, dipping at an angle of 50° to the

east, and striking parallel to the foliation of an adjoining hill

composed of the common mixture of quartz, feldspar, and imperfect

hornblende: this fact perhaps indicates that these volcanic rocks have

been metamorphosed, and their constituent parts rearranged, at the same

time and according to the same laws, with the granitic and metamorphic

formations of this whole region. In the valley of the Marmaraya, a few

miles south of the Tapas, a band of trappean and amygdaloidal rock is

interposed between a hill of granite and an extensive surrounding

formation of red conglomerate, which (like that at the foot of the S.

Animas) has its basis porphyritic with crystals of feldspar, and which

hence has certainly suffered metamorphosis.

[7] Near the Pan de Azucar there is some greenish porphyry, in one

place amygdaloidal with agate.

_Monte Video._—The rocks here consist of several varieties of gneiss,

with the feldspar often yellowish, granular and imperfectly

crystallised, alternating with, and passing insensibly into, beds, from

a few yards to nearly a mile in thickness, of fine or coarse grained,

dark-green hornblendic slate; this again often passing into chloritic

schist. These passages seem chiefly due to changes in the mica, and its

replacement by other minerals. At Rat Island I examined a mass of

chloritic schist, only a few yards square, irregularly surrounded on

all sides by the gneiss, and intricately penetrated by many curvilinear

veins of quartz, which gradually _blend_ into the gneiss: the cleavage

of the chloritic schist and the foliation of the gneiss were exactly

parallel. Eastward of the city there is much fine-grained,

dark-coloured gneiss, almost assuming the character of

hornblende-slate, which alternates in thin laminæ with laminæ of

quartz, the whole mass being transversely intersected by numerous large

veins of quartz: I particularly observed that these veins were

absolutely continuous with the alternating laminæ of quartz. In this

case and at Rat Island, the passage of the gneiss into imperfect

hornblendic or into chloritic slate, seemed to be connected with the

segregation of the veins of quartz.[8]

[8] Mr. Greenough (p. 78, “Critical Examination,” etc.) observes that

quartz in mica-slate sometimes appears in beds and sometimes in veins.

Von Buch also in his “Travels in Norway” (p. 236), remarks on

alternating laminæ of quartz and hornblende-slate replacing

mica-schist.

The Mount, a hill believed to be 450 feet in height, from which the

place takes its name, is much the highest land in this neighbourhood:

it consists of hornblendic slate, which (except on the eastern and

disturbed base) has an east and west nearly vertical cleavage; the

longer axis of the hill also ranges in this same line. Near the summit

the hornblende-slate gradually becomes more and more coarsely

crystallised, and less plainly laminated, until it passes into a heavy,

sonorous greenstone, with a slaty conchoidal fracture; the laminæ on

the north

and south sides near the summit dip inwards, as if this upper part had

expanded or bulged outwards. This greenstone must, I conceive, be

considered as metamorphosed hornblende-slate. The Cerrito, the next

highest, but much less elevated point, is almost similarly composed. In

the more western parts of the province, besides gneiss, there is

quartz-rock, syenite, and granite; and at Colla, I heard of marble.

Near M. Video, the space which I more accurately examined was about

fifteen miles in an east and west line, and here I found the foliation

of the gneiss and the cleavage of the slates generally well developed,

and extending parallel to the alternating strata composed of the

gneiss, hornblendic and chloritic schists. These planes of division all

range within one point of east and west, frequently east by south and

west by north; their dip is generally almost vertical, and scarcely

anywhere under 45°: this fact, considering how slightly undulatory the

surface of the country is, deserves attention. Westward of M. Video,

towards the Uruguay, wherever the gneiss is exposed, the highly

inclined folia are seen striking in the same direction; I must except

one spot where the strike was N.W. by W. The little Sierra de S. Juan,

formed of gneiss and laminated quartz, must also be excepted, for it

ranges between [N. to N.E.] and [S. to S.W.] and seems to belong to the

same system with the hills in the Maldonado district. Finally, we have

seen that, for many miles northward of Maldonado and for twenty-five

miles westward of it, as far as the S. de las Animas, the foliation,

cleavage, so-called stratification and lines of hills, all range N.N.E.

and S.S.W., which is nearly coincident with the adjoining coast of the

Atlantic. Westward of the S. de las Animas, as far as even the Uruguay,

the foliation, cleavage, and stratification (but not lines of hills,

for there are no defined ones) all range about E. by S. and W. by N.,

which is nearly coincident with the direction of the northern shore of

the Plata; in the confused country near Las Minas, where these two

great systems appear to intersect each other, the cleavage, foliation,

and stratification run in various directions, but generally coincide

with the line of each separate hill.

_Southern La Plata._—The first ridge, south of the Plata, which

projects through the Pampean formation, is the Sierra Tapalguen and

Vulcan, situated 200 miles southward of the district just described.

This ridge is only a few hundred feet in height, and runs from C.

Corrientes in a W.N.W. line for at least 150 miles into the interior:

at Tapalguen, it is composed of unstratified granular quartz,

remarkable from forming tabular masses and small plains, surrounded by

precipitous cliffs: other parts of the range are said to consist of

granite: and marble is found at the S. Tinta. It appears from M.

Parchappe’s[9] observations, that at Tandil there is a range of

quartzose gneiss, very like the rocks of the S. Larga near Maldonado,

running in the same N.N.E. and S.S.W. direction; so that the framework

of the country here is very similar to that on the northern shore of

the Plata.

[9] M. d’Orbigny’s “Voyage,” Part. Géolog., p. 46. I have given a

short account of the peculiar forms of the quartz hills of Tapalguen,

so unusual in a metamorphic formation, in my “Journal of Researches”

(2nd edit.), p. 116.

The Sierra Guitru-gueyu is situated sixty miles south of the S.

Tapalguen: it consists of numerous parallel, sometimes blended together

ridges, about twenty-three miles in width, and five hundred feet in

height above the plain, and extending in a N.W. and S.E. direction.

Skirting round the extreme S.E. termination, I ascended only a few

points, which were composed of a fine-grained gneiss, almost composed

of feldspar with a little mica, and passing in the upper parts of the

hills into a rather compact purplish clay-slate. The cleavage was

nearly vertical, striking in a N.W. by W. and S.E. by E. line, nearly,

though not quite, coincident with the direction of the parallel ridges.

The Sierra Ventana lies close south of that of Guitru-gueyu; it is

remarkable from attaining a height, very unusual on this side of the

continent, of 3,340 feet. It consists up to its summit, of quartz,

generally pure and white, but sometimes reddish, and divided into thick

laminæ or strata: in one part there is a little glossy clay-slate with

a tortuous cleavage. The thick layers of quartz strike in a W. 30° N.

line, dipping southerly at an angle of 45° and upwards. The principal

line of mountains, with some quite subordinate parallel ridges, range

about W. 45° N.: but at their S.E. termination, only W. 25° N. This

Sierra is said to extend between twenty and thirty leagues into the

interior.

_Patagonia._—With the exception perhaps of the hill of S. Antonio (600

feet high) in the Gulf of S. Matias, which has never been visited by a

geologist, crystalline rocks are not met with on the coast of Patagonia

for a space of 380 miles south of the S. Ventana. At this point (lat.

43° 50′), at Points Union and Tombo, plutonic rocks are said to appear,

and are found, at rather wide intervals, beneath the Patagonian

tertiary formation for a space of about three hundred miles southward,

to near Bird Island, in latitude 48° 56′. Judging from specimens kindly

collected for me by Mr. Stokes, the prevailing rock at Ports St. Elena,

Camerones, Malaspina, and as far south as the Paps of Pineda, is a

purplish-pink or brownish claystone porphyry, sometimes laminated,

sometimes slightly vesicular, with crystals of opaque feldspar and with

a few grains of quartz; hence these porphyries resemble those

immediately to be described at Port Desire, and likewise a series which

I have seen from P. Alegre on the southern confines of Brazil. This

porphyritic formation further resembles in a singularly close manner

the lowest stratified formation of the Cordillera of Chile, which, as

we shall hereafter see, has a vast range, and attains a great

thickness. At the bottom of the Gulf of St. George, only tertiary

deposits appear to be present. At Cape Blanco, there is quartz rock,

very like that of the Falkland Islands, and some hard, blue siliceous

clay-slate.

At Port Desire there is an extensive formation of the claystone

porphyry, stretching at least twenty-five miles into the interior: it

has been denuded and deeply worn into gullies before being covered up

by the tertiary deposits, through which it here and there projects in

hills; those north of the bay being 440 feet in height. The strata have

in several places been tilted at small angles, generally either to

N.N.W. or S.S.E. By gradual passages and alternations, the porphyries

change incessantly in nature. I will describe only some of the

principal

mineralogical changes, which are highly instructive, and which I

carefully examined. The prevailing rock has a compact purplish base,

with crystals of earthy or opaque feldspar, and often with grains of

quartz. There are other varieties, with an almost truly trachytic base,

full of little angular vesicles and crystals of glassy feldspar; and

there are beds of black perfect pitchstone, as well as of a

concretionary imperfect variety. On a casual inspection, the whole

series would be thought to be of the same plutonic or volcanic nature

with the trachytic varieties and pitchstone; but this is far from being

the case, as much of the porphyry is certainly of metamorphic origin.

Besides the true porphyries, there are many beds of earthy, quite white

or yellowish, friable, easily fusible matter, resembling chalk, which

under the microscope is seen to consist of minute broken crystals, and

which, as remarked in a former chapter, singularly resembles the upper

tufaceous beds of the Patagonian tertiary formation. This earthy

substance often becomes coarser, and contains minute rounded fragments

of porphyries and rounded grains of quartz, and in one case so many of

the latter as to resemble a common sandstone. These beds are sometimes

marked with true lines of aqueous deposition, separating particles of

different degrees of coarseness; in other cases there are parallel

ferruginous lines not of true deposition, as shown by the arrangement

of the particles, though singularly resembling them. The more indurated

varieties often include many small and some larger angular cavities,

which appear due to the removal of earthy matter: some varieties

contain mica. All these earthy and generally white stones insensibly

pass into more indurated sonorous varieties, breaking with a conchoidal

fracture, yet of small specific gravity; many of these latter varieties

assume a pale purple tint, being singularly banded and veined with

different shades, and often become plainly porphyritic with crystals of

feldspar. The formation of these crystals could be most clearly traced

by minute angular and often partially hollow patches of earthy matter,

first assuming a _fibrous structure_, then passing into opaque

imperfectly shaped crystals, and lastly, into perfect glassy crystals.

When these crystals have appeared, and when the basis has become

compact, the rock in many places could not be distinguished from a true

claystone porphyry without a trace of mechanical structure.

In some parts, these earthy or tufaceous beds pass into jaspery and

into beautifully mottled and banded porcelain rocks, which break into

splinters, translucent at their edges, hard enough to scratch glass,

and fusible into white transparent beads: grains of quartz included in

the porcelainous varieties can be seen melting into the surrounding

paste. In other parts, the earthy or tufaceous beds either insensibly

pass into, or alternate with, breccias composed of large and small

fragments of various purplish porphyries, with the matrix generally

porphyritic: these breccias, though their subaqueous origin is in many

places shown both by the arrangement of their smaller particles and by

an oblique or current lamination, also pass into porphyries, in which

every trace of mechanical origin and stratification has been

obliterated.

Some highly porphyritic though coarse-grained masses, evidently

of sedimentary origin, and divided into thin layers, differing from

each other chiefly in the number of embedded grains of quartz,

interested me much from the peculiar manner in which here and there

some of the layers terminated in abrupt points, quite unlike those

produced by a layer of sediment naturally thinning out, and apparently

the result of a subsequent process of metamorphic aggregation. In

another common variety of a finer texture, the aggregating process had

gone further, for the whole mass consisted of quite short, parallel,

often slightly curved layers or patches, of whitish or reddish finely

granulo-crystalline feldspathic matter, generally terminating at both

ends in blunt points; these layers or patches further tended to pass

into wedge or almond-shaped little masses, and these finally into true

crystals of feldspar, with their centres often slightly drusy. The

series was so perfect that I could not doubt that these large crystals,

which had their longer axes placed parallel to each other, had

primarily originated in the metamorphosis and aggregation of

alternating layers of tuff; and hence their parallel position must be

attributed (unexpected though the conclusion may be), not to laws of

chemical action, but to the original planes of deposition. I am tempted

briefly to describe three other singular allied varieties of rock; the

first without examination would have passed for a stratified

porphyritic breccia, but all the included angular fragments consisted

of a border of pinkish crystalline feldspathic matter, surrounding a

dark translucent siliceous centre, in which grains of quartz not quite

blended into the paste could be distinguished: this uniformity in the

nature of the fragments shows that they are not of mechanical, but of

concretionary origin, having resulted perhaps from the self-breaking up

and aggregation of layers of indurated tuff containing numerous grains

of quartz,—into which, indeed, the whole mass in one part passed. The

second variety is a reddish non-porphyritic claystone, quite full of

spherical cavities, about half an inch in diameter, each lined with a

collapsed crust formed of crystals of quartz. The third variety also

consists of a pale purple non-porphyritic claystone, almost wholly

formed of concretionary balls, obscurely arranged in layers, of a less

compact and paler coloured claystone; each ball being on one side

partly hollow and lined with crystals of quartz.

_Pseudo-dikes._—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;[10]

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.

[10] 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.

_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 Palæozoic 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°, 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,[11] by the parallelism in the clay-slate districts of

the shores and channels, ranging in a line between [W. 20° to 40° N.]

and [E. 20° to 40° S.].

[11] _Geographical Journal,_ vol. i, p. 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:—

Ancyloceras simplex, d’Orbigny, “Pal Franc,” Mount Tarn.

Fusus (in imperfect state), d’Orbigny, “Pal Franc,” Mount Tarn.

Natica, d’Orbigny, “Pal Franc,” Mount Tarn.

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

Lucina excentrica, G. B. Sowerby, Port Famine.

Venus (in imperfect state), G. B. Sowerby, Port Famine.

Turbinolia (?), G. B. Sowerby, Port Famine.

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

M. d’Orbigny states[12] that MM. Hombron and Grange found in this

neighbourhood an Ancyloceras, perhaps _A. simplex_, an Ammonite, a

Plicatula and Modiola. 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 Palæozoic

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.

[12] “Voyage,” Part Géolog., p. 242.

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

dikes[13] and by great masses of greenstone, often highly hornblendic:

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 laminæ 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.

[13] 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.

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 scoriæ, 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[14] to consist of

porphyritic lava. 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.[15] 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.

[14] Determined by Professor Jameson. Weddell’s “Voyage,” p. 169.

[15] See Mr. Brooke’s Paper in the _London Phil. Mag.,_ vol. x. This

mineral occurs in an ancient volcanic rock near Rome.

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-laminæ 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,_[16] seem, together with granite, chiefly to prevail along the

western coast as far north as the entrance of the St. of Magellan: 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.

[16] See the Paper by Captain King in the _ Geograph. Journal_; also a

Letter to Dr. Fitton in “Geolog. Proc.,” vol. i, p. 29; also some

observations by Captain Fitzroy, “Voyages,” vol. i, p. 375. I am

indebted also to Mr. Lyell for a series of specimens collected by

Lieutenant Graves.

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 laminæ ranging with extreme regularity, W.N.W.

and E.S.E., seldom varying more than one point of the compass from this

direction.[17] 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° and 90°, 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 laminæ 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-laminæ, though to the eye appearing straight, being

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

[17] 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-laminæ was

in two cases 45° and in two others 79°.

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

fine laminæ 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 laminæ 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[18] 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.” 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.

[18] _Geograph. Journal,_ vol. i, p. 170.

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 lat. 47° 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[19] 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. 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.

[19] 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.

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°

W. and S. 19° 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° W. and

the dip, generally at an angle of about 40° to W. 25° 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° to 22° W.] and [S. 11° to

22° 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° W. to even as

much as N. 45° W.: in an analogous manner at Vallenar Bay, where the

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

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

west of north. The average result from all the observations on this 200

miles of coast, is a strike of N. 19° W. and S. 19° 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° E., whereas, as we have just seen, the

average strike of the foliation is N. 19° W.

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

the foliation. At the northern point of Tres Montes (lat. 45° 52′) a

bold chain of granite, between two and three thousand feet in height,

runs from the coast far into the interior,[20] in an E.S.E. line, or

more strictly E. 28° S. and W. 28° N. 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°

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° to 20°, but sometimes mounting up to 30°. The southern flank

consists of bare granite. The mica-slate is penetrated by small

veins[21] of granite, branching from the main body. 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° 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 laminæ 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 laminæ, dipping at an angle of 40

degrees to W. 19° S., and let an axis of elevation traverse it in an E.

28° S. line, what will the position of the laminæ be on the northern

flank after a tilt, we will first suppose, of 45°? Mr. Hopkins informs

me, that the angle of the dip will be 28° 31′, and its direction to

north 30° 33′ west.[22] By varying the supposed angle of the tilt, our

previously inclined folia can be thrown into any angle between 26°,

which is the least possible angle, and 90°; 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.

[20] 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.

[21] 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.

[22] 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° 55′,

directed to west 35° 33′ 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°, would here be only

86° 50′ apart.

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°: 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° (in one spot, however, at 60°), 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° and 22° 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:[23] but here, as at Valdivia, a N.W. and S.E. strike seemed to

be the most frequent one. 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.

[23] I observed in some parts that the tops of the laminæ of the

clay-slate (_b_ of the diagram) under the superficial detritus and

soil (_a_) were bent, sometimes without being broken, as represented

in the accompanying diagram, which is copied from one given by Sir H.

De la Beche (p. 42 “Geological Manual”) of an exactly similar

phenomenon in Devonshire. Mr. R. A. C. Austen, also, in his excellent

paper on S.E. Devon (“Geolog. Transact.,” vol. vi, p. 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 Phil. Mag.,_ vol. xxi, p. 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.

[Illustration: Diagram described in note 23.]

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.[24]

[24] “Geolog. Trans.,” vol. vi, pp. 602 and 617. “Journal of

Researches” (2nd edit.), p. 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.[25]

[25] Humboldt (“Personal Narrative,” vol. iv, p. 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°: 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-laminæ range over wide areas with remarkable uniformity,

cutting straight through the planes of stratification,[26] but yet

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

to the outlines of the coast. 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.

[26] In my paper on the Falkland Islands (_Geological Journal,_ vol.

iii, p. 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 fact of the cleavage-laminæ 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,[27] where glaciers had passed over

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

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

caused by the competent laminæ being of slightly different degrees of

hardness. 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.”[28] 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.

[27] _London Phil. Mag._, vol. xxi, p. 182.

[28] “Geological Trans.,” vol. iii, p. 471.

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-laminæ, the folia preserve over very large areas a uniform

strike: thus Humboldt[29] 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° to N.W.; 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° W. and S. 19° 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-laminæ 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,[30] the

coincidence between its strike and that of the main stratification

seems sometimes to fail. Foliation and cleavage resemble each other in

the planes winding round concretions, and in becoming tortuous where

veins of quartz abound.[31] 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,[32] in

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

flanks inclined inwards. 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.[33]

[29] “Personal Narrative,” vol. vi, p. 59 _et seq._

[30] Cases are given by Mr. Jukes in his “Geology of Newfoundland,” p.

130.

[31] I have seen in Brazil and Chile concretions thus enfolded by

foliated gneiss; and Macculloch (“Highlands,” vol. i, p. 64) has

described a similar case. For analogous cases in clay-slate, see

Professor Henslow’s Memoir in “Cambridge Phil. Trans.,” vol. i, p.

379, and Macculloch’s “Class. of Rocks,” p. 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,” p. 78.

[32] Studer in _Edin. New Phil. Journal,_ vol. xxiii, p. 144.

[33] 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,[34] we clearly see

that folia of distinct minerals can originate through the metamorphosis

of a homogeneous fissile rock. 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.[35] 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-laminæ,

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-laminæ of closely allied rocks, which none would imagine had

ever been horizontal, dip at nearly the same angle, to nearly the same

point?

[34] I have described in “Volcanic Islands” a good instance of such a

passage at the Cape of Good Hope.

[35] See some excellent remarks on this subject, in D’Aubuisson’s

“Traité de Géog.,” tome i, p. 297. Also some remarks by Mr. Dana in

_Silliman’s American Journ.,_ vol. xlv, p. 108.

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.[36] 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.

[36] Macculloch states (“Classification of Rocks,” p. 364) states that

primary limestones are often found in irregular masses or great

nodules, “which can scarcely be said to possess a stratified shape!”

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-laminæ: 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 laminæ 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.[37]

[37] 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,[38] 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. 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-laminæ 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.

[38] In “Volcanic Islands.”

[Illustration: Geological sections through the Cordilleras.]

For enlargements of the above plate use the following links:

left section

center section

right section