Trap-pebbles of the boulder--Thickness of the earth's crust
unknown--Not of much consequence to the practical
geologist--Interior of the earth in a highly heated
condition--Proofs of this--Granite and hypogene
rocks--Trap-rocks; their identity with lavas and
ashes--Scenery of a trappean country--Subdivisions of
the trap-rocks--Intrusive traps--Trap-dykes-intrusive
sheets--Salisbury Crags--Traps of the neighbourhood of
Edinburgh--Amorphous masses--Contemporaneous trap-rocks of
two kinds--Contemporaneous melted rocks--Tests for their age
and origin--Examples from neighbourhood of Edinburgh--Tufas
or volcanic ashes--Their structure and origin--Example
of contemporaneous trap-rocks--Mode of interpreting
them--Volcanoes of Carboniferous times--Conclusion.
In the previous pages, allusion has been made to the trap-pebbles
imbedded in the boulder, to the various forms of decay exhibited by
granitic and trappean rocks, and to the elevation and depression of the
solid crust of the earth. Will the reader bear with me for but a few
pages more, while I seek to indicate one or two points of interest in
a branch of geology that would abundantly reward a diligent observer?
Since the days of Hutton, the investigation of what are called
_igneous_ rocks has fallen somewhat into the background, and geologists
have given themselves, perhaps too exclusively, to the study of organic
remains, so that while the palæontology of the British islands has
enjoyed an extensive exploration, but little has been done towards the
elucidation of our igneous formations and their accompanying phenomena.
Much remains to be accomplished, even in those districts usually
regarded as in a manner thread-bare, and he must be but an indifferent
observer who cannot add a few gleanings to the general stock of
information upon this branch of British geology.
Many conjectures have been formed, and many theories propounded, as
to the nature of the internal parts of our globe. Some have supposed
that there is an outer solid film or crust, some ten or twenty miles
thick, enveloping a vast ball of intensely heated matter; others have
attempted to show that the interior must be nearly solid throughout,
with, however, great lakes, or vesicles of gas and melted rock,
somewhat after the fashion, we may suppose, of the oil-holes in a
Gruyère cheese. But whether the heated material occupy the whole or
only parts of the internal area, is not of much consequence to the
practical geologist; he is content to believe that it exists, and in
sufficient quantity, too, to produce the most momentous changes on
the surface of the earth. We see the effects of this subterraneous
agent in earthquakes and volcanoes, and the geologist can tell us
of similar, as well as of other changes, effected by it during past
ages. Granite hills, and mountainous districts of mica-slate and
gneiss, bear evidence of what is termed _metamorphism_--a change in
the mineral structure of rocks, believed to have taken place through
the agency of heat deep in the interior of the earth; for no analogous
appearances have been detected in progress at the surface. Such rocks,
known as _metamorphic_, or _hypogene_, still form a difficult problem,
not likely to be satisfactorily solved until the chemist shall have
thoroughly investigated the subject; for it seems likely to be found,
after all, that long-continued chemical action, without a very alarming
degree of heat, may have produced even the most intense metamorphism.
But dropping this part of the subject, in which so much yet remains
to be discovered, let us look for a little at another branch of the
geologist's evidence, where we meet with no such hampering hypotheses
and doubtful conjectures, namely, the _trap_-rocks.
Every one knows that basalt, lava, pumice, scoriæ, and ashes, are
the various matters ejected from volcanoes. When these materials are
found interstratified among the various geological formations, they
are termed _trap_-rocks,--a name derived from the Swedish _trappa_, a
stair, in allusion to the step-like or terraced appearance which they
often present. They are of all ages, having been detected in the lower
Silurians of Wales, and in the deposits of all subsequent periods up
to the volcanic eruptions of the present day; thus evidencing, that
from the remotest times there have been Ætnas and Vesuvii slumbering
perhaps for ages, and then awakening to lay the surrounding districts
in ruins. I have already said that the rocks from which the geologist
has to compile his history, are mostly relics of the sea; hence most of
the trap-rocks which he meets with in his explorations are the products
of submarine eruptions. Far away down among the Silurian rocks, he can
trace the floor of a primeval ocean thickly covered with stone-lilies,
trilobites, and molluscs, and in following it out he marks how ashes
and lapilli, ejected from some submarine orifice, settled down amid the
organisms and well-nigh destroyed them, while at other times streams of
molten matter were poured out along the sea-bottom, and hardened into
masses of solid rock. He sometimes even encounters what seems the vent
whence these eruptions proceeded, filled up now by a boss or plug of
hardened trap, but he never can detect any trace of land. Some of these
oceanic volcanoes may, like Graham's Island in the Mediterranean, have
raised their tops above water, sending clouds of steam and cinders far
and wide through the air, but the waves would eventually wear down the
new-born land, and scatter its broken fragments along the floor of the
sea. Among the carboniferous rocks of Scotland, however, we meet with a
different condition of things. There, too, we can trace out submarine
lava-streams, and mark how showers of ashes destroyed the delicate
organisms of the deep; but we encounter, besides, undoubted traces
of a land, not parched and ruinous as though the igneous forces had
laid it waste for ever, but thickly clothed with vegetation of a more
luxuriant type than that which clusters over Vesuvius and Calabria, or
lies spread out across the "level plains of fruit-teeming Sicily."[74]
We have looked at the plants and animals of the Carboniferous era; its
rivers and deltas; its slow elevations and depressions of the ground.
It may, perhaps, complete the picture of that ancient period, if we
examine, though but briefly, its igneous eruptions, the more especially
since these may be regarded as, to a considerable extent, typical of
trap-rocks belonging to every age and every country.
[Footnote 74: Της καλλικἁρπον Σικελἱας λενροὑς γὑας. Æsch. _Prom.
Vinct._ 369--a passage graphically descriptive of an ancient eruption
of Ætna.]
Unless when deeply buried beneath drift-sand and clay, trappean regions
usually possess scenery of a marked kind. A green undulating country
stretches out as far as the eye can reach, diversified here and there
with bold abrupt crags and conical hills. The lower grounds show in the
winter season their rich brown loam, that waxes green as the spring
comes on, and ere summer's close spreads out its heavy crops of golden
grain. The higher ridges are for the most part thickly wooded, yet
the soil is often scanty, and, among the white stems of the beech, or
the matted roots of the fir and the elm, we may not unfrequently see
the rock protruding its lichen-crusted face, mottled with mosses and
liverworts, while some sluggish runnel collects in stagnant pools,
or trickles over the blocks with a thick green scum. Sometimes the
hill has never been planted, but stands up now, as it has done for
centuries; its western face craggy and precipitous, with bushes of
sloe-thorn and furze, and stray saplings of mountain-ash clinging to
the crevices, while its eastern slope sinks down into the rolling
country around with a green lumpy surface, through which, at many a
point, the grey time-stained rock may be seen. The whole district
suggests to the fancy a billowy sea, and, as one casts his eye from
some commanding hill-top athwart the wide expanse of hill and valley,
sweeping away in endless undulations, he is apt to bethink him of
some day far back in the past, when the verdant landscape around lay
barren and desolate, while the solid earth rocked and heaved in vast
ground-swells like a wide tempested ocean. Such is the aspect presented
by some of the more trappean regions of Scotland. But the origin of
this kind of scenery must be ascribed to the effects of denuding
currents in scooping out the softer strata into clefts and valleys, and
leaving the harder trap-rocks in prominent relief, rather than to any
great inequality of surface produced by the eruption of igneous matter;
for we shall find that the throwing out of sheets of lava and showers
of volcanic ashes was often a very quiet process after all.
Trap-rocks generally may be variously classified according to the
aspect under which we view them. Mineralogically they are _augitic_,
when the mineral _augite_ enters largely into their composition;
_hornblendic_, when the _augite_ is replaced by _hornblende_; and
_felspathic_, where _felspar_ forms the most marked constituent.
The first class includes all the dark homogeneous compounds called
_basalts_; the second, the hornblendic _greenstones_, or _diorites_;
and the third, the _felstones_, _porphyries_, and _tufas_.
Geologically, they are _beds_ when they are interstratified with the
contiguous rocks; and _dykes_ or _veins_ when they penetrate them like
walls, or in an irregular manner. The former class may be either of
the same age with the rocks among which they lie, or of a later date,
just as in a pile of books the centre one may either have been placed
there originally with the rest, or thrust in long afterwards. The
latter class must always be later than the rocks which they traverse,
for it is plain the rocks must have been in existence before trap-dykes
and veins could be shot through them. Hence geologists are accustomed
to speak of contemporaneous and subsequent trap-rocks: the one list
including all the tufas, and those melted rocks which can be shown to
have been erupted during the time when the limestones, sandstones, or
shales around them were forming; the other embracing all the dykes and
veins along with those beds of melted rock which have been intruded
between the strata. These and other distinctions will be better
understood from a few examples collected chiefly from the carboniferous
district of central Scotland.
The trap-rocks seen there exhibit a wide range of structure, texture,
colour, and general aspect. There are two pretty marked kinds--the
augitic or hornblendic, and the felspathic; the former being usually
of a more or less crystalline aspect; the latter, commonly dull, and
often without any crystals.[75] In the augitic traps, the crystals
are sometimes of large size and well-defined, so that the rock could
hardly be distinguished at first sight from an ordinary grey granite,
while at other times, and not unfrequently even in other portions of
the same mass, the stone assumes a black appearance without distinct
crystals. The former variety would be called a _greenstone_, the
latter a _basalt_; the chief components in either case being felspar
and hornblende, or felspar and augite, with a variable admixture
of other minerals, the shade of colour varying from a pale blue or
leek-green, through the different hues of grey, to a deep velvet black.
There are other traps, however, consisting entirely, or nearly so,
of felspar, whence they are known as _felstones_. Such rocks enjoy
a wide range of colour, some of them being pure white, others of a
bluish grey or dingy brown; and they may be seen graduating from a
pale yellow, or flesh-colour, to a brick-red or deep purple. When a
trap displays distinct disseminated crystals, usually of felspar, it
becomes a _porphyry_; when it shows rounded cavities, like those of
furnace-slag, it is said to be _vesicular_; and when these globular or
almond-shaped cavities are filled with carbonate of lime, chalcedony,
or other minerals, the rock forms an _amygdaloid_. Such peculiarities
of structure indicate to some extent the origin of the mass, and may
be found in any kind of trap. Thus we have porphyritic greenstones,
basalts, or felstones, and the same rocks may be likewise vesicular
or amygdaloidal. Some of them, such as many greenstones, display on
weathered surfaces that curious spheroidal structure already alluded
to; others are built up into geometric columns.
[Footnote 75: This distinction, though a sufficiently safe one in
some localities, must not be held as by any means universal in its
application, the felspathic traps being often as crystalline in aspect
as the augitic, and the augitic, on the other hand, as dull as the
felspathic.]
Such peculiarities of composition and structure form the basis of
a mineralogical classification of the igneous rocks, which is of
use in working out the geology of a district. The most convenient
subdivision for our present purpose, however, is that which proceeds
upon the origin and mode of occurrence of the trap-rocks. Viewed
thus, they resolve themselves into two great groups, the _intrusive_
and _contemporaneous_, both of which contain greenstones, basalts,
&c.,--the sole distinction between those of the one class and those of
the other, being the relation of age and mode of occurrence which they
bear to the surrounding rocks.
I. The _intrusive_ traps occur in the form of walls and veins,
sometimes in that of flat parallel beds, and often as huge amorphous
masses, to which no definite name can be given. But whatever shape they
may assume, they generally agree in presenting well-marked features,
whereby their origin can be readily ascertained. The rocks through
which they pass are more or less hardened, often contorted, and
sometimes traversed by innumerable cracks and rents, into some of which
the trap has penetrated in the form of veins.
A trap-dyke is a long wall of igneous matter, cutting more or less
perpendicularly through the surrounding rocks. Sometimes these dykes
attain a breadth of many yards, and may not unfrequently be traced for
miles running in a nearly straight line over hill and valley, easily
recognisable by a long smooth ridge, with the rock protruding here and
there from below where the soil is thin. It is interesting to follow
out one of these long ramparts from its beginning to its close, and
mark how undeviatingly it cuts through the rocks. No matter what may
be the nature of the stone encountered, hard conglomerate, friable
shale, compact limestone, or jointed fissile sandstone, all are broken
across, and the right line preserved throughout. Nay, I have seen a
still more curious instance of this persistency, where the dyke ran for
four miles through a set of mountain limestone and lower coal-measure
strata, and several enormous sheets of greenstone and basalt. Even
when passing through these traps the dyke remained perfectly distinct,
its crystalline structure and external configuration presenting a
well-marked contrast with those of the surrounding eminences. Of
course it must have been formed after all the rocks through which
it passed. The sandstones and shales must have settled down long
previously on some estuary bed or sea-bottom; the corals and shells
of the limestones, and the matted plants of the successive coal-seams
must have lived and died, perhaps thousands or millions of years
before, and their remains have hardened into stone, ere the continuity
of the strata was broken across by the long deep wall of greenstone.
Trap-dykes are accordingly appropriately termed _intrusive_. They
have been intruded among and must always be later than the rocks in
which they occur. In tracing out their character, more especially in a
trappean district, such as that of Linlithgowshire, where they abound,
we soon find other evidence of their intrusive nature. Where they pass
through limestone, they sometimes convert it into a white saccharine
marble; shales they bake into a sort of porcelain or burnt pottery; and
sandstones become semi-fused into a hard homogeneous quartz-rock. Nor
are the changes confined to the rocks traversed; the dykes themselves,
along their sides, become fine grained and hardened; occasionally, too,
the colour alters from the usual bluish or greenish-grey to black, or
to a brick-red, or dull-brown, similar to that of the altered shale
and sandstone, of which detached portions may be found adhering to
the outer walls of the dyke, or even embedded in its substance.
The central portion of the dyke may thus be markedly crystalline,
forming what we should call a greenstone, while the outside parts,
where the trap comes in contact with the adjacent rocks, are fine
grained and homogeneous, so as to become a true basalt. Sometimes,
too, these exterior edges are highly vesicular and amygdaloidal,
detached fragments closely resembling the slag of an iron-furnace, and
occasionally the dyke presents a columnar arrangement, the ends of
the hexagonal or polygonal columns abutting against the sandstone or
other rock on either side, and losing themselves towards the centre in
the general mass of the trap. Where the strata traversed are broken
and jointed, the dykes which cut them through may be seen in some
places throwing out lateral veins that accommodate themselves to all
the irregularities of the fissures. These minor portions exhibit for
the most part the same leading features with the parent mass, and the
result of the whole is a general baking of the beds, with sometimes not
a little contortion, and an amount of irregularity and disturbance,
apparent at once to the most inexperienced observer. (See Fig. 34.)
If the reader will verify these statements by actual exploration in
the field, he will probably not be long in arriving at the following
conclusions: trap-dykes must once have been in a melted state, as is
shown by their vesicular cavities and divergent veins; this liquid
condition must have been attended with the most intense heat, as may be
gathered from the burnt and baked appearance of the contiguous rocks;
they have, for the most part, especially where of large size, risen
from below along previously-formed dislocations--a circumstance which
may be inferred from their persistency in a straight line through beds
of very different resisting power, for had the liquid matter forced
a way for itself, it would have squirted between the beds along the
lines of least resistance, and not directly and for miles across them;
and hence, trap-dykes must be regarded not as themselves the agents
in dislocating and contorting a district, but merely as signs of the
parent force at work below.
All the features of these trap-dykes here stated may be observed in the
central district of Scotland, among rocks of Carboniferous age. But he
who would study trap-dykes on the great scale without quitting Britain,
should visit some of the more trappean islands of the Hebrides. He
will there find them intersecting glen and hill-side, in an intricate
network, standing up through the heather like ruined walls, and running
often for considerable distances up bald cliff-line, and across
precipitous ravine. In some localities, among such limestone districts
as that of Strath, detached eminences may be seen with congregated
dykes coursing their sides and summits, while the heathy interspaces
are cumbered with grey and white protruding blocks of marble, that give
to these green knolls the aspect of old time-wasted abbeys with their
clustering tombstones. The magnificent sections laid open in these
localities by the action of mountain streams, and by the waves of the
Atlantic, leave the student of igneous rocks nothing to desire save a
long lease of leisure.
Another form frequently assumed by the intrusive traps, is that of
wide beds or sheets intercalated with greater or less regularity among
stratified rocks (Fig. 34 _b_). They may be regarded as horizontal
dykes, the igneous matter, in place of cutting across the strata,
having forced a way for itself between them. Viewed in this light they
will be found exactly to correspond with ordinary dykes; the rocks on
which they rest, and those which lie above them being both altered like
those on either side of a dyke or vein. A well-known example of this
form of trap is that of Salisbury Crags, where a bed of greenstone
twenty to eighty feet thick is intercalated among sandstones, shales,
and coarse limestones, belonging to the Lower Carboniferous series. Its
under surface presents a remarkably even line, broken at intervals,
however, where the truncated ends of sandstone beds protrude up
into the greenstone, or where the latter cuts into the sandstone
below, occasionally enveloping detached fragments, and sending veins
through them. Along the line of contact both rocks undergo a change.
The greenstone becomes reddened, finer grained, and of a dull earthy
aspect. The sandstones and shales are also red, and excessively hard,
the former resembling a quartz rock, and the latter passing into a sort
of flinty chert or chalcedony. The sandstones above the trap, where
they can be examined, are also found to present the same hardened,
baked appearance, the most intense metamorphism being observable
in those parts which are completely surrounded by igneous matter.
These points were noted many years ago during the famous controversy
between the disciples of Hutton and Werner, the former viewing them
as demonstrative evidence of the igneous origin of the trap-rock, the
latter, on the other hand, professing to see nothing in the section
of the Crags at all militating against the theory that the rocks had
originated from deposition in water. Many a battle was fought in this
locality, and not a few of the trap-dykes and hills possess to the
geologist a classic interest, from having been the examples whence
some of the best established geological opinions were first deduced.
The contest between the Huttonians and Wernerians terminated long ago
in the acknowledged victory of the former; Hutton's doctrines are now
recognised all over the world. It is interesting, however, to walk
over the scenes of the warfare, and mark the very rocks among which it
raged, and from the peculiarities of which it took its rise. Basalts
and greenstones, sandstones and shales, with all their crumplings and
contortions, still stand up as memorials of powerful igneous action,
and of physical changes in the primeval past; and they have become to
the geologist memorials, too, of changes in the onward progress of his
science, where, out of conflicts perhaps yet more tumultuous than those
of ancient Nature, there emerged at last the clear demonstrable truth.
[Illustration: Fig. 34.--Intrusive Trap.]
In the accompanying section (Fig. 34), the more marked characters of
intrusive traps are exhibited. The main mass of igneous rock is the
dyke (_d_), rising through a dislocation or fault, which has thrown
down the beds on one side several feet below those on the other, as
is shown by the interruption of the shale and ironstone beds (_sh_).
The dyke gives off two ramifications, one of them cutting across the
beds obliquely as a vein (_v_); the other passing along the planes
of the shaly layers as a horizontal bed (_b_). The vein, it will be
noticed, produces considerable alteration in its progress, carrying up
and baking a portion of the shale (_sh_), and turning up the edges of
the beds on both sides, which get cracked and hardened along the line
of contact. The bed runs with some regularity for a short distance
through the shales, which show marks of great alteration at their
junction with the trap. Its under surface at one point is seen to have
involved a portion of the shale which has become in consequence highly
metamorphosed, while along the upper surface the bed has sent out a
short irregular vein that twists and otherwise alters the shales
above. These circumstances would suffice to show that even though we
did not find this bed in connexion with a mass of intrusive trap, it
must, nevertheless, have been thrust among previously-formed strata,
and could not have been contemporaneous, that is, poured out along the
sea-bottom before the shales above it were deposited.
But one other form needs to be mentioned here as characteristic of the
Carboniferous intrusive trap-rocks--that of great amorphous masses
which cut through the strata irregularly. They have not the wall-like
form of dykes, nor do they conform to the line of bedding of the rocks
among which they occur. They are sometimes irregular lumps, lying
above or among the strata, and probably connected with some vein or
dyke below. In other localities they look like the upper ends of vast
pillars which may descend into the very depths of Tartarus, as though a
great hole had been blown through the crust of the earth, and a column
of melted matter had risen to fill the cavity. Such masses are often
called _bosses_, and seem not unfrequently to have been the craters
of eruption whence great sheets of lava and showers of ashes were
ejected far and wide over the neighbourhood. They serve to connect
the intrusive traps, whose age is always more or less uncertain, with
the bedded traps properly so called, the geological date of which can
usually be sufficiently ascertained.
II. The bedded or contemporaneous trap-rocks consist of two well-marked
kinds. There are, 1st, the melted rocks, such as greenstones and
basalts and 2d, the tufas and volcanic ashes.
Those of the first-named class differ in no respect from the traps
already noticed, so far as regards mineralogical texture, general
structure, and appearance. In hand specimens the intrusive and bedded
greenstones and basalts cannot be distinguished, nor even when examined
in the field and in masses extending over considerable areas is it
always possible to say to which division any particular hill or crag
should be assigned. The reason of this resemblance is obvious. Where a
trap has either cut through or insinuated itself among rocks of earlier
date it is called intrusive, in relation to the rocks so traversed,
and of course we cannot be sure to what geological period it should be
referred, nor how long an interval may have elapsed between the time
when these rocks were forming and the time when the trap was intruded
among them. If, however, the igneous rock passed upward through these
same strata and then spread out as a flat sheet along the sea-bottom,
the part that came to the top would be termed contemporaneous
with the deposits going on at the time. Hence it follows that all
contemporaneous lava-form trap-rocks are at the same time intrusive as
regards the strata passed through in their progress to the surface. If
the sheet of melted matter that spread out below the water were in the
course of ages worn completely away, along with the strata subsequently
piled above it, so as to leave merely a neck or dyke filling up the
cavity through which the lava rose, we should pronounce the remnant
intrusive, and could form no certain conclusion as to its age or
as to whether its site had ever been a crater actively at work in
throwing out lava and ashes. The sole difference, therefore, between a
contemporaneous and an intrusive greenstone is simply this: the former
rose through a fissure until it reached the surface, and then rolled
out as a flat parallel sheet; the latter may have been erupted from
below at the same time, yet, owing to different circumstances, never
reached the surface, but spread out among or cut through the strata
underneath. And so, when we come to examine in quarries, ravines, and
other exposures, the remains of two such eruptions, we soon ascertain
the relative age of the former from that of the strata among which it
occurs, but as to the date of the latter we are wholly at a loss, for
it gives us no clue by which we can show whether it was erupted before
or after the other. We can but compare the mineralogical character
of the intruded with that of the contemporaneous masses in the same
district, and, from the resemblance which may be traced between them,
draw at the best but a doubtful inference as to their relative dates.
The contemporaneous traps always assume a bedded form, the intrusive
occasionally do so; and the question naturally arises here, what are
the tests whereby a bed of trap may be known to be contemporaneous and
not intrusive? The answer is happily a simple one. An intrusive mass
is found to alter to a greater or less extent the rocks in contact
with it; if it occur as a dyke, then the beds on either side have
been cut through and probably otherwise affected; if it take the form
of a bed or sheet, the strata lying above and below it will be found
to be both altered, showing evidently that a heated mass has been
interposed between them, and consequently that the igneous rock is of
later date than any of the strata among which it occurs. In the case
of a contemporaneous melted trap, however, the appearances presented
are different; it always takes the form of a flat bed corresponding
to all the inclinations and curvatures of the sandstones, shales,
limestones, or other strata among which it lies. If examined carefully,
it may be found not unfrequently baking and contorting the bed that
forms its pavement, but producing no change whatever on that which
composes its roof. It may be capped and underlaid by layers of shale,
and in such a case we might not improbably find the shale below it
highly baked, so as to resemble a sort of rude pottery, while the shale
above would present no sign of such metamorphism, but on the contrary
might display its delicate plants or shells down to the very surface
of the trap, and were the latter concealed from view we should never
suspect, from the aspect of this shale, that any igneous rock existed
in the neighbourhood. The inference to be drawn from such appearances
seems very obvious. Had the upper shale been in existence when the
greenstone or basalt was erupted, it would have suffered an alteration
similar to that produced on the shale below; and the fact, plain and
palpable, that it has undergone no such change, shows pretty clearly
that it was deposited at the bottom of the water after the trap had
cooled and consolidated, and that consequently the trap must be
intermediate in age between the beds on which it rests and those which
lie above it; in other words, that it is a _contemporaneous_ rock.
Hence, if we know the exact geological position and age of the shales,
we know also those of the associated trap, and can thus ascertain
that at a certain definite period in the past history of our planet a
particular district was the scene of volcanic action.
Examples of such contemporaneous traps abound among the carboniferous
rocks of central Scotland, especially in Fife and the Lothians (Fig.
35). I may refer again to the vicinity of Edinburgh as affording some
excellent illustrations. The eastern part of Arthur's Seat displays
a series of basalts and greenstones which can be proved to have been
thrown out during the times of the Lower Carboniferous rocks, at a
period long anterior to that of the Burdiehouse limestone. The Pentland
Hills exhibit on a much greater scale vast sheets of felspathic traps,
such as felstones and tufas, traceable in some cases for six or seven
miles, which were erupted at a still earlier period.[76] The trap
pebbles in our boulder consisted of light yellow and pink felstone,
and were derived, I make no doubt, from these Pentland Hill beds, when
what forms now the cone of Carnethy, rising well-nigh 1900 feet above
the sea, existed as one of a scattered archipelago of islets, or as a
sunken rock battered by the waves that scattered its shingle along the
floor of what may have been either a shallow sea or a shoaling estuary,
where eventually the sand and pebbles hardened into that bed of coarse
grey sandstone whence our boulder was derived.
[Footnote 76: The geology of Arthur's Seat and Pentland Hills was
admirably worked out more than quarter of a century ago by Mr. M'Laren.
His work (already referred to) is unfortunately now out of print.]
The second class of contemporaneous trap-rocks are the tufas or
volcanic ashes. They differ entirely in their aspect and origin from
any of the rocks already described. Greenstones, basalts, felstones,
and such like, were all melted rocks, thrust up from below as we see
lava thrown out by a modern volcano, being styled contemporaneous when
poured out along the sea-bottom or the land, and intrusive when they
never reached the surface but cut through the strata below. The tufas,
however, point to a totally different origin. They are of various
shades of colour, according to their chemical composition. In East
Lothian they assume a deep red hue; among the Pentland Hills they
are often flesh-coloured, while in Linlithgowshire they range from a
dull-brown to a pale leek-green, green being the prevailing tint. They
always show a dull uncrystalline surface, irregularly roughened by
included fragments of various rocks, such as trap, sandstone, shale,
and many others. These fragments or _lapilli_ vary in size from less
than a pin-head up to large bombs of several hundredweight, and from
being generally abundant give to the tufas one of their best-marked
characteristics. The smaller pieces are usually more or less angular,
and throughout the carboniferous series of Linlithgowshire consist
chiefly of a pale felspathic matter, lighter in shade and commonly
harder in texture than the matrix or paste in which they lie. In
some localities, where the included pieces are larger, they have a
rounded form, and often show a honey-combed vesicular surface, like
balls of hardened slag. Fragments of sandstone have not unfrequently
a semi-fused appearance, and plates of shale sometimes look like the
broken debris from a tile-work, although in many instances these
fragments may be found showing no trace whatever of alteration, being
undistinguishable from the neighbouring sandstones and shales from
which they probably came. I have seen in some of the coarser tufas, or
rather volcanic conglomerates, enormous masses of basalt and greenstone
buried deep in the surrounding green or red felspathic paste, and
showing on their more prominent edges the usual vesicular cavities.
In such conglomerates there is usually no division into beds; the
whole mass, indeed, forms a bed between lower and higher strata, but
internally it shows for the most part no trace of stratification. In
these confused assemblages one may occasionally light upon detached
crystals of augite or other mineral scattered irregularly through
the tufa. Their angles will be found often blunted, and the crystals
themselves broken, appearances which have likewise been noticed among
the ash of modern volcanoes. When the tufas are finer grained they
usually exhibit a well-marked stratification, and can often be split
up into laminæ like an ordinary fissile sandstone. Organic remains not
unfrequently abound in such laminated beds, and vary in their character
as widely as in any other stratified rock, being sometimes land-plants,
sometimes sea-shells.
Such are some of the more obvious characters of the volcanic ashes or
tufas, as developed among the carboniferous rocks of central Scotland.
Their great varieties of composition and general aspect render them
a somewhat difficult set of rocks to master, but when fairly and
fully understood they soon prove themselves to be by far the most
interesting section of the traps, for one needs seldom to hesitate a
moment as to their origin or date, while their fossil contents impart
to them an interest all their own. By comparing such rocks with the
consolidated ash or fine dust and _lapilli_ of a modern volcano, a
remarkable resemblance of external characters is found to subsist; and
this likeness holds sufficiently close, when pursued into details, to
show that the ancient and the modern rocks have resulted from the same
source, that, namely, of volcanic eruption. The ash of active burning
mountains falls down their sides loosely and incoherently, every
successive shower of dust or scoriæ settling without much regularity
on those that have gone before. The ash of the old carboniferous
eruptions, however, was showered for the most part over the sea or
across wide shoaling estuaries, at least it is only such portions of
it as fell there that have come down to our day. Settling down among
the mud and sand at the bottom, the volcanic matter accumulated in
wide horizontal beds, every marked inequality being smoothed down by
the currents until a series of regularly stratified layers came to be
formed, entombing any organisms that might find their way to the bottom
or be lying there at the time. The ash of terrestrial volcanoes has no
marked stratification because thrown out in open air, while that of
the carboniferous rocks of central Scotland is distinctly bedded from
having been deposited under water.
Tufas and contemporaneous melted traps are very generally found
together interstratified regularly with each other, and the inference
to be drawn from their juxtaposition is of course simply this, that
at one time liquid lava rolled along the bottom of the water, while
at another showers of volcanic dust and cinders settled down in
successive beds. This active play of the igneous forces took place
at the mouths of estuaries or farther to sea; and it is accordingly
sometimes not a little interesting to trace, amid the sediment that
accumulated below the water during the pauses between the eruptions,
well-preserved remains now of plants that had come drifting from the
land, anon of slim spirifers, and producti that swarmed upon the
hardened lava-streams, and amid the thickening volcanic mud that
slowly sank to the sea-bottom. Such a sequence of events will be made
plain from the following section, the materials of which are derived
from different parts of the trappean region of Linlithgowshire. The
undermost bed here shown (1) is one of marine limestone, abounding
with encrinal joints, corals, spirifers, and other undoubtedly marine
organisms. Above it comes a layer of tufa or volcanic ash (2) of a
dull green aspect, the boundary line between the two rocks lying as
clear as if the quarryman had marked it off with his foot-rule. The
upper part of the ash, however, does not show an equally clear line
of demarcation with the stratum above. On the contrary, it gradually
changes its character, becomes more calcareous as it goes up, with
here and there a stone-lily joint or a stray productus, until these
organisms increase so much in number that the rock insensibly passes
into an ordinary limestone (3) like that below. Next succeeds a thin
seam of ash (4) resting sharply on the limestone and overlaid by a bed
of shale (5) containing the same marine organisms. Another stratum of
ash (6) resembling those below follows the shale, and is surmounted by
a close compact greenstone (7) that hardens the ash on which it rests,
but produces no apparent alteration on the soft fissile shale (8)
above it. Next is a fourth seam of volcanic ash (9) resembling those
below it, but without any shells or crinoidal joints, the only fossils
observable being a few carbonized stems apparently of calamites and
lepidodendra. Above it comes a bed of white quartzy sandstone (10) with
similar vegetable remains, and then a layer of white stiff fire-clay
(11) with rootlets of stigmaria, above which lies a seam of coal (12).
A thin layer of soft blue shale (13) here intervenes, somewhat baked
along its upper portions by another bed of compact vesicular greenstone
(14), which displays in places a well-marked columnar structure. It is
surmounted by a highly characteristic ash (15) in which there occur
numerous large bombs chiefly of trap of different kinds, some of them
highly vesicular. Fragments of shale also occur, mingled here and there
with black carbonized fragments of coal-measure plants, but without any
of the shells and other marine organisms so abundant below. The topmost
bed is a grey carbonaceous sandstone (16), underlying a thin covering
of vegetable mould.
[Illustration: Fig. 35.--Contemporaneous Trap.]
Such is the skeleton, as it were, of the section; the mere dry bones
which remain to the geologist, and which he must study closely to
be able to give them life again. The lowest bed visible, with its
stone-lilies and molluscs, we readily recognise as marking an old
ocean-bed, so that the little episode in the primeval records of
our planet here presented to us opens, like the two great epics of
antiquity, within sound of the wide-roaring sea. The seam of ash which
follows shows, from the sharpness of its line of demarcation with the
limestone, how the denizens of the sea-bottom were suddenly destroyed
by a thick shower of volcanic dust that settled down over their
remains. The waters, however, soon cleared, and ere long stone-lilies
and producti were plentiful as ever, mingling their remains among the
upper layers of the soft muddy ash, and giving rise therefrom to a
sort of calcareous ash or ashy limestone, until in the course of time
the volcanic matter became wholly covered over by a seam of ordinary
limestone. The corals and stone-lilies were, however, anew destroyed
by the deposition of volcanic dust that settled over them as a seam
of ash, after which the water was again rendered turbid and muddy by
the inroad of foreign matter, which, brought down by rivers or by the
changing currents of the ocean, sank to the bottom and eventually
consolidated into a seam of shale. Thereafter the volcanic forces
began once more to eject a quantity of dust and scoriæ that fell into
the water and spread along the bottom as a stratum of ash, and to
pour out a current of lava which hardened into a great sheet visible
now as the undermost greenstone of the section. The emission of the
lava seems to have terminated the eruption, for the next stratum is
one of shale like that below the ash, so that the muddy sediment,
the deposition of which was interrupted for a while by the volcanic
products, began afresh to settle down along the sea-bottom. This
last condition of things seems to have continued for a considerable
period, seeing that the shale bed is relatively thick, and from its
fissile laminated structure indicates a slow and tranquil deposition.
Another eruption of volcanic dust and ashes again interrupted the
detrital deposits, and gave rise to another seam of tufa. This last
subterranean movement seems to have considerably altered the general
contour of the sea-bottom, and so elevated it, at least at one part,
that a thick accumulation of sand, and subsequently of clay, filled
it up to the level of the water or nearly so, giving rise to a dense
growth of the stigmaria and other coal-measure plants whose roots are
still seen imbedded in the fire-clay on which, as a soft muddy soil,
they originally grew. It is probable, however, that, notwithstanding
such elevations of the sea-bed, there was a general subsidence of the
ground during the accumulation of these strata, for we see that the
peaty morass, represented now by the coal-seam, ere long sank beneath
the waters, with the inroads of which it was unable to keep pace, while
there slowly silted over it a muddy sediment that hardened at length
into what is now a seam of shale. But this order of things had been in
existence for but a comparatively short period when the igneous forces
broke out again, ejecting a stream, of molten lava that spread along
the bottom of the shallow waters and hardened as before into a sheet
of greenstone. This was followed by an abundant shower of dust and
lapilli, along with numerous large masses of greenstone and basalt.
These falling into the water accumulated on the upper surface of the
lava-stream, then somewhat cooled, and formed in the end a stratum of
ash of a rubbly conglomeritic aspect. That the sheet of greenstone
really spread out along the sea-bottom before the ejection of the ash,
and was not intruded among the beds at a later period,--that, in short,
it must be regarded as a contemporaneous and not as an intrusive rock,
seems sufficiently shown by its great regularity and evenness, and by
the unaltered condition of the fine soft felspathic matter which covers
its upper surface. It was assuredly in a highly-heated condition when
poured out, as may be gathered from the baked aspect of the mud over
which it rolled; but it had cooled and solidified, at least along its
upper surface, ere buried beneath the shower of ashes. The last bed
exhibited in the section is a grey sandstone, with many carbonaceous
streaks and traces of land-plants, showing a pause in the volcanic
activity of the district, during which the streams from the land
brought down sandy sediment, with an abundant admixture of macerated
leaves, branches, and other drift-wood.
It thus appears that not only were the plains and hills of the
Carboniferous era richly clothed with vegetation, and its waters
crowded with animals, but that then, as now, subterranean forces were
at work, sometimes elevating, sometimes depressing the area alike of
the land and of the sea; while, not unfrequently, melted lava rose
from below, rolling along the bottom of the waters, and showers of
ashes were flung far and wide through the air, settling at last as a
thickening sediment along the floor of the sea, or athwart the marshy
swamps of the delta. Whether the interior of the land had burning cones
among its pine-covered hills we know not yet. Such, however, probably
existed; nay, there may have been among the higher peaks some "snowy
pillar of heaven," like the Ætna of Pindar, raising its smoking summit
among everlasting crags of ice in solitudes lifeless and bare.[77]
[Footnote 77: The highest points of New Zealand, nearly 10,000 feet
above the sea, are said to be clothed for two-thirds of their height
with ice and snow. If, therefore, during Carboniferous times, there
existed somewhere to the west of what is now central Scotland, a chain
cf hills 5000 or 6000 feet high, their summits might perhaps have been
as wintry us that of Mont Blanc.]
Our boulder has served us like the minstrels in modern Gothic
poetry, who appear between the cantos, and give an air of unity and
completeness to what would otherwise be often rambling and unconnected.
And now, at the close, it comes again before us, lying in its bed of
clay, clustered with mosses of brightest green, and overshadowed by its
flickering canopy of beechen leaves. Silent and senseless, the emblem,
seemingly, of calm repose and unchanging durability, what could we have
conceived it should have to chronicle, save the passing, perchance, of
many a wintry December and many a sultry June. Such, indeed, would be
the character of its records of the centuries that have passed away
since the birth of man, did any such record survive in its keeping.
But it rests there as the memorial of far earlier centuries, and of
an older creation; and though now surrounded with all that is lovely
or picturesque--the twinkling flowers on every side, the wide arch of
boughs overhead, and the murmuring streamlet in the dell below--and
though forming itself no unimpressive object in the scene, the boulder
looks out upon us unconnected with anything around. Like a sculptured
obelisk transported from the plains of Assyria to the streets of
London, it offers no link of association with the order of things
around it; its inscriptions are written in hieroglyphics long since
extinct, but of which the key yet remains to show us that the rocks of
our planet are not masses of dead, shapeless matter, but chronicles of
the past; and that all the varied beauty of green field and waving wood
is but a thin veil of gossamer spread out over the countless monuments
of the dead. We have raised one little corner of this gauze-like
covering, and tried to decipher the memorials of bygone creations,
traced in clear and legible characters on the boulder. First, there
lies spread out before us a wide arctic sea, studded with icebergs
that come drifting from the north. Here and there a bare barren islet
rises above the waste of waters, and the packed ice-floes often strand
along its shores, while at other parts great towering bergs, aground
in mid-ocean, keep rising and falling with the heavings of the surge,
and seem ever on the verge of toppling into the deep. But this scene,
so bleak and lifeless, erelong fades away, and we can descry a wide
archipelago of islands, green well-nigh to the water's edge, and
looking like the higher hill-tops of some foundered continent. The
waves are actively at work wearing down the shores, which present for
the most part an abrupt cliff-line to the west. This picture, too,
gets gradually dim, and when the darkness and haze have cleared away,
the scene is wholly new. For miles around there spreads out an expanse
of water, like a wide lake, thickly dotted with islets of every form
and size, clothed with a rich vegetation. Here a jungle of tall reeds
shoots out of the water, clustering with star-like leaves; there a
group of graceful trees, fluted like the columns of an ancient temple,
and crowned by a coronal of sweeping fronds, spread out their roots
amid the soft mud. Yonder lies a drier islet, rolling with ferns of
every shape and size, with here and there a lofty tree-fern, waving its
massive boughs high overhead. The vegetation, rank and luxuriant in the
extreme, strikes us as different from anything visible at the present
day, though, as our eyes rest on the muddy discoloured current, we
can mark, now and then, huge trunks, branchless and bare, that recall
some of the living pine-trees. The denizens of the water seem to be
equally strange. Occasionally a massive head, with sharp formidable
tusks, peers above the surface, and then the gleam of fins and scales
reveals a creature some twenty or thirty feet long. Glancing down into
the clearer spots, we can detect many other forms of the finny tribes,
all cased in a strong glistening armature of scales, and darting
about with ceaseless activity. Beyond this scene of almost tropical
luxuriance, on the one side, lies the blue ocean, with its countless
shells and corals, its stone-lilies and sea-urchins, and its large
predaceous fish; on the other side stretches a far-off chain of hills,
whose nether slopes, dark with pine-woods, sweep down into the rich
alluvial plains. And then this landscape, too, fades slowly away, and
thick darkness descends upon us. Yet through the gloom we feel ever
and anon the rambling earthquake, and see in the distance the glare
of some active volcano that throws a ruddy gleam amid the pumice and
ashes, ever dancing along the surface of the sea. And now this last
scene melts away like the rest, and dark night comes down in which we
can detect no ray of light, and beyond which we cannot go. The record
of the boulder can conduct us no further into the history of the past.
The same principles which have been pursued in the previous pages in
elucidating the history of the Carboniferous system, will conduct
the reader to the true origin and age of any group of rocks he may
encounter, whatever its nature, and wheresoever its locality. Let
him, therefore, in his country rambles, seek to verify them in valley
and hill-side, by lake and cataract, and along river-course and
sea-shore. Let him not be content with simply admiring the picturesque
grouping of rock-masses, but rather seek to interpret their origin
and history, tracing them step by step into the past, amid ages long
prior to man. Such a process will give him a yet keener relish for
the beauties of their scenery, by ever calling up to his mind some of
those striking contrasts with which geology abounds. In the stillness
of the mountain-glen, he will see on every side traces of the waves of
ocean, and when dipping his oars into the unruffled sea among groups
of wasted rocks, miles from shore, he will bethink him, perchance,
of some old forest-covered land, of which these battered islets are
the sole memorials. His enjoyment of the scenery of nature is thus
increased manifold, and he carries about with him a power of making
even the tamest landscape interesting. Cowper, in one of his exquisite
letters, remarks,--"Everything I see in the fields, is to me an object;
and I can look at the same rivulet or at a handsome tree, every day of
my life, with new pleasure." Had the sweet singer of Olney lived to
witness the results obtained by the geologists whom he satirized, he
would perhaps have sauntered along the Ouse with a new pleasure, and
have felt a yet more intense delight in casting his eyes athwart the
breadth of landscape that spreads out around-the "Peasant's nest."
Such, however, are after all only secondary incentives to the study of
the rocks. As a mental exercise, geology certainly yields to none of
the other sciences, for it addresses itself at once to the reasoning
powers and to the imagination, and may thus be made a source both
of intellectual training and of delightful recreation. Of none of
the sciences is it so easy to get a general smatter, yet none is so
difficult thoroughly to master, for geology embraces all the sciences.
In so wide a field, the student will therefore find ample room to
expatiate. In beginning the study, he may perhaps think it, as Milton
pictured the other paths of learning, "laborious, indeed, at the first
ascent; but else so smooth, so green, so full of goodly prospects and
melodious sounds on every side, that the harp of Orpheus was not more
charming." If time and taste disincline him to travel over the whole
of the broad field, there are delightful nooks to which he may betake
himself, replete with objects of beauty and interest, where he may
spend his leisure, and by so doing not merely delight himself, but
enlarge the bounds of human knowledge. No part of the domain can be too
obscure or remote to reward his attention; no object too trifling or
insignificant: for the march of science, though a stately one, proceeds
not by strides, but by steps often toilsome and slow; and she stands
mainly indebted for her progress not to the genius of a few gigantic
intellects, but to the united efforts of many hundred labourers, each
working quietly in his own limited sphere.
But the highest inducement to this study must ever be that so quaintly
put by old Sir Thomas Browne: "The world was made to be inhabited by
beasts, but studied and contemplated by man: 'tis the debt of our
reason we owe unto God, and the homage we pay for not being beasts;
without this the world is still as though it had not been, or as it
was before the sixth day, when as yet there was not a creature that
could conceive or say there was a world." Geology lifts off for us the
veil that shrouds the past, and lays bare the monuments of successive
creations that had come and gone long ere the human race began. She
traces out the plan of the Divine working during a vast cycle of ages-,
and points out how the past dovetails with the present, and how the
existing condition of things comes in as but the last and archetypal
economy in a long progressive series. By thus revealing what has gone
before, she enables us more fully to understand what we see around us
now. Much that is incomplete she restores; much that is enigmatical
she explains. She teaches us more fully man's true position in the
created universe, by showing that in him all the geologic ages meet
that he is the point towards which creation has ever been tending. How
far the facts brought to light by geology may bear upon the future,
will not, perhaps, be solved until that future shall have come. There
is, nevertheless, in the meanwhile, material enough for solemn and
earnest reflection, and as years go by the amount will probably be
always increasing. For we must ever be only learners here, and when all
earthly titles and distinctions have passed away, and we enter amid the
realities of another world, we shall carry with us this one common name
alone. It will, perhaps, be then as now, that only
"In contemplation of created things
By steps we may ascend to God."
And it can surely be no unmeet preparation for such a scene, in humble
faith to read the records of His doings which the Almighty has graven
on the rocks around us. Many problems meet us on every hand problems
which it seems impossible for us now to solve and as the circle of
science ever widens, its enveloping circumference of difficulty and
darkness widens in proportion. It is, doubtless, well that it should be
so; for we are thus taught to regard our present state as imperfect and
incomplete, and to long for that higher and happier one promised by the
Redeemer to those that love Him, when "we shall know thoroughly even as
we are thoroughly known."
TABLE OF FOSSILIFEROUS ROCKS.
LYELL'S _Elements_, p. 109.
1. RECENT. } POST-TERTIARY.
2. POST-PLIOCENE. }
}
3. NEWER PLIOCENE. } PLIOCENE. }
4. OLDER PLIOCENE. } }
} TERTIARY
5. UPPER MIOCENE. } MIOCENE. } or
6. LOWER MIOCENE. } } CAINOZOIC.
}
7. _a_ UPPER EOCENE. } EOCENE. }
7. _b_ MIDDLE EOCENE. } }
8. LOWER EOCENE. } }
9. MAESTRICHT BEDS. }
10. UPPER WHITE CHALK. }
11. LOWER WHITE CHALK. }
12. UPPER GREENSAND. } CRETACEOUS. }
13. GAULT. } }
14. LOWER GREENSAND. } }
15. WEALDEN. } }
}
16. PURBECK BEDS. } }
17. PORTLAND STONE. } } SECONDARY
18. KIMMERIDGE CLAY. } } or
19. CORAL RAG. } JURASSIC } MESOZOIC.
20. OXFORD CLAY. } }
21. GREAT or BATH OOLITE. } }
22. INFERIOR OOLITE. } }
23. LIAS. } }
}
24. UPPER TRIAS. } }
25. MIDDLE TRIAS, } }
or } TRIASSIC. }
MUSCHELKALK. } }
26. LOWER TRIAS. } }
27. PERMIAN, }
or } PERMIAN. }
MAGNESIAN LIMESTONE. } }
}
28. COAL-MEASURES. } }
29. CARBONIFEROUS } CARBONIFEROUS. }
LIMESTONE. } }
} PRIMARY
30. UPPER DEVONIAN. } DEVONIAN, or } or
31. LOWER " } OLD RED SANDSTONE. } PALÆOZOIC.
}
32. UPPER SILURIAN. } SILURIAN. }
33. LOWER " } }
}
34. UPPER CAMBRIAN. } CAMBRIAN. }
35. LOWER " } }
* * * * *
Transcriber Note
Minor typos corrected. Some images moved to nearest paragraph break. A
paragraph break was added to page 74 and 105 to accommodate placement
of Figures 17 and 26 respectively. The missing anchor for the footnote
on page 199 was added.