Town Geology

imitate the boy who, cliff-bred from his youth, used to spend stolen hours on the house-top, with his back against a chimney-stalk, transfigur

tains whence they came; perhaps to pleasant trips to the lakes and hills of Cumberland, Westmoreland, and North Wales; and

wherever there is a brick wall and a slate roof there need be no want of rich colour in an English landscape. But most beautiful is the hue of slate, when, shining wet in the sunshine after a summer shower, its blue is brought out in rich contrast by golden spots of circular lichen, whose spores,

ch has lasted long enough, and endured enough likewise, to bring out in it whatsoever latent capabilities of streng

with burn

n baths of h

d by the st

ape a

aught but an ugly lump of

l leave each reader to interpret it for himself. I shall confine myself now to pro

icts (at least in Snowdon) carry such a rich clay on them, wherever it is not masked by the ruins of other rocks. At Ilfracombe, in North Devon, the passage from slate below to clay above, may be clearly seen. Wherever the top of the slate beds, and the soil upon i

, how did the firs

s, the more perfectly elaborate, is the slate. The best slates of Snowdon-I must confine myself to the district which I know personally-are found in the so-called "Cambrian" beds. Below these beds but one series of beds is as yet known in the world, called the "Laurentian." They occur, to a thickness of some eighty thousand feet, in Labrador, Canada, and the Adirondack mountains of New Y

found, it is true, in the upper Cambrian beds. In the lower they have all but disappeared. Whether their traces have been obliterated by heat and pressure, and chemical action, during long ages; or whether, in these lower beds, we are actually reaching that "Primordial Zone" conceived of by M. Barrande, namely, rocks which existed before living things had begun to people this planet, is a question not yet answered. I believe the former theory to be the true one. That there was life, i

esent for the Cambrian

till those in the upper beds have become unlike those in the lower, and all are from the beginning more or less unlike any existing now on earth. Whole families, indeed, disappear entirely, like the Trilobites, which seem to have swarmed in the Silurian seas, holding the same place there as crabs and shrimps do in our modern seas. They vanish after the period of the coal, and their place is taken by an allied family of Crustaceans, of which only one form (as far as I am aware) lingers now on earth, namely, the "King Crab," or Limulus, of the Indian Seas, a well-known animal, of which specimens may sometimes be seen alive in English aquaria. So perished in the lapse of those same ages, the armour-plated or "Ganoid" fish which Hugh Miller made so justly famous-and which made him so justly famous in return-appearing first in the upper Silurian beds, and abounding in vast variety of strange forms in the old Red Sandstone,

ghout a seemingly wandering paragraph-surely there has been t

were the

ext see that the map is covered with a labyrinth of red patches and curved lines, signifying the outcrop or appearance at the surface of these volcanic beds. They lie at every conceivable slope; and the hills and valleys have been scooped out by rain and ice into every conceivable slope likewise. Wherefore we see, here a broad patch of red, where the back of a sheet of Lava, Porphyry, Greenstone, or what not is exposed; there a narrow line curving often with the curve of the hill-side, where only the edge of a similar sheet is exposed; and every possible variety of shape and attitude between these two. He will see also large spaces covered with little coloured do

its seeming confusion; and let him, if he be a courteous and grateful person, return due thanks to Professor Ramsay for having found

rocks, which he sees marked as Felspathic Traps, Quartz Porphyries, Greenstones, and so forth, got intermingled with beds which he is told to believe are volcanic ashes, and those again with fossil-bearing Silurian beds and Cam

o himself what must go on in the case of a submarine eruption, such as broke out off the coast of Icel

ch rushes an enormous jet of high-pressure steam and other gases, which boils up through the sea, and forms a

them also veins of hardened lava, which had burrowed out through the soft ashes of the cone. Of those lava veins I will speak presently. What I want the reader to think of now is the immense quantity of ash which the steam-mitrailleuse hurls to so vast a height into the air, that it is often drifted many miles down to leeward. To give two instances: The jet of steam from Vesuvius, in the eruption of 1822, rose more than four miles into the air; the jet from the Souffrière of St. Vincent in the West Indies, in 1812, probably rose higher; certainly it met the N.E. trade-wind, for it poured down a layer of ashe

ed (as is usual in geological maps) red. Let us go down to the

e molten lava would rise in the bore, and flow out over the ashes and the sea-bottom-perhaps in one direction, perhaps all round. Then, usually, the volcano, having vented itself, would be quieter for a ti

might have layers on layers of sediment deposited, with live shells, etc., living in them, which would be converted into fossils when they died; and so we should have fossilifer

y beds are laid down by water, the volc

ly this, which has o

long horizontally. This process accounts for the very puzzling, though very common case in Snowdon and elsewhere, in which we find lavas interstratified with rocks which are plainly older than those lavas. Perhaps when that is done the vol

ich have come from different directions. The ashes blown out of the two craters may mingle also, and so, in the course of ages, the result may be such a confusion of ashes, lavas, and sedimentary rocks as we find througho

ppened to Sabrina Island-the cone is sunk again by earthquakes, and gnawn down at the same time by the sea-waves, till nothing is left but a shoal under water. But where have all its vast heaps of ashes gone? To be s

ll this to do

the beginning in order to end at the end. Let me first make my readers clearly understand that all our slate-bearing mountains, and most also of the non-slate-bearing ones likewise, are formed after the fashion which I have described, namely, beneath the sea. I do not say

ancy that in any known part of these islands craters are to be still seen, such

lakes about their tops are true craters. I have been told, for instance, that that wonderful little blue Glas Llyn, under the highest cliff of Snowdon, is the

ised to find the Government geologists declaring that the Llyn on Cader Idris is not one either. The fact is, that the crater, or rather

st resistance among the overlying rocks. But where are these vents? Buried deep under successive eruptions, shifted probably from their places by successive upheavings and dislocations; and if we wanted to find them we should have to quarry the mountain range all over, a mile deep, before we hit upon here and there a tap-root of ancient lava, connecting the upper and the nether worlds. There are such tap-roots, probably, under each of our British mountain ranges. But Snowdon, certainly, does not owe its shape to the fact of one of these old fire vents being under it. It owes its sh

ntains, since first their strata were laid down at the bottom of the sea: I shall give fa

ow such a heap of beds as I have described c

a number of pieces of cloth, or any such stuff; lay them on each other and then squeeze them together at each end. They will arrange themselves in folds, just as the beds of the cliff have done. And if, instead of cloth, you take some more brittle matter, you will find that, as you squeeze on, these folds will tend to snap at the poi

open space between them. Now if you could contrive to squeeze into them from below a paste, which would harden in the cracks and between the layers, and so keep them permanently apart, you would make them into a fair likeness of an aver

r here are the palpable effects of it. And the simplest general cause which I can give

-to enter into the various theories which philosophers

st probable, is the theory of M. Elie de

have been once in a state of intense heat th

ing, and giving off

s it cools, its cr

g but tiny wrinkles, compared with the whole mass of the earth), wrinkles, I say, must

the rocks out, and probably cracking them, and inserting melting lava into those cracks from below. On the other hand, if the internal heat leaves that spot again, and it cools, then it must con

question-If these upheaved beds were once horizontal, should

n, and one which adm

e determined generally by the fossils, usually shells, peculiar to them: so that if we find the same fashion of shells, and still more the same species of shells, in two beds in dif

r no beds which are found in one place upheaved, broken, and altered by heat, which are not found

faulted boulder clay and chalk of the Isle of Moen in Denmark-downwards through all the strata, do

ancient Silurian rocks of Wales,

, unbroken, and still soft, because undisturbed by volcanic rooks and earthquakes. At the bottom of them all, near Petersburg, Sir Roderick found a shale of dried mud (to quote his own words), "so soft and incoherent that it is even used by sculptors for modelling, although it underlies the great mass

teran, I know none more valuable for its bearing on the whole questi

under Snowdon, an immense wedge of porphyry has been thrust up, in what is now the bottom of the valley, between rocks far newer than it, on one side to a height of eight hundred, on the other to a height of eighteen hundred feet-half the present height of Snowdon. Nay, the very slate beds of Snowdonia have not forced their way up from under the mountain-without long and fearful struggles. They are set in places upright on end, then horizontal again, then sunk

can only be estimated by looking at them on the section which may be found at the end of Professor Ramsay's "Geological Survey of North Wales." But anyone who will study that section, and use (as with the map) a little imagination and common sense, will see that between the heat of that Porphyry, which must have been pou

e slates on the roof, and may dispo

ach slate is one of the little horizontal beds of it, perhaps just what was laid down in a single tide. We should have a right to do so, because that would be true of most sedimentary rocks. But it would not be true of slate. The plane of bedding in slate has nothing to do with the plane of cleavage. Or, more plainly, the mud of which the slate is made may have been de

freely, will run through a whole mountain at the same angle, though the beds t

er, if he geologises about slate quarries much, may see with his own eyes. The fossils in the slate are often distorted into quaint shapes, pulled out long if they lie along the plane of cleavage, or squeezed together,

ay be even now going on, in the more ancient rocks, which is similar to that which produces single crystals; and similar, too, to that which produced the jointed crystals of basalt, i.e. lava, at the Giant's Causeway, in Ireland, and Staffa, in the Hebrides. Two philosophers-Mr. Robert Were Fox and Mr. Robert Hunt-are of opinion that the force which has determined the cleavage of slates may be that o

not have happened without a proportionately enormous pressure, and therefore heat; and next, that the best slates are invariably found in the oldest beds-

in mountains composed of these elder rocks upon the lie of the strata, or beds, but has been carved out by great forces, long after those beds were not only laid down and hardened, but faulted and tilted on end. S

hat case, slope up to it. They slope up from it, to the north-west in one direction, and the south-south-west in the other; and Snowdon is a mere insignificant boss, left hanging on one slope of what was once an enormous trough, or valley, of strata far older than itself. By restoring these strata, in the direction of the angles, in which they crop out, and vanish at the surface, it is found that to the north-west-the direction of the Menai Straits-they must once have risen to a height of at least six or seven thousand feet; and more, by restoring them, specially the ash-bed of Snowdon, towards the south-east-which can be done by the guida

tons-gone? Where is it now? I know not. But if I dared to hazard a g

een ground off, such as would make fine sandstones if they had the chance. So that many a grain of sand in Chester walls was probably once blasted out of the bowels of the earth

te? Mud the slate was, and into mud it has returned. Why not? Some of the richest red marl land I know, is, as I have said, actually being made now, out of the black slates of Ilfracombe, wherev

ot find one for themselves? I do not say that it is the cause, but it is at least a causa vera, one which would

are; and their remains, being washed away by the sea the last, would be washed on to the top of the remains of the Silurians; and so (as in most cases) the remains of the older rock, when redeposited by water, would lie on the remains of the younger rock. And do they not see that (if what I just said is true) these slates would grind up into red

all be required. But to him, for that very reason, all that can be given should be given; he should have every facility for learning what he can about this earth, its composition, its capabilities; lest his intellect, crushed and fettered by that artificial drudgery which we for a time miscall civilisation, should begin to fancy, as too many do already, that the world is composed mainly of bricks and deal, and governed by acts of parliament. If I shall have awakened any townsmen here and there to think seriously of the complexity, the antiquity, the grandeur, the true poetry, of the commonest objects around them, even the stones beneat

tno

e," No. XXV. (

d to the members of the Natural

eological Society, June, 1870. This article is so remarkable, not only for its sound scientific matter, but for the vividness and poetic b

Antiquity of Man