The World Before the Deluge Part 35

"In the Carpathians and the Caucasus the existence of ancient glaciers of great extent has also been observed.

"In the Sierra Nevada, in the south of Spain, mountains upwards of 11,000 feet high, the valleys which descend from the Picacho de Veleta and Mulhacen have been covered with ancient glaciers during the Quaternary epoch."

There is no reason to doubt that at this epoch all the British islands, at least all north of the Thames, were covered by glaciers in their higher parts. "Those," says Professor Ramsay, "who know the Highlands of Scotland, will remember that, though the weather has had a powerful influence upon them, rendering them in places rugged, jagged, and cliffy, yet, notwithstanding, their general outlines are often remarkably rounded and flowing; and when the valleys are examined in detail, you find in their bottoms and on the sides of the hills that the mammillated structure prevails. This rounded form is known, by those who study glaciers, by the name of _roches moutonnees_, given to them by the Swiss writers. These mammillated forms are exceedingly common in many British valleys, and not only so, but the very same kind of grooving and striation, so characteristic of the rocks in the Swiss valleys, also marks those of the Highlands of Scotland, of c.u.mberland, and Wales.

Considering all these things, geologists, led by Aga.s.siz some five or six and twenty years ago, have by degrees come to the conclusion, that a very large part of our island was, during the glacial period, covered, or nearly covered, with a thick coating of ice in the same way that the north of Greenland is at present; and that by the long-continued grinding power of a great glacier, or set of glaciers nearly universal over the northern half of our country, and the high ground of Wales, the whole surface became moulded by ice."

Whoever traverses England, observing its features with attention, will remark in certain places traces of the action of ice in this era. Some of the mountains present on one side a naked rock, and on the other a gentle slope, smiling and verdant, giving a character more or less abrupt, bold, and striking, to the landscape. Considerable portions of dry land were formerly covered by a bluish clay, which contained many fragments of rock or "boulders" torn from the old c.u.mbrian mountains; from the Pennine chain; from the moraines of the north of England; and from the Chalk hills--hence called "boulder" clay--present themselves here and there, broken, worn, and ground up by the action of water and ice. These erratic blocks or "boulders" have clearly been detached from the parent rock by violence, and often transported to considerable distances. They have been carried, not only across plains, but over the tops of mountains; some of them being found 130 miles from the parent rocks. We even find, as already hinted, some rocks of which no prototypes have been found nearer than Norway. There is, then, little room for doubting the fact of an extensive system of glaciers having covered the land, although the proofs have only been gathered laboriously and by slow degrees in a long series of years. In 1840 Aga.s.siz visited Scotland, and his eye, accustomed to glaciers in his native mountains, speedily detected their signs. Dr. Buckland became a zealous advocate of the same views. North Wales was soon recognised as an independent centre of a system which radiated from lofty Snowdon, through seven valleys, carrying with them large stones and grooving the rocks in their pa.s.sage. In the pa.s.s of Llanberis there are all the common proofs of the valley having been filled with glacier ice. "When the country was under water," says Professor Ramsay, "the drift was deposited which more or less filled up many of the Welsh valleys. When the land had risen again to a considerable height, the glaciers increased in size: although they never reached the immense magnitude which they attained in the earlier portion of the icy epoch. Still they became so large that such a valley as the Pa.s.s of Llanberis was a second time occupied by ice, which ploughed out the drift that more or less covered the valley. By degrees, however, as we approach nearer our own days, the climate slowly ameliorated, and the glaciers began to decline, till, growing less and less, they crept up and up; and here and there, as they died away, they left their terminal and lateral moraines still as well defined in some cases as moraines in lands where glaciers now exist. Frequently, too, ma.s.ses of stone, that floated on the surface of the ice, were left perched upon the rounded _roches moutonnees_, in a manner somewhat puzzling to those who are not geologists.

"In short, they were let down upon the surface of these rocks so quietly and so softly, that there they will lie, until an earthquake shakes them down, or until the wasting of the rock on which they rest precipitates them to a lower level."

It was the opinion of Aga.s.siz, after visiting Scotland, that the Grampians had been covered by a vast thickness of ice, whence erratic blocks had been dispersed in all directions as from a centre; other geologists after a time adopted the opinion--Mr. Robert Chambers going so far as to maintain, in 1848, that Scotland had been at one time moulded by ice. Mr. T. F. Jamieson followed in the same track, adducing many new facts to prove that the Grampians once sent down glaciers in all directions towards the sea. "The glacial grooves," he says, "radiate outward from the central heights towards all points of the compa.s.s, although they do not strictly conform to the actual shape and contour of the minor valleys and ridges." But the most interesting part of Mr.

Jamieson's investigations is undoubtedly the ingenious manner in which he has worked out Aga.s.siz' a.s.sertion that Glenroy, whose remarkable "_Parallel Roads_" have puzzled so many investigators, was once the basin of a frozen lake.

Glenroy is one of the many romantic glens of Lochaber, at the head of the Spey, near to the Great Glen, or the valley of the Caledonian Ca.n.a.l, which stretches obliquely across the country in a northwesterly direction from Loch Linnhe to Loch Ness, leaving Loch Arkaig, Loch Aich, Glen Garry, and many a highland loch besides, on the left, and Glen Spean, in which Loch Treig, running due north and south, has its mouth, on the south. Glenroy opens into it from the north, while Glen Gluoy opens into the Great Glen opposite Loch Arkaig. Mr. Jamieson commenced his investigations at the mouth of Loch Arkaig, which is about a mile from the lake itself. Here he found the gneiss ground down as if by ice coming from the east. On the hill, north of the lake, the gneiss, though much worn and weathered, still exhibited well-marked striae, directed up and down the valley. Other markings showed that the Glen Arkaig glacier not only blocked up Glen Gluoy, but the mouth of Glen Spean, which lies two miles or so north of it on the opposite side.

At Brackletter, on the south side of Glen Spean, near its junction with Glen Lochy, glacial scores pointing more nearly due west, but slightly inclining to the north, were observed, as if caused by the pressure of ice from Glen Lui. The south side of Glen Spean, from its mouth to Loch Treig, is bounded by lofty hills--an extension of Ben Nevis, the highest of these peaks exceeding 3,000 feet. Numerous gullies intersect their flanks, and the largest of these, Corry N'Eoin, presents a series of rocky amphitheatres, or rather large caldrons, whose walls have been ground down by long-continued glacial action: the quartz-veins are all shorn down to the level of the gneiss, and streaked with fine scratches, pointing down the hollows and far up the rocks on either side. During all these operations the great valley was probably filled up with ice, which would close Glen Gluoy and Glen Spean, and might also close the lowest of the lines in Glenroy. But how about the middle and upper lines?

A glacier crossing from Loch Treig, and protruding across Glen Spean, would cut off Glens Glaibu and Makoul, when the water in Glenroy could only escape over the Col into Strathspey, when the first level would be marked.

Now let the Glen Treig glacier shrink a little, so as to let out water to the level of the second line by the outline at Makoul, and the theory is complete. When the first and greatest glacier gave way, Glenroy would be nearly in its present state.

The glacier, on issuing from the gorge at the end of Loch Treig, would dilate immensely, the right flank spreading over a rough expanse of syenite, the neighbouring hills being mica-schists, with veins of porphyry. Now the syenite breaks into large cuboidal blocks of immense size. These have been swept before the advancing glacier along with other debris, and deposited in a semicircle of mounds having a sweep of several miles, forming circular bands which mark the edges of the glacier as it shrunk from time to time under the influence of a milder climate.

This moraine, which was all that was wanting to complete the theory laid down by Aga.s.siz, is found on the pony-road leading from the mouth of Loch Treig towards Badenoch. A mile or so brings the traveller to the summit-level of the road, and beyond the hill a low moor stretches away to the bottom of the plain. Here, slanting across the slope of the hill towards Loch Treig, two lines of moraine stretch across the road. At first they consist of mica-schists and bits of porphyry, but blocks of syenite soon become intermingled. Outside these are older hillocks, rising in some places sixty and seventy feet high, forming narrow steep-sided mounds, with blocks fourteen feet in length sticking out of the surface, mixed with fragments of mica-schist and gneiss. The inner moraine consists, almost wholly, of large blocks of syenite, five, ten, fifteen, and five-and-twenty feet long.

[Ill.u.s.tration: Fig. 198.--Parallel roads of Glenroy; from a sketch by Professor J. Phillips.]

The present aspect of Glenroy is that of an upper and lower glen opening up from the larger Glen Spean. The head-waters of Lochaber gather in a wild mountain tract, near the source of the Spey. The upper glen is an oval valley, four miles long, by about one broad, bounded on each side by high mountains, which throw off two streams dividing the mica-schist from the gneissic systems; the former predominating on the west side, and the latter on the east. The united streams flow to the south-west for two miles, when the valley contracts to a rocky gorge which separates the upper from the lower glen. Pa.s.sing from the upper to the lower glen, a line is observed to pa.s.s from near the junction of the two streams, on a level with a flat rock at the gorge, and also with the uppermost of the three lines of terraces in the lower glen. This line girdles the sides of the hills right and left, with a seemingly higher sweep, and is followed by two other perfectly parallel and continuous lines till Glenroy expands into Glen Spean, which crosses its mouth and enters the great glen a little south of Loch Lochy. At the point, however, where Glenroy enters Glen Spean, the two upper terraces cease, while the lower of the three appears on the north and south side of Glen Spean, as far as the pa.s.s of Glen Muckal, and southward a little way up the Gubban river and round the head of Loch Treig.

In Scotland, and in Northern England and Wales, there is distinct evidence that the Glacial Epoch commenced with an era of continental ice, the land being but slightly lower than at present, and possibly at the same level, during which period the Scottish hills received their rounded outlines, and scratched and smoothed rock-surfaces; and the plains and valleys became filled with the stiff clay, with angular scratched stones, known as the "Till," which deposit is believed by Messrs. Geikie, Jamieson, and Croll to be a _moraine profonde_, the product of a vast ice-sheet.

In Wales, Professor Ramsay has described the whole of the valleys of the Snowdonian range as filled with enormous glaciers, the level of the surface of the ice filling the Pa.s.s of Llanberis, rising 500 feet above the present watershed at Gorphwysfa. In the Lake District of c.u.mberland and Westmorland, Mr. De Rance has shown that a vast series of glaciers, or small ice-sheets, filled all the valleys, radiating out in all directions from the larger mountains, which formed centres of dispersion, the ice actually pushing over many of the lesser watersheds, and scooping out the great rock-basins in which lie the lakes Windermere, Ullswater, Thirlmere, Coniston Water, and Wast.w.a.ter, the bottoms of which are nearly all below the sea-level. The whole of this district, he has shown, experienced a second glaciation, after the period of great submergence, in which valley-glaciers scooped out the marine drift, and left their _moraines_ in the Liza, Langdale, and other valleys, and high up in the hills, as at Harrison's Stickle, where a tarn has been formed by a little _moraine_, acting as a dam, as shown by Professor Hull.

In Wales, also, valley-glaciers existed after the submergence beneath the Glacial sea. Thus in Cwm-llafar, under the brow of Carnedd Dafydd, and Carnedd Llewelyn, Professor Ramsay has shown that a narrow glacier, about two miles in length, has ploughed out a long narrow hollow in the drift (which "forms a succession of terraces, the result of marine denudation, during pauses in the re-elevation of its submersion) to a depth of more than 2,000 feet."[108]

[108] Professor Ramsay, "The Old Glaciers of North Wales." Longman, 1860.

The proofs of this great submergence, succeeding the era of "land-ice,"

are constantly acc.u.mulating. Since 1863, when Professor Hull first divided the thick glacial deposits of Eastern Lancashire and Cheshire into an Upper Boulder Clay, and Lower Boulder Clay divided by a Middle Sand and Gravel, the whole of which are of marine origin, these subdivisions have been found to hold good, by himself and Mr. A. H.

Green, over 600 square miles of country around Manchester, Bolton, and Congleton; by Mr. De Rance over another 600 square miles, around Liverpool, Preston, Blackpool, Blackburn, and Lancaster, and also in the low country lying between the c.u.mberland and Welsh mountains and the sea.

In Ireland, also, the same triplex arrangement appears to exist.

Professors Harkness and Hull have identified the "Limestone and Manure Gravels" of the central plain, as referable to the "Middle Sand and Gravel," and the "Lower Boulder Clay" rests on a glaciated rock-surface along the coasts of Antrim and Down, and is overlain by sand, which, in 1832, was discovered by Dr. Scouler to be sh.e.l.l-bearing. At Kingstown the three deposits are seen resting on a moutonneed surface of granite, scored from the N.N.W.

In Lancashire and on the coast of North Wales, between Llandudno and Rhyl, Mr. De Rance has shown that these deposits often lie upon the denuded and eroded surface of another clay, of older date, which he believes to be the product of land-ice, the remnant of the _moraine profonde_, and the equivalent of the Scotch "Till." He also shows that the Lower Boulder Clay never rises above an elevation of fifty or eighty feet above the sea-level; and that the Middle Sand and Shingle rests directly upon the rock, or on the surface of this old Till.

Near Manchester the Lower Boulder Clay occasionally rests upon an old bed of sand and gravel. It is extremely local, but its presence has been recorded in several sections by Mr. Edward Binney, who was the first to show, in 1842,[109] that the Lancashire Boulder Clays were formed in the sea, and that the erratic pebbles and boulders, mainly derived from the c.u.mberland Lake Districts, were brought south by means of floating ice.

[109] In 1840 Dr. Buckland described the occurrence of boulders of Criffel Granite between Shalbeck and Carlisle, and attributed their position to the agency of ice floating across the Solway Firth.

Most of the erratic pebbles and boulders in the Lancashire clays are more or less scratched and scored, many of them (though quite rounded) in so many directions that Mr. De Rance believes the c.u.mberland and Westmoreland hills to have been surrounded by an ice-belt, which, occasionally thawing during summer or warm episodes, admitted "breaker action" on the gradually subsiding coast, wearing the fragments of rocks brought down by rivers or by glaciers into pebbles that, with the return of the cold, became covered with the "ice-belt," which, lifted by the tides, rolled and dinted the pebbles one against another, and gradually allowed them to be impressed into its ma.s.s, with which they eventually floated away.

The Middle Sands and Shingles in England have also afforded a great number of sh.e.l.ls of mollusca. At Macclesfield they have been described by Messrs. Prestwich and Darbishire as occurring at an elevation of 1,100 to 1,200 feet above the level of the sea.[110]

[110] Mr. Darbishire records seventy species from Macclesfield and Moel Tryfaen, taken together, of which 6 are Arctic, and 18 are not known in the Upper Crag.

Among other proofs of glacial action and submersion in Wales may be mentioned the case of Moel Tryfaen, a hill 1,400 feet high, lying to the westward of Caernarvon Bay, and six or seven miles from Caernarvon. Mr.

Joshua Trimmer had observed stratified drift near the summit of this mountain, from which he obtained some marine sh.e.l.ls; but doubts were entertained as to their age until 1863, when a deep and extensive cutting was made in search of slates. In this cutting a stratified ma.s.s of loose sand and gravel was laid open near the summit, thirty-five feet thick, containing sh.e.l.ls, some entire, but mostly in fragments. Sir Charles Lyell examined the cutting, and obtained twenty species of sh.e.l.ls, and in the lower beds of the drift, "large heavy boulders of far-transported rocks, glacially polished and scratched on more than one side:" underneath the whole, the edges of vertical slates were exposed to view, exhibiting "unequivocal marks of prolonged glaciation." The sh.e.l.ls belonged to species still living in British or more northern seas.

From the gravels of the Severn Valley, described by Mr. Maw, thirty-five forms of mollusca have been identified by Mr. Gwyn Jeffreys. In the Shingle beds of Leyland, Euxton, Chorley, Preston, Lancaster, and Blackpool,[111] Mr. De Rance has obtained nearly thirty species.

[111] The typical species in West Lancashire are _Tellina Balthica_, _Cardium edule_, _C. aculeatum_, _C. rustic.u.m_, _Psammobia ferroensis_, _Turritella terebra_.

In Eastern Yorkshire, Mr. Searles V. Wood, Jun., has divided the glacial deposits into "Purple Clay without Chalk," "Purple Clay with Chalk," and "Chalky Clay," the whole being later than his "Middle Glacial Sands and Gravel," which, in East Anglia, are overlain by the "Chalky Clay," and rest unconformably upon the "Contorted Drift" of Norfolk, the Cromer Till, and the Forest Bed. His three Yorkshire clays are, however, considered by most northern geologists to be the representatives of the "Upper Boulder Clay" west of the Pennine Chain, the "Chalky Clay" having been formed before the country had sufficiently subsided to allow the sandstones and marls, furnishing the red colouring matter, to have suffered denudation; while the "Purple Clay without Chalk, and with Shap Granite," was deposited when all the chalk was mainly beneath the sea, and the granite from Shap Fell, which had been broken up by breaker-action during the Middle Sand era, was floated across the pa.s.ses of the Pennine Chain and southwards and northwards. A solitary pebble of Shap granite has been found by Mr. De Rance at Hoylake, in Cheshire; and many of Criffel Granite, in that county, and on the coast of North Wales, by Mr. Mackintosh, who has also traced the flow of this granite in the low country lying north and south of the c.u.mberland mountains.

At Bridlington, in Yorkshire, occurs a deposit at the base of the "Purple Clay," with a truly Arctic fauna. Out of seventy forms of mollusca recorded by Mr. S. V. Wood, Jun., nineteen are unknown to the Crag--of these thirteen are purely arctic, and two not known as living.

Sh.e.l.ls have been found in the Upper Boulder Clay of Lancashire, at Hollingworth Reservoir, near Mottram, by Messrs. Binney, Bateman, and Prestwich, at an elevation of 568 feet above the sea, consisting of _Fusus Bamffius_, _Purpura lapillus_, _Turritilla terebra_, and _Cardium edule_. The clay is described by Mr. Binney as sandy, and brown-coloured, with pebbles of granite and greenstone, some rounded and some angular. All the above sh.e.l.ls, as well as _Tellina Balthica_, have been found in the Upper Clay of Preston, Garstang, Blackpool, and Llandudno, by Mr. De Rance, who has also found all the above species (with the exception of _Fusus_), as well as _Psammobia ferroensis_, and the siliceous spiculae of marine sponges, in the Lower Boulder Clay of West Lancashire. He has described the ordinary red Boulder Clay of Lancashire as extending continuously through Cheshire and Staffordshire into Warwickshire, gradually becoming less red and more chalky, everywhere overlying intermittent sheets of "sands and shingle-beds,"

one of which is particularly well seen at Leamington and Warwick, where it contains Pectens from the Crag, _Gryphaea_ from the Lias, and chalk fossils and flints. The latter have also been found by Mr. Lucy in the neighbourhood of Mount Sorrel, a.s.sociated with bits of the Coral Rag of Yorkshire. The gravels of Leicester, Market Harborough, and Lutterworth were long ago described by the Rev. W. D. Conybeare as affording "specimens of the organic remains of most of the Secondary Strata in England."

The Rev. O. Fisher, F.G.S., has paid much attention to the superficial covering usually described as "heading," or "drift," as well as to the contour of the surface, in districts composed of the softer strata, and has published his views in various papers in the _Journal of the Geological Society_ and in the _Geological Magazine_. He thinks that the contour of the surface cannot be ascribed entirely to the action of rain and rivers, but that the changes in the ancient contour since produced by those changes can be easily distinguished. He finds the covering beds to consist of two members--a lower one, entirely dest.i.tute of organic remains, and generally unstratified, which has often been forcibly indented into the bed beneath it, sometimes exhibiting slickenside at the junction.

There is evidence of this lower member having been pushed or dragged over the surface, from higher to lower levels, in a plastic condition; on which account he has named it "The Trail."

The upper member of the covering beds consists of soil derived from the lower one, by weathering. It contains, here and there, the remains of the land-sh.e.l.ls which lived in the locality at a period antecedent to cultivation. It is "The Warp" of Mr. Trimmer.

Neither of these acc.u.mulations occur on low flats, where the surface has been modified since the recent period. They both alike pa.s.s below high-water mark, and have been noticed beneath estuarine deposits.

Mr. Fisher is of opinion that land-ice has been instrumental in forming the contour of the surface, and that the trail is the remnant of its _moraine profonde_. And he has given reasons[112] for believing that the climate of those lat.i.tudes may have been sufficiently rigorous for that result about 100,000 years ago. He attributes the formation of the superficial covering of Warp to a period of much rainfall and severe winter-frosts, after the ice-sheet had disappeared.

[112] _Geological Magazine_, vol. iii., p. 483.

The phenomena which so powerfully affected our hemisphere present themselves, in a much grander manner, in the New World. The glacier-system appears to have taken in America the same gigantic proportions which other objects a.s.sume there. Nor is it necessary, in order to explain the permanent existence of this icy mantle in temperate climates, to infer the prevalence of any very extraordinary degree of cold. On this subject M. Ch. Martins thus expresses himself: "The mean temperature of Geneva is 9 5 Cent. Upon the surrounding mountains the limit of perpetual snow is found at 8,800 feet above the level of the sea. The great glaciers of the valley of Chamounix descend 5,000 feet below this line. Thus situated, let us suppose that the mean temperature of Geneva was lowered only 4, and the average became 5 5; the decrease of temperature with the height being 1 c. for every 600 feet, the limit of perpetual snow would be lowered by 2,437 feet, and would be 6,363 feet above the level of the sea. We can readily admit that the glaciers of Chamounix would descend below this new limit, to an extent at least equal to that which exists between their present limit and their lower extremity. Now, in reality, the foot of these glaciers is 5,000 feet above the ocean; with a climate 4 colder, it would be 2,437 feet lower; that is to say, at the level of the Swiss plain. Thus, the lowering of the line of perpetual snow to this extent would suffice to bring the glacier of the Arve to the environs of Geneva.... Of the climate which has favoured the prodigious development of glaciers we have a pretty correct idea; it is that of Upsala, Stockholm, Christiana, and part of North America, in the State of New York.... To diminish by four degrees the mean temperature of a country in order to explain one of the grandest revolutions of the globe, is to venture on an hypothesis not bolder than geology has sometimes permitted to itself."[113]

[113] _Revue des Deux Mondes._

In proving that glaciers covered part of Europe during a certain period, that they extended from the North Pole to Northern Italy and the Danube, we have sufficiently established the reality of this _glacial period_, which we must consider as a curious episode, as well as certain, in the history of the earth. Such ma.s.ses of ice could only have covered the earth when the temperature of the air was lowered at least some degrees below zero. But organic life is incompatible with such a temperature; and to this cause must we attribute the disappearance of certain species of animals and plants--in particular, the Rhinoceros and the Elephant--which, before this sudden and extraordinary cooling of the globe, appear to have limited themselves, in immense herds, to Northern Europe, and chiefly to Siberia, where their remains have been found in such prodigious quant.i.ties. Cuvier says, speaking of the bodies of the quadrupeds which the ice had seized, and in which they have been preserved, with their hair, flesh, and skin, up to our own times: "If they had not been frozen as soon as killed, putrefaction would have decomposed them; and, on the other hand, this eternal frost could not have previously prevailed in the place where they died; for they could not have lived in such a temperature. It was, therefore, at the same instant when these animals perished that the country they inhabited was rendered glacial. These events must have been sudden, instantaneous, and without any gradation."[114]

[114] "Oss.e.m.e.nts fossiles. Discours sur les Revolutions du Globe."

[Ill.u.s.tration: Fig. 199.--Fissurella nembosa.

(Living sh.e.l.l.)]

How can we explain the _glacial period_? We have explained M. Adhemar's hypothesis, to which it may be objected that the cold of the glacial period was so general throughout the Polar and temperate regions on both sides of the equator, that mere local changes in the external configuration of our planet and displacement of the centre of gravity scarcely afford adequate causes for so great a revolution in temperature. Sir Charles Lyell, speculating upon the suggestion of Ritter and the discovery of marine sh.e.l.ls spread far and wide over the Sahara Desert by Messrs. Escher von der Linth, Desor, and Martins--which seem to prove that the African Desert has been under water at a very recent period--infers that the Desert of Sahara const.i.tuted formerly a wide marine area, stretching several hundred miles north and south, and east and west. "From this area," he adds, "the south wind must formerly have absorbed moisture, and must have been still further cooled and saturated with aqueous vapour as it pa.s.sed over the Mediterranean. When at length it reached the Alps, and, striking them, was driven into the higher and more rarefied regions of the atmosphere, it would part with its watery burthen in the form of snow; so that the same aerial current which, under the name of the Fohn, or Sirocco, now plays a leading part with its hot and dry breath, sometimes, even in the depth of winter, in melting the snow and checking the growth of glaciers, must, at the period alluded to, have been the princ.i.p.al feeder of Alpine snow and ice."[115] Nevertheless, we repeat, no explanation presents itself which can be considered conclusive; and in science we should never be afraid to say, _I do not know_.

[115] Lyell's "Elements of Geology," p. 175.