A Manual of Elementary Geology Part 22

Warren a true gauge for the s.p.a.ce occupied by the intervertebrate substance, so as to enable him to form a correct estimate of the entire length. The tusks when discovered were 10 feet long, but a part only could be preserved. The large proportion of animal matter in the tusk, teeth, and bones of some of these fossil mammalia is truly astonis.h.i.+ng. It amounts in some cases, as Dr. C. T. Jackson has ascertained by a.n.a.lysis, to 27 per cent., so that when all the earthy ingredients are removed by acids, the form of the bone remains as perfect, and the ma.s.s of animal matter is almost as firm, as in a recent bone subjected to similar treatment.

It would be rash, however, to infer from such data that these quadrupeds were mired in _modern_ times, unless we use that term strictly in a geological sense. I have shown that there is a fluviatile deposit in the valley of the Niagara, containing sh.e.l.ls of the genera _Melania_, _Lymnea_, _Planorbis_, _Valvata_, _Cyclas_, _Unio_, and _Helix_, &c., all of recent species, from which the bones of the great Mastodon have been taken in a very perfect state. Yet the whole excavation of the ravine, for many miles below the Falls, has been slowly effected since that fluviatile deposit was thrown down.

Whether or not, in a.s.signing a period of more than 30,000 years for the recession of the Falls from Queenstown to their present site, I have over or under estimated the time required for that operation, no one can doubt that a vast number of centuries must have elapsed before so great a series of geographical changes were brought about as have occurred since the entombment of this elephantine quadruped. The freshwater gravel which incloses it is decidedly of much more modern origin than the drift or boulder clay of the same region.[138-A]

Other extinct animals accompany the _Mastodon giganteus_ in the post-glacial deposits of the United States, among which the _Castoroides ohioensis_, Foster and Wyman, a huge rodent allied to the beaver, and the _Capybara_ may be mentioned. But whether the "loess," and other freshwater and marine strata of the Southern States, in which skeletons of the same Mastodon are mingled with the bones of the Megatherium, Mylodon, and Megalonyx, were contemporaneous with the drift, or were of subsequent date, is a chronological question still open to discussion.

It appears clear, however, from what we know of the tertiary fossils of Europe--and I believe the same will hold true in North America--that many species of testacea and some mammalia, which existed prior to the glacial epoch, survived that era. As European examples among the warm-blooded quadrupeds, the _Elephas primigenius_ and _Rhinoceros tichorinus_ may be mentioned. As to the sh.e.l.ls, whether fresh water, terrestrial, or marine, they need not be enumerated here, as allusion will be made to them in the sequel, when the pliocene tertiary fossils of Suffolk are described. The fact is important, as refuting the hypothesis that the cold of the glacial period was so intense and universal as to annihilate all living creatures throughout the globe.

That the cold was greater for a time than it is now in certain parts of Siberia, Europe, and North America, will not be disputed; but, before we can infer the universality of a colder climate, we must ascertain what was the condition of other parts of the northern, and of the whole southern, hemisphere at the time when the Scandinavian, British, and Alpine erratics were transported into their present position. It must not be forgotten that a great deposit of drift and erratic blocks is now in full progress of formation in the southern hemisphere, in a zone corresponding in lat.i.tude to the Baltic, and to Northern Italy, Switzerland, France, and England.

Should the uneven bed of the southern ocean be hereafter converted by upheaval into land, the hills and valleys will be strewed over with transported fragments, some derived from the antarctic continent, others from islands covered with glaciers, like South Georgia, which must now be centres of the dispersion of drift, although situated in a lat.i.tude, agreeing with that of the c.u.mberland mountains in England.

Not only are these operations going on between the 45th and 60th parallels of lat.i.tude south of the line, while the corresponding zone of Europe is free from ice; but, what is still more worthy of remark, we find in the southern hemisphere itself, only 900 miles distant from South Georgia, where the perpetual snow reaches to the sea-beach, lands covered with forests, as in Terra del Fuego. There is here no difference of lat.i.tude to account for the luxuriance of vegetation in one spot, and the absolute want of it in the other; but among other refrigerating causes in South Georgia may be enumerated the countless icebergs which float from the antarctic zone, and which chill, as they melt, the waters of the ocean, and the surrounding air, which they fill with dense fogs.

I have endeavoured in the "Principles of Geology," chapters 7. and 8., to point out the intimate connexion of climate and the physical geography of the globe, and the dependence of the mean annual temperature, not only on the height of the dry land, but on its distribution in high or low lat.i.tudes at particular epochs. If, for example, at certain periods of the past, the antarctic land was less elevated and less extensive than now, while that at the north pole was higher and more continuous, the conditions of the northern and southern hemispheres might have been the reverse of what we now witness in regard to climate, although the mountains of Scandinavia, Scotland, and Switzerland, may have been less elevated than at present. But if in both of the polar regions a considerable area of elevated dry land existed, such a concurrence of refrigerating conditions in both hemispheres might have created for a time an intensity of cold never experienced since; and such probably was the state of things during that period of submergence to which I have alluded in this chapter.

_Alpine erratics._--Although the arctic regions const.i.tute the great centre from which erratics have travelled southwards in all directions in Europe and North America, yet there are some mountains, as I have already stated, like those of North Wales and the Alps, which have served as separate and independent centres for the dispersion of blocks. In ill.u.s.tration of this fact, the Alps deserve particular attention, not only from their magnitude, but because they lie beyond the ordinary limits of the "northern drift" of Europe, being situated between the 44th and 47th degrees of north lat.i.tude.

On the flanks of these mountains, and on the Subalpine ranges of hills or plains adjoining them, those appearances which have been so often alluded to, as distinguis.h.i.+ng or accompanying the drift, between the 50th and 70th parallels of north lat.i.tude, suddenly reappear, to a.s.sume in a more southern country their most exaggerated form. Where the Alps are highest, the largest erratic blocks have been sent forth, as, for example, from the regions of Mont Blanc and Monte Rosa, into the adjoining parts of France, Switzerland, Austria, and Italy, while in districts where the great chain sinks in alt.i.tude, as in Carinthia, Carniola, and elsewhere, no such rocky fragments, or a few only and of smaller bulk, have been detached and transported to a distance.

In the year 1821, M. Venetz first announced his opinion that the Alpine glaciers must formerly have extended far beyond their present limits, and the proofs appealed to by him in confirmation of this doctrine were afterwards acknowledged by M. Charpentier, who strengthened them by new observations and arguments, and declared, in 1836, his conviction that the glaciers of the Alps must once have reached as far as the Jura, and have carried thither their moraines across the great valley of Switzerland. M.

Aga.s.siz, after several excursions in the Alps with M. Charpentier, and after devoting himself some years to the study of glaciers, published, in 1840, an admirable description of them, and of the marks which attest the former action of great ma.s.ses of ice over the entire surface of the Alps and the surrounding country.[140-A] He pointed out that the surface of every large glacier is strewed over with gravel and stones detached from the surrounding precipices by frost, rain, lightning, or avalanches. And he described more carefully than preceding writers the long lines of these stones, which settle on the sides of the glacier, and are called the lateral moraines; those found at the lower end of the ice being called terminal moraines. Such heaps of earth and boulders every glacier pushes before it when advancing, and leaves behind it when retreating. When the Alpine glacier reaches a lower and warmer situation, about 3000 or 4000 feet above the sea, it melts so rapidly that, in spite of the downward movement of the ma.s.s, it can advance no farther. Its precise limits are variable from year to year, and still more so from century to century; one example being on record of a recession of half a mile in a single year. We also learn from M. Venetz, that whereas, between the eleventh and fifteenth centuries, all the Alpine glaciers were less advanced than now, they began in the seventeenth and eighteenth centuries to push forward so as to cover roads formerly open, and to overwhelm forests of ancient growth.

These oscillations enable the geologist to note the marks which they leave behind them as they retrograde, and among these the most prominent, as before stated, are the terminal moraines, or mounds of unstratified earth and stones, often divided by subsequent floods into hillocks, which cross the valley like ancient earth-works, or embankments made to dam up the river. Some of these transverse barriers were formerly pointed out by Saussure below the glacier of the Rhone, as proving how far it had once transgressed its present boundaries. On these moraines we see many large angular fragments, which, having been carried along on the surface of the ice, have not had their edges worn off by friction; but the greater number of the boulders, even those of large size, have been well rounded, not by the power of water, but by the mechanical force of the ice, which has pushed them against each other, or against the rocks flanking the valley.

Others have fallen down the numerous fissures which intersect the glacier, where, being subject to the pressure of the whole ma.s.s of ice, they have been forced along, and either well rounded or ground down into sand, or even the finest mud, of which the moraine is largely const.i.tuted.

As the terminal moraines are the most prominent of all the monuments left by a receding glacier, so are they the most liable to obliteration; for violent floods or debacles are often occasioned in the Alps by the sudden bursting of what are called glacier-lakes. These temporary sheets of water are caused by the damming up of a river by a glacier which has increased during a succession of cold seasons, and, descending from a tributary into the main valley, has crossed it from side to side. On the failure of this icy barrier, the acc.u.mulated waters are let loose, which sweep away and level all transverse mounds of gravel and loose boulder below, and spread their materials in confused and irregular beds over the river-plain.

Another mark of the former action of glaciers, in situations where they exist no longer, is the polished, striated, and grooved surfaces of rocks already alluded to. Stones which lie underneath the glacier and are pushed along by it, sometimes adhere to the ice, and as the ma.s.s glides slowly along at the rate of a few inches, or at the utmost two or three feet, per day, abrade, groove, and polish the rock, and the larger blocks are reciprocally grooved and polished by the rock on their lower sides. As the forces both of pressure and propulsion are enormous, the sand, acting like emery, polishes the surface; the pebbles, like coa.r.s.e gravers, scratch and furrow it; and the large stones scoop out grooves in it. Another effect also of this action, not yet adverted to, is called "roches moutonnees."

Projecting eminences of rock are smoothed and worn into the shape of flattened domes, where the glaciers have pa.s.sed over them.

Although the surface of almost every kind of rock, when exposed in the open air, wastes away by decomposition, yet some retain for ages their polished and furrowed exterior; and, if they are well protected by a covering of clay or turf, these marks of abrasion seem capable of enduring for ever.

They have been traced in the Alps to great heights above the present glaciers, and to great horizontal distances beyond them.

There are also found, on the sides of the Swiss valleys, round and deep holes, with polished sides, such holes as waterfalls make in the solid rock, but in places remote from running waters, and where the form of the surface will not permit us to suppose that any cascade could ever have existed. Similar cavities are common in hard rocks, such as gneiss, in Sweden, where they are called _giant caldrons_, and are sometimes 10 feet and more in depth; but in the Alps and Jura they often pa.s.s into spoon-shaped excavations and prolonged gutters. We learn from M. Aga.s.siz that hollows of this form are now cut out by streams of water, which flow along the surface of glaciers, and then fall into fissures which are open to the bottom. Here, forming a cascade, the stream cuts a round cavity in the rock with the gravel and sand, which it either finds there or carries down with it, and causes to rotate; and, as it usually happens that the glacier is advancing, a locomotive cascade is produced, which converts the first circular hole into a deep groove.

Another effect of a glacier is to lodge a ring of stones round the summit of a conical peak which may happen to project through the ice. If the glacier is lowered greatly by melting, these circles of large angular fragments, which are called "perched blocks," are left in a singular situation near the top of a steep hill or pinnacle, the lower parts of which may be dest.i.tute of boulders.

_Alpine blocks on the Jura._--Now some or all the marks above enumerated,--the moraines, erratics, polished surfaces, domes, striae, caldrons, and perched rocks, are observed in the Alps at great heights above the present glaciers, and far below their actual extremities; also in the great valley of Switzerland, 50 miles broad; and almost everywhere on the Jura, a chain which lies to the north of this valley.

The average height of the Jura is about one third that of the Alps, and is now entirely dest.i.tute of glaciers, yet it presents almost everywhere similar moraines, and the same polished and grooved surfaces, and water-worn cavities. The erratics, moreover, which cover it, present a phenomenon which has astonished and perplexed the geologist for more than half a century. No conclusion can be more incontestible than that these angular blocks of granite, gneiss, and other crystalline formations, came from the Alps, and that they have been brought for a distance of 50 miles and upwards across one of the widest and deepest valleys of the world, so that they are now lodged on the hills and valleys of a chain composed of limestone and other formations, altogether distinct from those of the Alps. Their great size and angularity, after a journey of so many leagues, has justly excited wonder; for hundreds of them are as large as cottages; and one in particular, celebrated under the name of Pierre a Bot, rests on the side of a hill about 900 feet above the lake of Neufchatel, and is no less than 40 feet in diameter.

It will be remarked that these blocks on the Jura offer an exception to the rule before laid down, as applicable in general to erratics, since they have gone from south to north. Some of the largest ma.s.ses of granite and gneiss have been found to contain 50,000 and 60,000 cubic feet of stone, and one limestone block near Devens, which has travelled 30 miles, contains 161,000 cubic feet, its angles being sharp and unworn.[143-A]

Von Buch, Escher, and Studer have shown, from an examination of the mineral composition of the boulders, that those on the western Jura, near Neufchatel, have come from the region of Mont Blanc and the Valais; those on the middle parts of the Jura from the Bernese Oberland; and those on the eastern Jura from the Alps of the small cantons, Glaris, Schwytz, Uri, and Zug. The blocks, therefore, of these three great districts have been derived from parts of the Alps nearest to the localities in the Jura where we now find them, as if they had crossed the great valley in a direction at right angles to its length: the most western stream having followed the course of the Rhone; the central, that of the Aar; and the eastern, that of the two great rivers, Reuss and Limmat. The non-intermixture of these groups of travelled fragments, except near their confines, was always regarded as most enigmatical by those who adopted the opinion of Saussure, that they were all whirled along by a rapid current of muddy water rus.h.i.+ng from the Alps.

M. Charpentier first suggested, as before mentioned, that the Swiss glaciers once reached continuously to the Jura, and conveyed to them these erratics; but at the same time he conceived that the Alps were formerly higher than now. M. Aga.s.siz, on the other hand, instead of introducing distinct and separate glaciers, imagines that the whole valley of Switzerland was filled with ice, and that one great sheet of it extended from the Alps to the Jura, when the two chains were of the same height as now relatively to each other. Such an hypothesis labours under this difficulty, that the difference of alt.i.tude, when distributed over a s.p.a.ce of 50 miles, gives an inclination of no more than two degrees, or far less than that of any known glaciers. It has, however, since received the able support of Professor James Forbes, in his excellent work on the Alps, published in 1843.

In the theory which I formerly advanced, jointly with Mr. Darwin[143-B], it was suggested that the erratics may have been transferred by floating ice to the Jura, at the time when the greater part of that chain, and the whole of the Swiss valley to the south, was under the sea. At that period the Alps may have attained only half their present alt.i.tude, and may yet have const.i.tuted a chain as lofty as the Chilian Andes, which, in a lat.i.tude corresponding to Switzerland, now send down glaciers to the head of every sound, from which icebergs, covered with blocks of granite, are floated seaward.[144-A] Opposite that part of Chili where the glaciers abound is situated the island of Chiloe, 100 miles in length, with a breadth of 30 miles, running parallel to the continent. The channel which separates it from the main land is of considerable depth, and 25 miles broad. Parts of its surface, like the adjacent coast of Chili, are overspread with recent marine sh.e.l.ls, showing an upheaval of the land during a very modern period; and beneath these sh.e.l.ls is a boulder deposit, in which Mr. Darwin found large travelled blocks. One group of fragments were of granite, which had evidently come from the Andes, while in another place angular blocks of syenite were met with. Their arrangement may have been due to successive crops of icebergs issuing from different sounds, to the heads of which glaciers descend from the Andes. These icebergs, taking their departure year after year from distinct points, may have been stranded repeatedly, in equally distinct groups, in bays or creeks of Chiloe, and on islets off the coast, so as afterwards to appear, some on hills and others in valleys, when that country and the bed of the adjacent sea had been upheaved. A continuance in future of the elevatory movement, in the region of the Andes and of Chiloe, might cause the former chain to rival the Alps in alt.i.tude, and give to Chiloe a height equal to that of the Jura. The same rise might dry up the channel between Chiloe and the main land, so that it would then represent the great valley of Switzerland.

In the course of these changes, all parts of Chiloe and the intervening strait, having in their turn been a sea-sh.o.r.e, may have been polished and scratched by coast-ice, and by innumerable icebergs running aground and grating on the bottom.

If we apply this hypothesis to Switzerland and the Jura, we are by no means precluded from the supposition that, in proportion as the land acquired additional height, and the bed of the sea emerged, the Jura itself may have had its glaciers; and those existing in the Alps, which had at first extended to the sea, may, during some part of the period of upheaval, have been prolonged much farther into the valleys than now. At a later period, when the climate grew milder, these glaciers may have entirely disappeared from the Jura, and may have receded in the Alps to their present limits, leaving behind them in both districts those moraines which now attest the former extension of the ice.[144-B]

_Meteorites in drift._--Before concluding my remarks on the northern drift of the Old World, I shall refer to a fact recently announced, the discovery of a meteoric stone at a great depth in the alluvium of Northern Asia.

Erman, in his Archives of Russia for 1841 (p. 314.), cites a very circ.u.mstantial account drawn up by a Russian miner of the finding of a ma.s.s of meteoric iron in the auriferous alluvium of the Altai. Some small fragments of native iron were first met with in the gold-was.h.i.+ngs of Petropawlowsker in the Mra.s.sker Circle; but though they attracted attention, it was supposed that they must have been broken off from the tools of the workmen. At length, at the depth of 31 feet 5 inches from the surface, they dug out a piece of iron weighing 17-1/2 pounds, of a steel-grey colour, somewhat harder than ordinary iron, and, on a.n.a.lysing it, found it to consist of native iron, with a small proportion of nickel, as usual in meteoric stones. It was buried in the bottom of the deposit where the gravel rested on a flaggy limestone. Much brown iron ore, as well as gold, occurs in the same gravel, which appears to be part of that extensive auriferous formation in which the bones of the mammoth, the _Rhinoceros tichorhinus_, and other extinct quadrupeds abound. No sufficient data are supplied to enable us to determine whether it be of Post-Pliocene or Newer Pliocene date.

We ought not, I think, to feel surprise that we have not hitherto succeeded in detecting the signs of such aerolites in older rocks, for, besides their rarity in our own days, those which fell into the sea (and it is with marine strata that geologists have usually to deal), being chiefly composed of native iron, would rapidly enter into new chemical combinations, the water and mud being charged with chloride of sodium and other salts. We find that anchors, cannon, and other cast-iron implements which have been buried for a few hundred years off our English coast have decomposed in part or entirely, turning the sand and gravel which enclosed them into a conglomerate, cemented together by oxide of iron. In like manner meteoric iron, although its rusting would be somewhat checked by the alloy of nickel, could scarcely ever fail to decompose in the course of thousands of years, becoming oxide, sulphuret or carbonate of iron, and its origin being then no longer distinguishable. The greater the antiquity of rocks,--the oftener they have been heated and cooled, permeated by gases or by the waters of the sea, the atmosphere or mineral springs,--the smaller must be the chance of meeting with a ma.s.s of native iron unaltered; but the preservation of the ancient meteorite of the Altai, and the presence of nickel in these curious bodies, renders the recognition of them in deposits of remote periods less hopeless than we might have antic.i.p.ated.

FOOTNOTES:

[134-A] Geol. Trans. 2d series, vol. vi. p. 135. Mr. Smith of Jordanhill had arrived at similar conclusions as to climate from the sh.e.l.ls of the Scotch Pleistocene deposits.

[134-B] Proceedings of Geol. Soc. No. 63. p. 119.

[135-A] Travels in N. America, vol. ii. p. 141.

[135-B] Ibid. p. 99. chap. xix.

[136-A] Bulletin Soc. Geol. de France, tom. iv. 2de ser. p. 1121.

[138-A] See Travels in N. America, vol. i. chap. ii.

[140-A] Aga.s.siz, Etudes sur les Glaciers.

[143-A] Archiac, Hist. des Progres, &c. vol. ii. p. 249.

[143-B] See Elements of Geology, 2d ed. 1841.

[144-A] Darwin's Journal, p. 283.

[144-B] More recently Sir R. Murchison, having revisited the Alps, has declared his opinion that "the great granitic blocks of Mont Blanc were translated to the Jura when the intermediate country was under water."--Paper read to Geol. Soc. London, May 30, 1849.

CHAPTER XIII.

NEWER PLIOCENE STRATA AND CAVERN DEPOSITS.

Chronological cla.s.sification of Pleistocene formations, why difficult--Freshwater deposits in valley of Thames--In Norfolk cliffs--In Patagonia--Comparative longevity of species in the mammalia and testacea--Fluvio-marine crag of Norwich--Newer Pliocene strata of Sicily--Limestone of great thickness and elevation--Alternation of marine and volcanic formations--Proofs of slow acc.u.mulation--Great geographical changes in Sicily since the living fauna and flora began to exist--Osseous breccias and cavern deposits--Sicily--Kirkdale--Origin of stalact.i.te--Australian cave-breccias--Geographical relations.h.i.+p of the provinces of living vertebrata and those of the fossil species of the Pliocene periods--Extinct struthious birds of New Zealand--Teeth of fossil quadrupeds.

Having in the last chapter treated of the boulder formation and its a.s.sociated freshwater and marine strata as belonging chiefly to the close of the Newer Pliocene period, we may now proceed to other deposits of the same or nearly the same age. It should, however, be stated that it is difficult to draw the line of separation between these modern formations, especially when we are called upon to compare deposits of marine and freshwater origin, or these again with the ossiferous contents of caverns.

If as often as the carca.s.ses of quadrupeds were buried in alluvium during floods, or mired in swamps, or imbedded in lacustrine strata, a stream of lava had descended and preserved the alluvial or freshwater deposits, as frequently happened in Auvergne (see above, p. 80,), keeping them free from intermixture with strata subsequently formed, then indeed the task of arranging chronologically the whole series of mammaliferous formations might have been easy, even though many species were common to several successive groups. But when there have been oscillations in the levels of the land, accompanied by the widening and deepening of valleys at more than one period,--when the same surface has sometimes been submerged beneath the sea, after supporting forests and land quadrupeds, and then raised again, and subject during each change of level to sedimentary deposition and partial denudation,--and when the drifting of ice by marine currents or by rivers, during an epoch of intense cold, has for a season interfered with the ordinary mode of transport, or with the geographical range of species, we cannot hope speedily to extricate ourselves from the confusion in which the cla.s.sification of these Pleistocene formations is involved.

At several points in the valley of the Thames, remnants of ancient fluviatile deposits occur, which may differ considerably in age, although the imbedded land and freshwater sh.e.l.ls in each are of recent species. At Brentford, for example, the bones of the Siberian Mammoth, or _Elephas primigenius_, and the _Rhinoceros tichorhinus_, both of them quadrupeds of which the flesh and hair have been found preserved in the frozen soil of Siberia, occur abundantly, with the bones of an hippopotamus, aurochs, short-horned ox, red deer, rein-deer, and great cave-tiger or lion.[147-A] A similar group has been found fossil at Maidstone, in Kent, and other places, agreeing in general specifically with the fossil bones detected in the caverns of England. When we see the existing rein-deer and an extinct hippopotamus in the same fluviatile loam, we are tempted to indulge our imaginations in speculating on the climatal conditions which could have enabled these genera to co-exist in the same region. Wherever there is a continuity of land from polar to temperate and equatorial regions, there will always be points where the southern limit of an arctic species meets the northern range of a southern species; and if one or both have migratory habits, like the Bengal tiger, the American bison, the musk ox, and others, they may each penetrate mutually far into the respective provinces of the other. There may also have been several oscillations of temperature during the periods which immediately preceded and followed the more intense cold of the glacial epoch.

The strata bordering the left bank of the Thames at Grays Thurrock, in Ess.e.x, are probably of older date than those of Brentford, although the a.s.sociated land and freshwater sh.e.l.ls are nearly all, if not all, identical with species now living. Three of the sh.e.l.ls, however, are no longer inhabitants of Great Britain; namely, _Paludina marginata_ (fig.

112. p. 127.), now living in France; _Unio littoralis_ (fig. 29. p.