Part 35 (2/2)
[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 Lancas.h.i.+re 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 s.h.i.+ngles 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 Darbis.h.i.+re as occurring at an elevation of 1,100 to 1,200 feet above the level of the sea.[110]
[110] Mr. Darbis.h.i.+re 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 s.h.i.+ngle beds of Leyland, Euxton, Chorley, Preston, Lancaster, and Blackpool,[111] Mr. De Rance has obtained nearly thirty species.
[111] The typical species in West Lancas.h.i.+re are _Tellina Balthica_, _Cardium edule_, _C. aculeatum_, _C. rustic.u.m_, _Psammobia ferroensis_, _Turritella terebra_.
In Eastern Yorks.h.i.+re, 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 Yorks.h.i.+re 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, furnis.h.i.+ng 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 Ches.h.i.+re; 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 Yorks.h.i.+re, 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 Lancas.h.i.+re, 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 Lancas.h.i.+re. He has described the ordinary red Boulder Clay of Lancas.h.i.+re as extending continuously through Ches.h.i.+re and Staffords.h.i.+re into Warwicks.h.i.+re, gradually becoming less red and more chalky, everywhere overlying intermittent sheets of ”sands and s.h.i.+ngle-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 Yorks.h.i.+re. 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.
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