CHAPTER XIV.

INFLUENCE OF A GENERAL INCREASE OF COLD.-MODIFICATIONS OF TEMPERATURE FROM CHANGES IN THE DISTRIBUTION OF LAND AND SEA.ERRATIC BLOCKS.-EFFECTS OF GRADUAL RISE OF THE SEA BOTTOM STREWED WITH ICE-TRANSPORTED DETRITUS.-EFFECTS OF A SUPPOSED DEPRESSION OF THE BRITISH ISLANDS.-INCREASE OF ALPINE GLACIERS. -TRANSPORT OF ERRATIC BLOCKS BY GLACIERS.-FORMER EXISTENCE OF GLACIERS IN BRITAIN.-ELEVATION OF BOULDERS BY COAST ICE DURING SUBMERGENCE OF LAND. ERRATIC BLOCKS OF THE ALPS.-ERRATIC BLOCKS OF NORTHERN EUROPE.-ERRATIC BLOCKS OF AMERICA.

THE geological effects now due to ice being as previously represented, it becomes desirable to consider those which would probably arise either from a general diminution of temperature on the surface of the globe, or from partial changes of that temperature. With respect to the first we have to look to some general cause common to the whole globe. Whatever the conditions for the distribution of temperature may have formerly been, we see that the influence of the sun now causes the heat of the tropics, and the different exposure of the polar parts of the earth's surface to it, the great variations of seasons there experienced. Any changes of sufficient importance, therefore, in the influence of the sun, which should produce a corresponding change on the face of the earth, so that the line of perpetual snow, as it is termed, should descend lower towards the sea in the equatorial, and cut its level at less high latitudes in the polar regions, would materially alter the climates of many parts of the world. Geological effects due to ice would be more widely spread than they now are, and the equatorial space within which ice-transported masses of rock and other detritus cannot be borne, would be more limited. Glaciers, where they could be formed, would not only become more extended than they now are in certain mountainous regions, but ranges of mountains, amid which they do not at present occur, the line of

perpetual snow not descending sufficiently low, would contain them; so that, in the one case, mineral matter would be distributed by them over a wider area; and in the other, over districts where no transport of the kind exists at the present time. Fragments angular, subangular, and rounded, would be distributed by riverice and coast-ice, where none such are now formed, and sea-bottoms would then be strewed over by them, where, at present, nothing of the kind is carried. Animal and vegetable life would be adjusted to the new conditions (that adapted to the colder climates of the earth moving more towards the equator), its remains, at least such as were preserved, spreading over those of the animals and plants which flourished in the same regions under higher temperatures.

The like general effects would be expected if, without supposing a diminished influence of the sun, our whole solar system, moving through space, should pass from the temperature now inferred to be that of the portion amid which that system takes its course (p. 206) to one less high. And it may well deserve the attention of the geologist to consider the effects which would follow such a change, even to the amount of a few degrees, as commonly measured by thermometers. In his observations on the distribution of masses of rock, apparently ice-borne to their present positions, and about to be noticed, it is very desirable that he should regard the subject generally as well as locally, so that, whatever may eventually appear the right inference to be drawn from the facts recorded, such as may bear upon the former should not be omitted in the search for the latter. As regards the evidence of many climates having remained much the same, with certain modifications, during those comparatively few revolutions of our planet round the sun, of which we have any records, and from which we may infer that the climates generally of the surface of the globe have not suffered material alteration since the historical period, as it has been termed, the geological observer will soon perceive that he is forced to consider it as affording him very limited aid in his inquiries respecting the former climatal conditions of the earth.

The present different conditions as to the production of ice capable of transporting mineral matter, in the manner above noticed, in the northern and southern cold regions of the globe, are sufficient to prove that partial changes of great importance may arise from differences on the surface of the earth itself. Every-day experience in geological research will show the observer that he has to consider the surface of the earth to have been in an unquiet state from remote geological times to the present, and that while

he so often stands, amid stratified deposits, on ancient sea-bottoms now elevated to various altitudes above the ocean level, many a region shows that its area has more than once been beneath that level and above it. Thus, although a mass of land may now rise above the sea-level at the South Pole, separated by a broad band of ocean from other great masses of land to the northward, producing certain effects as regards the climate of that part of the globe, and the northern polar regions are otherwise circumstanced, it by no means follows that such has always been the case, even in more recent geological times. If we change the conditions of the two polar regions, a difference of results is obtained of an important geological character. Mr. Darwin has skilfully touched upon the effects which would follow such a modification of conditions, and which require to be borne in mind in researches of this kind.*

In like manner any elevation or depression of a considerable area of dry land, which should raise parts of it above, or lower others, now above, beneath the line of perpetual snow, would produce modifications in the transport of mineral matter which could be effected

*He transports, in imagination, parts of the southern region to a corresponding latitude in the north. "On this supposition," he observes," in the southern provinces of France, magnificent forests, intwined by arborescent grasses, and the trees loaded with parasitical plants, would cover the face of the country. In the latitude of Mont Blanc, but on an island as far eastward as Central Siberia, tree-ferns and parasitical orchidea, would thrive amidst the thick woods. Even as far north as Central Denmark, humming birds might be seen fluttering about delicate flowers, and parrots feeding amidst the evergreen woods, with which the mountains would be clothed down to the water's edge. Nevertheless, the southern part of Scotland (only removed twice as far to the eastward) would present an island "almost wholly covered with everlasting snow, and having each bay terminated by ice-cliffs, from which great masses, yearly detached, would sometimes bear with them fragments of rock. This island would only boast of one land-bird, a little grass and moss; yet, in the same latitude, the sea might swarm with living creatures. A chain of mountains, which we will call the Cordillera, running north and south, through the Alps (but having an altitude much inferior to the latter), would connect them with the central part of Denmark. Along this whole line nearly every deep sound would end in 'bold and astonishing glaciers.' In the Alps themselves (with their altitude reduced by about half), we should find proofs of recent elevations, and occasionally terrible earthquakes would cause such masses of ice to be precipitated into the sea, that waves, tearing all before them, would heap together enormous fragments, and pile them up in the corner of the valleys. At other times, icebergs, charged with no inconsiderable blocks of granite, would be floated from the flanks of Mont Blanc, and then stranded in the outlying islands of the Jura. Who, then, will deny the possibility of these things having taken place in Europe during a former period, and under circumstances known to be different from the present, when, on merely looking to the other hemisphere, we see they are under the daily order of events?" Mr. Darwin then calls attention to the island groups, “situated in the latitude of the south part of Norway, and others in that of Ferroe. These, in the middle of summer, would be buried under snow, and surrounded by walls of ice, so that scarcely a living thing of any kind would be supported on the land."-Narrative of the Surveying Voyages of the Adventure and Beagle, vol. iii. p. 291.

by ice. If the region comprising the Alps were raised 3,000 feet above its present relative level, the area fitted for the formation of glaciers would be greatly extended, many a valley would be filled with ice, and many a mountain would contribute its glacier, not so filled or contributing at the present moment. Blocks and minor fragments of rocks would be ice-borne over, and left at distances from the main range not now attained; and, under the supposition of a gradual rise of land, many modifications would attend the change in the perpetual snow line, whence the glaciers for the time took their rise. Many a ravine and mountain side would be grooved and scratched not now touched by glaciers, and huge masses of rock be accumulated in heaps or lines, in localities where no ice now transports such masses. Assuming a depression of the same area, if we take the present relative levels only into consideration, the transport of glacier-borne blocks and fragments of rock, with the polishing, grooving, and scratching of valleys and their sides by the moving ice, would be limited to the areas now occupied by glaciers, duly allowing for their extension and contraction within the range of the present climatal conditions.

Thus, by the elevation and depression of large areas of dry land, very varied conditions for the existence, extension, or contraction of glaciers, with their geological consequences, may arise, without reference to those due to floating ice, excepting such as could be formed in great lakes, such as that of Geneva, for example, where effects similar to those observed in northern America would be produced. On the shores of such lakes coast ice would be formed, enclosing fragments of the rocks, and the shingles of beaches, to be borne away, should circumstances permit, if raised to an altitude permitting a depression of temperature sufficient for the production of such ice. There is also no difficulty in imagining conditions under which glaciers could protrude into large fresh-water lakes, carrying rock fragments with them, and having their extremities broken off and floated away with their detrital loads, under proper depths of water, as now takes place in the sea in the polar regions. Such masses of ice, though not moved onwards by streams of tide or ocean currents, would still be under the influence of the winds, to be driven to, and stranded in minor depths, where the ice could melt, and leave any blocks or fragments entangled in or resting upon them.

With respect to the distribution of ice-borne blocks of rock upon lakes, Sir Roderick Murchison has called attention to effects which would follow the lowering of lakes in regions where ice could be

formed of sufficient thickness and importance for the transport of detritus.*

When the depression of an area of dry land, with the needful modifications of surface, in climates where glaciers had been formed, was such that the sea entered amid the valleys in which these streams of ice occurred, the change might or might not, according to the general climatal conditions produced, affect the glaciers. Should the change in the northern be of an order to introduce the climate of the southern hemisphere, it has been above seen (p. 240), the cold might be so increased, that Alpine glaciers became more extended, delivering icebergs into surrounding seas, so that, as Mr. Darwin has remarked (note, p. 253), they might float away, and be stranded on the Jura, then an island range.

Hitherto we have regarded these alterations of level as slowly produced, so that the changes, of whatever kind, were gradual, causing no sudden alteration of conditions. This, however, is far from necessary in geological reasoning, there being evidence connected not only with actual mountain ranges, but also with many a district wherein the rocks are broken and contorted, which would lead us to infer, with every allowance for the repeated effects resulting from the multiplied application of minor forces, that considerable forces had often been somewhat suddenly called into action. The waves produced during the disturbances of the land, known to us as earthquakes, and which will be noticed hereafter, are sufficient to show how, in that mode alone, glaciers, protruding into the sea, or great lakes of fresh water, may be lifted at their ends, and their fragments, with any load of detritus they may sustain, be whirled about and stranded in unusual situations. Greater waves would produce greater results, and when we unite them with land suddenly depressed beneath the sea-level, even only a few hundred feet, in such regions as those of Victoria Land and South Georgia, or of Greenland and Iceland, we have the means of removing ice and producing a complicated mixture of blocks and minor fragments of rock of great geological importance. In like manner, the sudden elevation of land, covered by snow and glaciers, if accompanied by the transmission of heat through fissures then formed, or by the increased temperature of the supporting mineral matter from the protrusion of igneous rocks among it, so that the snow and ice were suddenly and in part melted, would be productive of no slight geological effect, more especially if the glaciers of the land so acted upon protruded, or nearly so, into the sea.

Geology of Russia in Europe and the Ural Mountains,” vol. i. p. 568.