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line is about 8000 feet, or 6000 below the summit). On Kenya formerly this line should have been not far from 13,500 feet above the sea, and its present level must be about 15,000 feet; a difference which roughly corresponds with a lowering of temperature amounting to 5°1

Other instances might be given, but as the results would be similar, it is hardly necessary to cite them. They seem to justify the following inferences :

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(1.) That in districts where the present temperature is abnormally high-as on the western coast of Britain—a fall of not less than 20° may be sometimes requisite, but that in more normal regions (in the Northern Hemisphere) one of from 12° to 15° is generally sufficient. (2.) That in the Southern Hemisphere a less reduction is generally demanded. (3.) That in some of the instances which have been quoted the reduction in temperature cannot have exceeded (supposing other conditions unchanged) the estimates given above, because in such case glaciers would have been produced larger than those which the moraines and other traces prove to have existed. From this it follows that districts where the glaciers have never been other than small are better guides in any attempt to estimate the temperature of their

1 Dr. J. W. Gregory, the explorer of Kenya, is, however, more favourable to the idea that the refrigeration was produced by an uprising of the region, and not by any general climatal change. Quart. Jour. Geol. Soc., 1. (1894), p. 526. Perhaps the estimate made of the change of temperature given above may prove to be slightly too small.

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epoch than districts where the glaciers have reached a great size, because it is difficult to connect the enlargement of the snow region with the length of the glaciers proceeding from it. (4.) That the temperature seems to have been lowered to a greater or less extent in all parts of the earth, though whether this occurred in strict contemporaneity cannot be ascertained, and the remark possibly may not be true of equatorial regions.

Some authorities maintain that the Glacial Epoch was interrupted by one or more intervals of considerable duration when the climate became comparatively mild. In many places masses of till and boulder clay are parted, as already described, by thick and widespread beds of sand and gravel; in some the latter are associated with laminated clays, peats, and lignites. Such intercalations have been observed in Scotland,1 and still more markedly in the neighbourhood of the Alps; for instance, at Utznach and Dürnten, on the northern side of the Lake of Zürich. At the former place the distinctly stratified deposits are nearly 100 feet thick, and rest upon Miocene rock; at the latter, where the lignite alone is from 5 to 12 feet in thickness, these deposits are underlain by a stony clay, which some consider to be morainic in origin, though this is not universally admitted. At Wetzikon, however,

1 See Professor J. Geikie's "Great Ice Age," chap. xi.

2 The gravels associated with the lignites are probably about the same age as those described on p. 32.

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near Lake Pfäffikon, in the same district, where the lignitic beds, according to Professor Heer, range from 13 to 30 feet, striated stones and erratics occur in the clay below. In all three places the overlying deposits are morainic. Somewhat similar lignites have been found in other parts of Switzerland, in Savoy, and on the south side of the Alps, as at Carignano, Lanzo, &c.1 With these occur remains (among them bones of Elephas antiquus) indicative of a less severe type of climate; and the remarkable breccia at Hötting, in the valley of the Inn, contains the leaves, &c., of palms. The age, however, of this last deposit is not beyond dispute. That the glaciers were liable to important oscillations seems to be proved, but whether the evidence suffices to establish interglacial epochs, in the usual sense of the words, is more doubtful. When the snow-fields, as in the Alps, were much more extensive than they are at present, the glaciers which radiated from them would be more sensitive to minor climatal changes. Even now they oscillate considerably. But during But during a Glacial Epoch, an inch, either more or less, of precipitation might mean a considerable advance or retreat of the ice in the lowlands. Moreover, if the changes of level during this epoch amounted, as many geologists maintain, sometimes to 1500 feet, or even more, this alone would

1 See for others Sir H. Howorth, "The Glacial Nightmare," p. 467. 2 Their movements seem to exhibit a certain periodicity, the duration apparently being from about thirty to forty years.

make a great difference in the size of the glaciers.1 In Britain, where there are no high mountains, such a subsidence, as already indicated, would probably exterminate every glacier, and might make all the difference between an ice-sheet and a snow-capped group of hilly islands; but the occasional erratics, and the intercalated patches of boulder clay in the sandy beds, mentioned above, indicate (on this hypothesis) the presence of considerable masses of floating ice, so that the advocates of a submergence cannot allow of any marked improvement in the climate. Indeed, if they are right in supposing the movement downwards to have been so great, the time of maximum depression may even have corresponded with that of the most intense cold. Hence the question of the occurrence of interglacial epochs, or of prolonged interruptions to the continuity of the one great Glacial Epoch, is more dependent on that of the level of the land than might at first sight be supposed, and cannot be ascertained, so far as concerns Britain, until that question is settled, or until the relation already mentioned that between the area of the feeding-ground and the length of a glacier-is more accurately known.

1 A general subsidence of 2000 feet in the Alpine region, as it now exists, would raise the snow-line to the present level of 10,000 feet, and almost, if not entirely, efface the glaciers in such a region as the Graians. The peak of the Grivola, in all probability, would look down upon two or three shrunken glaciers of "the second order," like those which now nestle in the heads of the glens around the craggy summit of Mount Emilius.

CHAPTER II

POSSIBLE CAUSES OF A GLACIAL EPOCH

ASSUMING a low temperature to have prevailed generally in the Northern Hemisphere during the Glacial Epoch, as has been described in preceding chapters, and the like to have happened, though not of necessity at precisely the same time, in the Southern Hemisphere, what explanation can be given of these changes of climate? Some have suggested that the solar system in its journey through space traverses regions of varying temperature. These, however, have only a hypothetical existence, so that we may pass on to explanations which appear more worthy of serious consideration. Of them one group seeks the cause on the earth itself, the other in its position relative to the sun.

As regards the first, a glance at a chart representing the annual isotherms shows that they depart widely from the parallels of latitude, especially in the temperate and circumpolar regions of the Northern Hemisphere. The line of 32° F.1 in Central North

Here, as elsewhere, unless it is expressly stated, the annual isotherm is to be understood.

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