It may be remarked that this theory, provided the water be assumed to have been salt or brackish, demands quite as great an oscillation in the level of the land as that on which Charpentier had speculated, the only difference being that the one hypothesis requires us to begin with a subsidence of 2500 or 3000 feet, and the other with an elevation to the same amount. We should also remember that the crests or watersheds of the Alps and Jura are about 80 miles apart, and if once we suppose them to have been in movement during the glacial period it is very probable that the movements at such a distance may not have been strictly uniform. If so the Alps may have been relatively somewhat higher, which would have greatly facilitated the extension of Alpine glaciers to the flanks of the less elevated chain.
Five years before the publication of the memoir last mentioned, M. Guyot had brought forward a great body of new facts in support of the original doctrine of Charpentier, that the Alpine glaciers once reached as far as the Jura and that they had deposited thereon a portion of their moraines.*
(* "Bulletin de la Societe des Sciences Naturelles de
Neufchatel" 1845.)
The scope of his observations and argument was laid with great clearness before the British public in 1852 by Mr. Charles Maclaren, who had himself visited Switzerland for the sake of forming an independent opinion on a theoretical question of so much interest and on which so many eminent men of science had come to such opposite conclusions.*
(* "Edinburgh New Philosophical Magazine" October 1852.)
M. Guyot had endeavoured to show that the Alpine erratics, instead of being scattered at random over the Jura and the great plain of Switzerland, are arranged in a certain determinate order strictly analogous to that which ought to prevail if they had once constituted the lateral, medial, and terminal moraines of great glaciers. The rocks chiefly relied on as evidence of this distribution consist of three varieties of granite, besides gneiss, chlorite-slate, euphotide, serpentine, and a peculiar kind of conglomerate, all of them foreign alike to the great Strath between the Alps and Jura and to the structure of the Jura itself. In these two regions limestones, sandstones, and clays of the Secondary and Tertiary formations alone crop out at the surface, so that the travelled fragments of Alpine origin can easily be distinguished and in some cases the precise localities pointed out from whence they must have come.
(FIGURE 42. MAP SHOWING THE SUPPOSED COURSE OF THE ANCIENT AND NOW
EXTINCT GLACIER OF THE RHONE, AND THE DISTRIBUTION OF THE ERRATIC
BLOCKS AND DRIFT CONVEYED BY IT TO THE GREAT VALLEY OF SWITZERLAND
AND THE JURA.)
The accompanying map or diagram (Figure 42) slightly altered from one given by Mr. Maclaren will enable the reader more fully to appreciate the line of argument relied on by M. Guyot. The dotted area is that over which the Alpine fragments were spread by the supposed extinct glacier of the Rhone. The site of the present reduced glacier of that name is shown at A. From that point the boulders may first be traced to B, or Martigny, where the valley takes an abrupt turn at right angles to its former course. Here the blocks belonging to the right side of the river or derived from c d e have not crossed over to the left side at B, as they should have done had they been transported by floating ice, but continue to keep to the side to which they belonged, assuming that they once formed part of a right lateral moraine of a great extinct glacier. That glacier, after arriving at the lower end of the long narrow valley of the upper Rhone at F, filled the Lake of Geneva, F, I, with ice. From F, as from a great vomitory, it then radiated in all directions bearing along with it the moraines with which it was loaded and spreading them out on all sides over the great plain. But the principal icy mass moved straight onwards in a direct line towards the hill of Chasseron, G (precisely opposite F), where the Alpine erratics attain their maximum of height on the Jura, that is to say 2015 English feet above the level of the Lake of Neufchatel or 3450 feet above the sea. The granite blocks which have ascended to this eminence G came from the east shoulder of Mont Blanc h, having travelled in the direction B, F, G.