150 EKEATICS OF THE JUEA. [Ch. XII 



In the theory which I formerly advanced, jointly with Mr. Darwin,* 

 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 nnder the sea. At that 

 period the Alps maj^ have attained only half their present altitude, and 

 may yet have constituted a chain as lofty as the Chilian Andes, which, 

 in a latitude corresponding to Switzerland, now send down glaciers to 

 the head of every sound, from which icebergs, covered with blocks of 

 granite, are floated seaward.f 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 

 oversjDread with recent marine shells, showing an upheaval of the land 

 during a very modern period ; and beneath these shells 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, tabng 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 that the stones trans- 

 ported by them might hereafter appear, some on hills and others in valleys, 

 should that country and the bed of the adjacent sea be ever 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 altitude, 

 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-shore, may have been polished and scratched by 

 coast-ice, and by innumerable icebergs running agTOund 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 ol 

 upheaval, have been prolonged much farther into the valleys than now. 

 At a later period, when the chmate 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 

 jnoraines which now attest the greater extension of the ice in former times.J 



* See Elements of Geology, 2d ed. 1841. -j- Darwin's Journal, p. 283. 



:{: More recently Sir R. Murchison, having revisited the Alps, has declared hia 



