Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

ADDENDA. 621 nels between the Alpine ranges, and not dropped in the intervening spaces. If any pointed rock came so near the surface that a float- ing mass of ice, thus charged, grounded on it, the block would, when the ice melted, be there left. But it may be asked, would the blocks usually be deposited on the bare surface of the rocky bottom off the shore, or on an intervening layer of gravel or sediment ? From what I have observed when passing in boats through the channels of Tierra del Fuego, and from frequent examinations of the armings of the lead used in sounding, I feel nearly sure that absolutely bare submarine rock is not very common. Moreover, where matter is depositing near a shore, the finer the particles are, the further they are drifted : in approaching a coast I have actually traced every step in the series, from the finest sand to large pebbles. But as the land in any case is slowly elevated, the same forces which carried the large pebbles to a certain distance from the beach, and the smaller ones to a still further distance, will, after each little elevation, carry them somewhat further : — a layer of little pebbles thus covering the sand, and a layer of large pebbles the smaller ones. Hence, when the part near the shore is converted into dry land, a section of the bed which was origi- nally the bottom of the sea will necessarily show solid rock covered by sand, this by fine pebbles, and these again by others, gradually increasing in size. Such then, I conclude, must have been the nature of the sub- littoral deposits of the Alps, during their assumed slow elevation. Finally, as icebergs of large size would seldom be driven up on the beach of a sheet of water, if, like the channel between the Jura and the Alps, it were pro- tected from the open sea, any fragments of rock transported by them would have been dropped some way outside, and therefore when upraised with the whole country, they would be found in most cases reposing on beds (where the loose matter had not been subsequently removed), charac- terized by the order of superposition just described. Such is the explanation I would suggest of the very curious facts observed by M. Agassiz. I make no assumptions which are not sup- ported by strong analogies and the foundation of the theory — namely, a change of climate of a peculiar kind — can be shown by reasoning, inde- pendent of the existence of erratic blocks, to be probable in a high degree : whether this is the case with the theo'y of M. Agassiz, I leave the reader to decide. Having said thus much on the scratched rocks of the Alps, I am tempted to make a few remarks on those of Scotland, described by Sir James Hall* in his celebrated paper {Edinburgh Phil- Transact., vol. vii.) * Sir James Hall believes that erratic boulders were transported by debacles, when embedded in ice. lie seems to have been led to this opinion, by a clear perception of the difficulty of supposing the existence