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 ahsolutely hare 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 theory 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. He seems to have been led to this 

 opinion, by a clear perception of the difficulty of supposing the existence 



