26 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 19, 



equal to that of Snowdon. I think it very probable, however, that 

 the manner in which the blocks are distributed along the sides of 

 most of the valleys in which they abound, may be justly regarded as 

 indicative of a position of the surface of the ocean much higher than 

 at present, with reference to the surface of the land. On the south 

 side of Loch Etive from Tyanuilt to the sea, and on the hills about 

 Oban, the blocks range to the height of 300 or 400 feet, and possibly 

 higher, according to a rough estimate by the eye. The elevation at 

 which they are found also on the island of Kerrara is much the same. 

 Again, on the eastern side of Loch Fyne I have observed them at 

 apparently about the same altitude, and also on the eastern side of 

 the Cobler, above the shores of Loch Long. Whether we suppose 

 the blocks at these heights to have been deposited there by floating 

 ice or transported by water, after having been brought down in part 

 from their original sources by glaciers, it would appear probable, that 

 the surface of the sea stood at a relative elevation somewhat exceed- 

 ing that of the upper limit of the blocks, at least during the time of 

 the transport of those which are now placed near to that limit. 



That the depression of the southern Highlands has been, however, 

 much greater at some period than that here contemplated, is rendered 

 extremely probable by evidence aiforded by the adjoining districts. 

 The Till in these districts is not unfrequently found, I believe, at the 

 height of 500 or 600 feet, and in some cases at the height of 1000 or 

 1200 feet or upwards, above the present sea-level. These facts, 

 assuming the Till to have been of marine origin, prove the great de- 

 pression to which the lower lands of Scotland must have been sub- 

 jected ; and I know of no reason for supposing the Highlands not to 

 have been subjected to a similar depression. 



4. On the Modes of Transport of the Blocks. 



The three modes of transport, — by glaciers, by floating ice, and by 

 currents of water, are all, I conceive, now pretty generally recognized 

 by geologists as possible modes in which the transport of blocks may 

 have been actually efl'ected. The difficulty consists in distinguishmg 

 the effects produced by these agencies respectively. In attempting 

 to distinguish the action of glaciers, one inquiry is, the heights of the 

 sources to which the blocks can be traced ; for, if it should appear 

 sufficiently probable that those heights are greater than the relative 

 elevation which the sea attained during the period of dispersion, we 

 must attribute the first removal of the blocks from their original sites 

 to the action of glaciers. On the north-western side of Ben Cruachan 

 the granitic mass forms the bed of the upper portion of Loch Etive 

 from Tyanuilt for many miles, and rises to the summit of the moun- 

 tains on either side, so that there is no determinate elevation from 

 which the blocks of that locality must necessarily have proceeded. 

 The blocks of Loch Fyne also present an exactly similar case. In 

 other cases the blocks must have descended from at least a certain 

 height. The grey blocks on the shores of Loch Lomond, which have 

 had their origin in Ben Nime, must have descended from points 



