and on Boulders transported by Floating Ice. 185 



nature and reddish colour. These beds closely resemble 

 those of Shropshire and Staffordshire, in which are found (as 

 I have myself observed in very many places) fragments of sea- 

 shells, and which every one, I believe, since the publication 

 of Mr. Murchison's chapters on the drift of these counties, 

 admits are of submarine origin. It may therefore be con- 

 cluded that the layers of coarse and argillaceous sand, and of 

 gravel, with far- transported pebbles and boulders, do not owe 

 their origin to an inundation, but were deposited when the 

 summit of Moel Tryfan stood submerged beneath the surface 

 of the sea. As there are no marks of the passage of glaciers 

 over this mountain (which indeed from its position could 

 hardly have happened), we must suppose that the boulders 

 were transported on floating ice ; and this accords with the 

 remote origin of some of the pebbles, and with the presence 

 of the sea-shells. Within the central valleys of Snowdonia, 

 the boulders appear to belong entirely to the rocks of the 

 country. May we not conjecture that the icebergs, grating 

 over the surface, and being lifted up and down by the tides, 

 shattered and pounded the soft slate-rocks, in the same man- 

 ner as they appear to have contorted the sedimentary beds of 

 the east coast of England (as shown by Mr. Lyell)*, and of 

 Tierra del Fuego ? Although I was unable to find any beds 

 on Moel Faban likely to preserve sea-shells, yet, considering 

 the absence of the marks of the passage of glaciers over it, I 

 cannot doubt that the boulders on its surface were transported 

 on floating ice. 



The drifting to and fro, and grounding of numerous icebergs 

 during long periods near successive uprising coast-lines, the 

 bottom being thus often stirred up and fragments of rock 

 dropped on it, will account for the sloping plain of unstratified 

 till, occasionally associated with beds of sand and gravel, which 

 fringes to the west and north the great Caernarvonshire 

 mountains. 



In a paper read before the Geological Society f, I have re- 

 marked that blocks of rock are transported by floating ice un- 

 der different conditions; 1st, by the freezing of the sea, in 

 countries where the climate does not favour the low descent 

 of glaciers ; 2nd, by the formation of icebergs by the descent 

 of glaciers into the sea, from mountains not very lofty, in la- 

 titudes (for instance in that of Geneva, or of the mouth of the 

 Loire, in the southern hemisphere) where the surface of the 



* " On the Boulder Formation of Eastern Norfolk ;" Phil. Mag., S. 3, 

 vol. xvi. May 1840, p. 351. 



t May 5th. 1841, " On the distribution of the Erratic Boulders, and on 

 the contemporaneous unstratified deposits of South America.'' [Phil. Mag. 

 S. 3, vol. xix. p. 536.] 



