and on Boulders transported by Floating Ice, 359 



Moel Tryfan stood submerged beneath the surface of tlie 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 trans- 

 ported on floating ice ; and this accords with the remote ori- 

 gin 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 manner as they ap- 

 pear to have contorted the sedimentary beds of the east coast 

 of England (as shewn by Mr Lyell*) and of Terra 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 ice- 

 bergs 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 Caer- 

 narvonshire mountains. 



In a paper read before the Geological Society,! I have 

 remarked that blocks of rock are transported by floating ice 

 \mder diff*erent conditions ; 1^^, by the freezing of the sea, in 

 countries where the climate does not favour the descent of 

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

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

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

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

 sea never freezes ; and, 3c/, by these two agencies united 



* "On the Boulder-Formation of Eastern Norfolk," Phil. Mag. § 3, 

 vol. xvi. May 1840, p. 351. 



t May 5, 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). 



