358 Mr Darwin on the Ancient Glaciers of Caernarvonshire, 



Tryfaii (S. E. of Caernarvon), I ascended this hill. Its height 

 is 1192 feet* above the sea ; it is strewed with boulders of 

 foreign rock, most of them apparently from the neighbom-ing 

 mountains ; but near the summit I found the rounded chalk- 

 flints.t and small pieces of white granite, alluded to by Dr 

 Buckland. Its form is conical, and it stands isolated ; wher- 

 ever the bare rock protrudes its surface is jagged, and shews 

 no signs of being in any part worn into bosses. The contrast 

 between the superficial part of the bare rock on this hill and 

 on Moel Faban, with that of the rocks within the great cen- 

 tral valleys of Caernarvonshire, is very remarkable ; it is a con- 

 trast of precisely the same kind as may be observed in these 

 same valleys by ascending on either side above the reach of 

 the ancient glaciers. A little way down the hill, a bed two 

 or three feet in thickness, of broken fragments of slate mixed 

 with a few imperfectly rounded pebbles and boulders of many 

 kinds of rock, is seen in several places to rest on the slate, the 

 upper surface of which, to the depth of several feet, has been 

 disintegrated, shattered, and contorted in a very curious man- 

 ner. The laminated fragments, however, sometimes partially 

 retain their original position. 



I did not succeed in finding any fragments of shells, but 

 near the summit of the hill, on the eastern or inland side, I 

 found beds at least twenty feet in thickness of irregularly 

 stratified gravel and boulders, with distinct and quite defined 

 layers of coarse yellow sand, and others of a fine argillaceous 

 nature and reddish colour. These beds closely resemble those 

 of Shropshire and Stafibrdshire, 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 concluded, that 

 the layers of coarse and argillaceous sand, and of gra,vel, with 

 far transported pebbles and boulders, do not owe their origin 

 to an inundation, but were deposited when the summit of 



* Murchison's Silurian System, p. 528. 



t I may mention, that at Little Madely, in Staffordshire, I have found 

 chalk-flints in the gravel beds, associated with existing species of sea- 

 shells. 



