and the Boulders transported hy Floating Ice. 861 



beneath the level of the sea, inland creeks of salt water miist 

 have stretched far up or quite through these valleys, and where 

 they were deep, the glaciers (as at present in Spitzbergen), 

 would have extended, floating on the surface of the water, ready 

 to become detached in large portions. From the presence 

 of boss-formed rocks low down in the valley of Nant>Francon, 

 and on the shores of the lakes of Llanberis (310 feet above 

 the sea), it is evident that glaciers filled the valleys after the 

 land had risen to nearly its present height ; and these glaciers 

 must have swept the valleys clean of all the rubbish left by 

 the sea. As far as my very limited observations serve, I sus- 

 pect that boss or dome-formed rocks will serve as one of the best 

 criterions between the effects produced by the passage of 

 glaciers and of icebergs.t Dr Bucklandhas described, in de- 

 tail, the marks of the passage of glaciers along nearly the whole 

 course of the great central Welch valleys ; I observed that these 

 marks were evident at the height of some hundred feet on the 

 mountain-sides above the water-sheds, where the streams 

 flowing into the sea at Conway, Bangor, Caernarvon, and Tre- 

 madoc, divide : hence, it appears, that a person starting from 

 any one of these four places (or from some way up the valley 

 where the glacier ended), might formerly, without getting 

 off the ice, have come out at either of the other three places, 

 or low down in the valleys in which they stand. The moun- 

 tains at this period must have formed islands, separated from 

 each other by rivers of ice, and surrounded by the sea. The 

 thickness of the ice in several of the valleys has been great. 

 In the vale of Llanberis I ascended a very steep mountain, 

 E.NE. of the upper end of the upper lake, which slightly pro- 



* Dr Martens on the Glaciers of Spitzbergen, Edin. New Phil. Journ. 

 1041, vol. xxx.p. 2C8. 



t In the Appendix to my Journal of Researches (1839), I endeavoured to 

 ^hew that many of the appearances attributed to debacles, and to the move- 

 ments of glaciers on solid land, would, in all probability, be produced by the ac- 

 tion of stranded icebergs. I have stated (p. CI 9) on the authority of Dr Rich- 

 ardson, that the rocky bods of the rivers in North America which convey ice 

 are smoothed and polished ; and that (p.,C20) the icebergs on the Arctic shore 

 drive before them every pebble, and leave the submarine ledges of rock ab- 

 solutely bare. 



TOL. XXXIII. NO. LXVI. OCTOBER 1842. A a 



