GLACIERS IN GREAT BRITAIN AND IRELAND. 137 



Ireland, aiid containing, as shown by Mr.Trimmerj* marine 

 shells, have been deposited at an elevation of 1392 feet on 

 Moel Tryfane. 



The observations of Dr. Buckland were followed by 

 those of Mr. Darwin,t and more recently by those of Pro- 

 fessor A. C. Ramsay. J This author has shown that many 

 of the tarnSj such as Llyn Llydaw and Llyu Idwal, have 

 been produced partly through the damming up of the 

 waters by moraines, as Agassiz had previously shown to 

 be the case in the Alps, and Lyell in Forfarshire. The 

 same author, in order to account for the fact that several 

 of the mountain tarns, as those near the summits of Cader 

 Idris, Moel Wynne and Snowdon, are in the form of 

 basins hollowed in solid rock, has suggested an explana- 

 tion which may be called "the scooping theory/^ These 

 tarns are generally surrounded through half their circum- 

 ference by precipitous walls of rock; and Professor Ramsay 

 supposes, that solid masses of ice, descending from these 

 heights, charged with imbedded fragments of rock, have 

 actually scooped these hollowSj which are so numerous in 

 all mountain districts. 



But there is one interesting fact brought out by Mr. 

 Ramsay, and which, according to my own observation, is 

 repeated amongst the valleys of the Lake district. Taking 

 the moraine of Llyn Idwal as one of several examples, he 

 shows that it is situated at about looo feet below the ele- 

 vation attained by the Northern Drift. Now if this moraine 

 had been formed previous to the deposition of this marine 

 deposit (which attains an elevation of 2300 feet), it would 

 most certainly have been entirely obliterated. It is there- 

 fore evident that moraines of this kind belong to a period 



* Mr. Trimmer, on the Diluvial Deposits of Caernarvonshire, Froceedings 

 of the Geological Society, vol. i. p. 331. 



t PMlosopMcal Magazine, vol. xxi. p. 180. 



X FeaTcs, Passes and Glaciers, 4th edition j and (Quarterly Journal of 

 the Geological Society, vol. viii. 



SBR. Ill, VOL, I, T 



