351 Mr Darwiii uti the Amicnt Glaciers of Caernarvonshire, 



the glacier during the last stage of its existence. At this pe- 

 riod, a small and distinct glacier descended from a narrow but 

 lofty gorge on the north-western end of the lake ; and here* 

 remnants of a terminal moraine may be traced in the little 

 mounds, forming a broken semicircle round a rushy plain, 

 scarcely more than a hundred yards in diameter. The rocks are 

 smooth, mammillatcd, and scored, all round the lake, and at 

 some little depth beneath the surface of the water, as I could 

 both see and feel. Similar marks occur at great heights on 

 all sides, far above the limits of the moraines just described, and 

 were produced at the time when the ice poured in a vast 

 stream over the rocky barrier bounding the northern end of 

 the amphitheatre of Lake Idwell. I may here mention, that 

 about eighty yards west of the spot where the river escapes 

 from the lake, through a low mound of detritus, probably once 

 a terminal moraine, there is an example of a boulder broken, 

 as described by Charpentier and Agassiz, into pieces, from 

 falling through a crevice in the ice. The boulder now consists 

 of four great tabular masses, two of which rest on their edges, 

 and two have partly fallen over against a neighbouring- 

 boulder. From the distance, though small in itself, at which 

 the four pieces are separated from each other, they must have 

 been pitched into their present position with great force ; and 

 as the two upright thin tabular pieces are placed transversely to 

 the gentle slope on which they stand, it is scarcely possible to 

 conceive that they could have been rolled down from the 

 mountain behind them ; one is led, therefore, to conclude 

 that they were dropped nearly vertically from a height into 

 their present places. 



The rocky and steep barrier over which the ice from the 

 amphitheatre of Lake Idwell flowed into the valley of Nant 

 Francon, presents from its summit to its very foot (between 

 400 and 500 feet), the most striking examples of boss or dome 

 formed rocks ; so much so, that they might have served as 

 models for some of the plates in Agassiz' work on Glaciers. 

 When two of the bosses stand near and are separated only by 

 a little gorge, their steep rounded sides are generally distinct- 

 ly scored with lines, slightly dipping towards the great valley 

 in front. The summit of the bosses is comparatively seldom 



