182 Mr. Darwin on the Ancient Glaciers of Caernarvonshire, 



to the different degrees of hardness of the laminae, smooth and 

 gentle furrows have been produced by the grinding of the ice, 

 transversely to the scores, and to the probable course of the 

 glacier. Here, as well as in some few other places, I noticed 

 an appearance which made it vividly clear that these bosses 

 had been formed by some process quite different from ordinary 

 aqueous or atmospheric erosion ; it is the abrupt projection 

 from the smooth surface of a boss of a piece of rock a few yards 

 square, and one or two feet in height, with its surface smoothed 

 and scored like the boss on which it stands, but with its sides 

 jagged : if a statuary were to cut a small figure out of a larger 

 one, the abrupt projecting portions, before he quite completed 

 his work, might be compared to these masses of rock : how it 

 comes that the glacier, in grinding down a boss to a smaller 

 size, should ever leave a small portion apparently untouched, 

 I do not understand. 



On the summit of some of the bosses on this barrier there 

 are perched boulders : but this phenomenon is seen far more 

 strikingly close to Capel-Curig, where almost every dome of 

 rock south of the Inn is surmounted by one or more large 

 angular masses of foreign rock. The contrast between the 

 rude form of these blocks, and the smooth mammillated domes 

 on which they rest, struck me as one of the most remarkable 

 effects produced by the passage of the glaciers. On the sides 

 of the mountains above Capel-Curig, I observed some bould- 

 ers left sticking on very narrow shelves of rocks, and other 

 boulders of vast size scattered in groups. The largest boulder 

 I noticed there was about 26 feet in length by 12 in breadth, 

 and buried to an unknown thickness. 



Proceeding down the great straight valley of Nant-Francon, 

 which must formerly have conveyed the united glaciers from 

 Lakes Idwell and Ogwyn, we continue to meet with boss- 

 formed rocks till below the village of Bethesda. From this 

 point towards Bangor these boss-formed rocks become rare ; 

 at least it is certain that a large number of hummocks of rock 

 with rugged surfaces project, whereas higher up in this valley, 

 and in all the great central valleys of Snowdonia, such un- 

 ground hummocks are not to be met with. At Bethesda, un- 

 stratified masses of whitish earth, from ten to forty feet in 

 thickness, full of boulders mostly rounded, but some angular, 

 from one to four feet square, are first met with. This deposit 

 is interesting from the boulders being deeply scored, like the 

 rocks in situ over which a glacier has passed. The scores are 

 sometimes irregular and crooked, but generally quite parallel, 

 as I distinctly saw over the entire side of one large block. 

 Some of the blocks were scored only on one side, others on 



