356 Mr Darwin on (he Ancient Glaciers of Oaernarvonshire^ 



of Snowdonia, such ungi'ound hummocks are not to be met 

 with. At Bethesda, unstratified 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 

 iu'st met with. This deposit is interesting from the boulders 

 being deeply scored, like the rocks in situ over which a gla- 

 cier 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 two sides, but from the 

 difficulty of turning over the larger ones, I do not know which 

 case is most common. I saw one large block on which the 

 scores on the opposite sides were all parallel ; and another 

 irregularly conical, four feet in length, of which three-fourths 

 of the circumference was marked with parallel striae, converg- 

 ing towards the apex. In the smaller elongated blocks, from 

 six to twelve inches in diameter, I observed that the stria? 

 were generally, if not always, parallel to their longer axis, 

 which shews that, Avhen subjected to the abrading force, they 

 arranged themselves in lines of least resistance ; but of three 

 large blocks which remained imbedded in a perpendicular 

 cliff, the vertical sides of two were scored in horizontal lines, 

 and of the third in an oblique direction. These several facts, 

 especially tlie parallel striae on the upper and lower surfaces, 

 shew that the boulders were not scored on the spot where they 

 are now imbedded, as seems to have been the case with the 

 boulders described by Mr Maclaren,* in the till near Edin- 

 burgh. The contrast is very striking in the state of the sur- 

 face of these boulders, and those which lie scattered high up 

 on the sides of the adjoining hills and of the great central 

 valleys, or are perched on the worn bosses of naked rock ; 

 such boulders, as I particularly noticed, present no signs of 

 scores or stria?, as might have been anticipated, if, as is siqi- 

 posed, they were transported on the surface of the glaciers. 

 In the quarries which I examined, namely, below Bethesda, 

 and at some little height on the eastern side of the village, 

 the till rested on slate-rocks, not worn into bosses. 1 found, 

 m mvKf 



,* Geology of Fife and the Lothians, p. 212. 



