B. Mackintosh — Age of Floating Ice. 21 



angular stones, mainly felspathic. On the way to Caernarvon there 

 are thousands of large boulders, chiefly porphyry and felstone. 

 Between Caernarvon and Bangor the lower brown Boulder-clay, in 

 places passing into pinel, may be seen in the railway cuttings, and 

 a little blue clay under the brown may be detected near Port 

 Dinorwic. 



From Bangor to Marchhjn-maior. — Between these two places the 

 drift is chiefly the lower brown Boulder-clay, with an occasional 

 greyish tinge towards the surface. It varies from a loose stony and 

 gravelly clay to hard pinel, its general character, however, being 

 nearly the same as that of the lower brown clay of the coasts 

 of Anglesey, Cumberland, Lancashire, and Cheshire. It is generally 

 full of stones and boulders, consisting of directly or indii-ectly local 

 poiphyry, grit, etc. The boulders, though often much rounded at 

 the lower levels, are, in most places, seldom well glaciated. In the 

 valley near Pentir, and elsewhere, this clay is overlain by mounds 

 and plateaux of stratified sand and gravel (excepting where the 

 latter rest on rock), which were probably in part washed out of the 

 clay during the rise of the land,^ for though a great thickness of sedi- 

 ment can of course only be amassed on a subsiding sea-bottom, yet 

 such comparatively thin, and often sandbank-like, deposits as those 

 under notice would be more likely to be accumulated during emer- 

 gence. Away from the coasts of N.W. Wales I have nowhere seen 

 sand and gravel regularly and persistently interpolated between a 

 lower and upper Boulder-clay, but in the area under consideration 

 an upper red or foxy-coloured loam or clay may often be seen resting 

 on the brown Boulder-clay. This may be observed at the Turbary 

 W. of the Penrhyn slate quarries, where the lower clay attains a 

 great thickness. From the Turbary the two drifts slope smoothly 

 upwards to the small lake called Marchlyn-mawr, 2000 feet above 

 the sea-level. As Professor Eamsay has suggested, the lake-basin 

 was probably ground out by a glacier ; for — though at first it may 

 seem unlikely that a glacier which, on ascending, smooths a pro- 

 jecting boss of rock, leaving its lee-side jagged, should excavate 

 a rock-basin at the bottom of a previously-existing cwm — it must be 

 taken into consideration that in a cwm a short glacier would press 

 " downwards and outwards," that the constant melting of its fore 

 part by the sea would leave the entrance to the cwm comparatively 

 or entirely beyond the reach of the grinding ice, and that the 

 entrance in consequence would remain as a barrier to the lake. The 

 basin of Marchlyn-mawr (like several other lake-basins in North 

 Wales) aj^pears to me as if it had been formed while the land was 

 sinking, and as if it had been partly filled with marine drift, and 

 occupied (after the rise of the land) by a second small glacier, which 

 ploughed out the greater part of the drift and left the striking 

 moraine of angular loose blocks which rests on and conceals the 

 inner termination of the marine drift immediately in front of the 



1 According to Professor Ramsay, the sTiell-bearing' sand and gravel of North 

 "Wales was arranged while emerging, or during terrestrial oscillations of level {Old 

 Glaciers of North Wales, p. 95) . 



