154 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



ascended from the quarry to the top, where a space of ground had 

 been cleared of the soil and detritus, preparatory to further quarry- 

 ing. Here I was much struck by the polish and undulations of 

 the surface of the rock ; and a detail of my examination of these 

 phenomena will leave no doubt, I think, that they are attributable 

 to the action of glaciers. 



The slope of the mountain at Porth-Treiddyn is to the north 

 and north-east ; and the dip of the rocks is in the same direction, 

 at an inclination of from 17° to 22°, but the angle increases as you 

 ascend. 



At the spot which had been cleared, above the quarry, the de- 

 tritus lying on the surface of the rock was from five to twelve feet 

 thick. It consisted of various soils and gravel, of blocks of rock 

 similar to the flags, and of boulders of porphyry and greenstone, 

 in some of which the felspar crystals were very distinct. Some 

 of these boulders were from three to four feet long, and from two 

 to three feet in diameter.* 



It is here, especially, that we have presented to us a perfect 

 type of the glacier action. The surface of the uncovered portion 

 of the rock, when it has not been disturbed by the workmen's 

 tools, is rounded and polished in the most extraordinary manner. 

 The surface is furrowed ; and the furrows, where the rock is 

 uneven, are from 1 to 2 feet deep, with their edges beautifully 

 rounded off. On the broader slopes, strias are very distinct. With 

 a few exceptions, which I shall presently take occasion to notice, 

 the furrows, striae, scoops, grooves, and undulations, all shape their 

 course, not in the direction which water would take, that is to say, 

 in the direction in which the mountain slopes, northwards ; but in 

 a diagonal or slanting direction, towards the valley of the Glas- 

 llyn, that is to say, towards the east or north-east. 



There are a few channels, from 1 to 2 feet deep, which run 

 in a north-westerly direction ; these channels appear to have 

 arisen from the form of the rock compelling the superincumbent 

 glacier, with its included blocks, to move that way, until the 

 obstacle was overcome, when the moving mass resumed its original 

 course. 



With this example of glacier action for our guide, other places 

 where, from lapse of time combined with other causes, the 

 traces of that action have become somewhat obscure, and might 

 consequently appear doubtful, will be regarded as affording clear 

 evidence that there also the same cause has been in operation. 



The same phenomena are traceable from Porth-Treiddyn higher 

 up the mountain, to the very top, which is rather more than half 

 a mile from the spot where these effects are first visible. In 

 cutting the new road to the quarry, many rocks have been ex- 

 posed, which show, by their polished faces, that the whole side of 

 the mountain has been acted on. 



* On the southern side of this mountain, and very much below the level of 

 the quarry, may be seen broken and but seldom rounded masses of these rocks, 

 in which felspar, hornblende, and shorl are disseminated. 



