42 Ice Marks in North Wales. [Jan., 



a little downward in the direction of least resistance, and this 

 alternate action, combined with the character of the rock, which is 

 here chiefly basaltic and splits into rude tabular and columnar 

 masses, seems sufficient to have produced that mass of blocks heaped 

 confusedly on the very summit of the mountain, which almost 

 always suggests to the mind of the non-geological visitor some 

 tremendous convulsion of nature, and makes him readily accept 

 the popular theory that the vast hollow of Llyn Cai is a volcanic 

 crater. 



In the ' Geological Magazine ' for September, 1866, Mr. Macin- 

 tosh maintains that the action of the sea has had most to do with 

 the formation of the valleys, cwms, and rocky surfaces of the Welsh 

 mountains; and he particularly instances Mynydd y Grader, a 

 rounded rocky mountain between Cader Idris and the town of 

 Dolgelly, as offering unmistakable evidence of a " seaworn summit." 

 I therefore devoted an afternoon to an examination of this moun- 

 tain, and was much surprised to find all over it what appeared to 

 me the most unmistakable evidences of " ice- work." The mountain 

 is composed of greenstone and lower Silurian flags, with veins and 

 masses of quartz. It is very rugged and uneven, consisting of 

 rounded lumps and knolls with numberless hollows and little 

 valleys between them. These are all more or less thickly covered 

 with angular blocks, slabs, and columnar masses, some standing on 

 the very summits of the knolls, others lying on steep slopes ; but 

 there is no arrangement of them in lines or layers, there are no 

 water-worn pebbles or boulders, no sand or shingle, nor, as far as I 

 could see, any sign whatever of the action of the sea. On the 

 other hand, the whole mountain offered the finest possible examples 

 of roclies moutonnees, the smooth slopes always facing Cader Idris, 

 from which the glacier had come. Some of these ice-ground sur- 

 faces were as smooth as a pavement although formed on the out- 

 cropping edges of the hard Silurian rocks, an effect which the sea 

 never produces. There is an angular block containing about twenty 

 cubic yards of stone, standing on the slope of one of the highest 

 bosses of the mountain, with no precipice from which it could have 

 come nearer than Cader Idris, more than a mile off, with a valley 

 between. Owing to the exposed situation of the roclies moutonuees, 

 their surfaces have been much weathered, and I did not succeed in 

 finding good groovings or scratches, though I have no doubt such 

 could be found by a more careful search. With this exception, the 

 evidences of recent glacial action are seldom to be seen more plainly 

 than upon this mountain. 



The phenomena of existing glaciers and icebergs have been 

 now so carefully studied, and the various effects which they produce 

 are for the most pari so well known, that there is no longer any 



