1867.] Ice Marks in North Wales. 39 



on rock-surfaces are almost straight lines parallel to each other, and 

 show the direction in which the glacier moved. Nothing is more 

 striking than to trace for the first time over miles of country these 

 mysterious lines, ruled upon the hardest rocks, and always pointing 

 in the same direction. In the neighbourhood of Llanberis they are 

 so abundant, that it seems strange they were not observed, compared, 

 and speculated on long before their true nature was known. The 

 lines vary from fine scratches to grooves in which one's finger may 

 he, and even to troughs a foot or more in diameter. Sometimes 

 on very hard rock the grooves are polished by the intense pressure 

 of a hard smooth pebble. On the east side of Llyn y gader is an 

 even slope of near a thousand feet at an angle of about 45° called 

 the Fox's Path, and covered with loose fragments of rock which roll 

 away under one's feet at every step. It descends from a saddle 

 between two eminences of Cader Idris, and was probably long the 

 path of avalanches or small glaciers. In the bed of a torrent which 

 descends this slope I found, recently exposed, a large piece of 

 yellowish porphyry, one surface of which (about 5 in. by 3 in.) was 

 slightly ridged and furrowed, and highly polished. The rock is 

 very heavy and excessively hard, and this fragment is of itself a 

 striking proof both of the presence of ice and of its power as a 

 grinding and polishing agent. I presume that the piece formed 

 part of the rocky bed over which the ice once slid, that it had been 

 split and loosened by atmospheric action and then covered up and 

 preserved by sediment and stones, till the torrent exposed it again, 

 and would soon have destroyed its polished surface had I not been 

 lucky enough to hit upon it. 



But it is not only the surface of rocks in situ that are thus 

 marked. The pebbles, boulders, and fragments embedded in the 

 glacier are themselves equally scratched, but as they are capable of 

 shifting their position the grooves and striae on them are not always 

 parallel to each other. It is this kind of material that contributes 

 largely to form the drift, and in some localities almost every boulder 

 and pebble is more or less marked. On close examination we can 

 often find proof that the grooves are really ancient markings by 

 their correspondence in appearance with old surfaces of the stone, 

 although this is sufficiently evident to anyone who sees their number, 

 and the variety of rude masses which bear them. These various 

 classes of markings are all found abundantly wherever glaciers 

 now exist, and as no other mode of explaining their occurrence has 

 ever been suggested, they may be considered to form the best and 

 most convincing of all the various proofs of the former existence of 

 glaciers and icebergs in places where they are not now found. 



5th. Boulders and Perched Blocks. — As a glacier in its 

 passage down a valley covers many irregularities of ground, some- 



