ORIGIN OF THE ROOKS AND SCENERY OF NORTH WALES. 217 



pebbles of about tbe same size as tbose in the Bunter conglomerate. 

 The matrix of tbe rock is very much altered, and occasionally resembles 

 the quartz-felsite above described. To quote from notes made on the 

 spot : " The included fragments frequently attain the size of an egg, and 

 consist of well rounded pebbles of grey and liver coloured quartzite, a 

 cherty rock of greenish colour, a dark slate, a compact greyish felspathic 

 rock, a quartz-felsite similar to that already described, and a granitoid 

 rock composed mainly of pinkish felspar and quartz. The whole mass is so 

 oompact that the pebbles split as readily as the matrix." As regards the 

 origin of this rock, a glance is sufficient to convince one that the pebbles 

 have been formed by water action ; their unmistakeable outline renders 

 this certain. The conglomerate, in all probability, accumulated along the 

 shore line of a sea or lake in some former period of the world's history. 

 The pebbles found in the rock are relics of the old land surface of this 

 period, and from them we can learn something of its geological structure. 

 I would call your attention especially to three of the rocks found as 

 pebbles in the conglomerate : the quartzite, quartz-felsite, and the 

 granitoid rock, composed of quartz and felspar. 



The quartzite pebbles vary in colour, from liver-colour to grey, the 

 former closely resembling the characteristic pebbles of the Bunter. Under 

 the microscope the quartzite presents all the usual characters of this 

 rock, the grains are in many cases fairly rounded, thus showing that the 

 constituent grains of the pebble owe their form to the agency which has 

 shaped the pebble itself. The old pre-Cambrian land then consisted in 

 part of quartzite, or of a conglomeratic rock cotaining quartzite pebbles 

 derived from a still older continent. 



The next rock we notice is the quartz-felsite. The occurrence of 

 this rock as a pebble in the Cambrian conglomerate is most significant 

 when considered in connection with the fact that it occurs in situ in the 

 immediate neighbourhood as an old lava current. The presence of 

 conglomerate itself raises the presumption that we are not far from an 

 old shore line, while the occurrence of this particular rock, both in situ 

 and as a pebble in a later deposit, renders it almost certain that the rock 

 in situ formed a portion of the land surface of the period to which the 

 conglomerate belongs. This point I shall again refer to in dealing with 

 the former changes in the physical geography of North Wales. 



The third rock that we have to consider is the one composed of 

 quartz and felspar. It is especially interesting because, like the quartz- 

 felsite, it occurs in situ at no very great distance. It may be seen 

 at Caernarvon, only five or six miles away, and possibly it occurs even 

 nearer than this. 



In introducing the conglomerate, I mentioned that the size of the 

 included fragments is liable to considerable variation, and the rock, for 

 instance along the shores of Llyn Padarn, may be found almost shading 

 off into a grit, this again into a still finer grit, and this again into a slate. 

 I propose now to consider the origin of these grits and slates. 



The finer conglomerates and the coarser grits consist mainly of quartz 

 fragments embedded in a matrix of finer particles ; some of these frag- 



