ROCKS TN N.W. CAERNARVONSHIRE. 277 



igueous rock, than to the north, where it abuts against the sedi- 

 meutar}', and is thickest in the bays of the curve, probably indicating 

 that this curve is near the true line of deposition. The most critical 

 part of this line is at Gorsbach, vt^here an older conglomerate comes 

 in, which we are only prevented from mistaking for the Arenig by 

 carefully tracing the line on both sides and comparing the character 

 of the rock step by step, as is done below. 



We have thus marked off for our study a definite area of rocks, 

 of which as yet we only know that they are Pre- Arenig. This area 

 we must now examine. 



The southern portion, as marked off by Prof. Ponney*, is cer- 

 tainly distinct from the rest, but it loses its character as we pass 

 northwards. Thus, at Ty-coch the section shows a coarse-crys- 

 talled granite, only cracked and disturbed. As we pass into the 

 so-called " Crug beds" of Prof. Hughes, /. e. at Crug Parm, we find 

 a beautiful radiating granophyr, much cracked and filled with 

 quartz. In the ravine west of Tan-y-maes the rock is porphyritic, 

 with idiomorphic quartz-crystals, and a matrix partly micropeg- 

 matitic, partly mosaic ; and at the extreme tongue by Tafarn-y- 

 grisiau only the latter matrix is left, and the whole is very dirty. 

 All these have a family likeness, and may well be considered parts 

 of the same intrusive mass. In the side of the main road south of 

 Dinorwic, between the two branches of the road which goes over the 

 hill, is seen the rock into which we may suppose this porphyry 

 intrudes. It has a dirty unstratified aspect, like an indurated mud, 

 but really consists of irregular fragments of quartz, in a quartzose 

 but dusty matrix, much lined and infiltrated. Immediately we cross 

 Prof. Ponney's line and pass to the east over Dinorwic, the change 

 in the character of the rock, as seen under the microscope, is 

 marvellous and complete. Whether close above Dinorwic or at 

 Pant-yr-fallan-fach, on the eastern slope of the hills, or in the valley 

 to the east, or still further east near Tan-yr-allt, every rock is a 

 fresh quartz-porphyry, whose ground-mass is crowded with streaming 

 microliths, exhibiting fluxion-structure to perfection. The contrast 

 between these and the Caernarvon group of rocks is all the more 

 marked from the latter being in their most altered state and the 

 former in their freshest, where they are found side by side. The 

 subdivision, therefore, of the great mass of quartz-porphyry in 

 this district into two distinct parts, of different ages, we owe entirely 

 to the microscopic researches of Prof. Bonney. 



I do not think that these felsites all belong to one outburst, 

 whether as an intrusion or a flow ; first, because there are varieties 

 of the porphyry, some containing only scattered quartzes, others 

 crowded with quartz and felspar, and some, as in the ravine near 

 the " d" of the Tyn-y-coed, with a spherulitic matrix. Moreover, 

 as pointed out by Prof. Hughes, there are breccia-beds, though I 

 have only found one, running north-eastwards by Tan-yr-allt. 



Probably the most important point to determine with regard to 

 this porphyry is the nature and cause of the long tongue which is 

 ^ Quart. Journ, Geol. See. vol. xxxv. 1879. 



