PKE-CAMBKIAN EOCKS OF ST. DAVID'S. 275 



eruptive; for the .strata adjacent to it present examples of the 

 induration and silicification so commonly, though not universally, 

 observable along the borders of a granite boss. 



In describing the sections that exhibit the actual contact of the 

 eruptive and sedimentary rocks, I would first allude to a fact of 

 some importance which hitherto appears to have escaped notice. 

 In the course of my examination my colleague and I observed that 

 as it crosses the valley of the Allan above Porth-lisky, the granite 

 sends out a tongue-like projection across the river at the ford, and 

 that this projection is separated, by an intervening mass of Cambrian 

 shales and sandstones, from another projecting tongue of granite 

 lying further north. This northern portion may cross the river as 

 a narrow belt and thereby connect the main mass of Bryn-y-garn 

 with the lesser area that extends to Porth-lisky. As it contracts, 

 however, to a breadth of not more than eighty yards on the west 

 side of the Allan, there seems to be hardly any room for it to pass 

 across the valley. Though, no doubt, continuous underneath, the 

 granite mass is probably divided at the present surface into two 

 separate areas by intervening Cambrian strata. (See Map, Plate VIII. 

 p. 268.) 



The Bryn-y-garn granite mass projects for a few yards into the 

 Cambrian grits and shales on the right bank of the Allan. I had 

 several yards of the actual contact of the rocks laid bare at the foot of 

 the hill, and found that the granite distinctly overlies the grit (fig. 3.), 



Pig. 3. — Section of Junction of Granite tuitJi Cambrian Strata. 

 Right bank of Allan River, Porth-clais. 



GRAM1TC / COWCLOMFR^TE 



the line of junction being a wavy surface inclined at an angle of 

 about bb° *. The grits are much indurated ; and their bedding is 



did not penetrate any of these beds " (Quart. Journ. Geol. Soc. vol. xxxiii. 

 p, 229, 1877). I cannot conceive in what direction this further examination 

 was carried, nor how the very clear proofs of intrusion could have been missed 

 or misunderstood. 



* This junction was bared, with Mr. Topley's assistance, on the occasion of 

 my second visit. Some portions of this grit are so coarse as to pass into a 

 quartzose conglomerate, which may be the conglomerate band above the vol- 

 canic group. This is about the place where that band should come in, next the 

 green and red beds seen at Porth-clais. Its position is suggested in Section 

 fig. 1, p. 268. 



