Vol. 65.] THE ORDOVICTAN AND SILURIAN ROCKS OP CONWAY. 179 



Area West of the Penmaenbach Fault. 



As noted long ago by Mr. Harker, 1 the ground west of the 

 Penmaenbach Fault does not seem to be occupied by intrusive 

 rock as the 1-inch map would indicate, but is mainly composed of 

 creamy rhyolites. They appear to be very similar in general 

 character to those forming the main mass of Conway Mountain, 

 although they are decidedly thicker and have more mudstones and 

 ■ash-bands intercalated amongst them. They strike north-north- 

 westwards, and it seems possible to recognize the two Upper 

 Modular Rhyolites overlain by the Siliceous Ashy Grits. Also, 

 though the mudstone is not well exposed naturally, it is obvious that 

 ■the reservoir-tank lying close to the Penmaenbach Fault has been 

 -excavated in a shaly mudstone-band ; and, from the heaps of material 

 lying round about, it was possible, after much searching, to collect 

 and identify a few specimens of Climacograptus scharenbergi, Lapw. 

 and CI. antiquus, Lapw. 



It does, however, appear that the main mass of Penmaenbach is 

 composed of a different rock, and the mapping suggests that this 

 may be filling a neck. This neck, if neck it be, is now filled with 

 a rock which has many of the characters of a rhyolite, although it 

 differs from the general mass of the rhyolitic lavas of the series. 

 It is most like one of the Lower Banded Rhyolites, but the ground- 

 mass is rather more coarsely crystalline, a fact which may well be 

 accounted for by its mode of occurrence. It is more distinctly 

 porphyritic near its edge, where there also appears to have been 

 greater silicification. 2 



The ground to the west of the Penmaenbach Fault is much 

 faulted, and the lavas are highly silicified, especially as they 

 approach the edge of the neck. 



(2) Cadnant or Dicranoffraptus-Shales. 



The best exposures of the Cadnant or Bicranograptus-Shales are 

 •near the town of Conway, in the district known as Cadnant Park ; 

 they used also to be well seen at various points near the old copper- 

 mines at Derwendeg, though these are now largely built over. 



The best section of all is displayed in the railway-cutting north- 

 west of Conway Station, where the entire thickness of the beds 



1 'Bala Volcanic Series of Caernarvonshire ' 1889, p. 22. 



2 All the lavas have undergone extensive silicification, and though in a 

 rock-slice it is often very difficult, if not impossible, to distinguish between 

 primary and secondary quartz, it is fairly obvious in the field that the 

 silicification is highly intensified in the neighbourhood of the larger faults. 

 The creamy rhyolites become almost like quartzites, and the nodular rhyolites 

 lose much of their conspicuously nodular character, the curved edges of the 

 ' shells ' being only discernible in places, and the rock having a very brecciated 

 appearance; as it is traced away from the fault-line, however, it gradually 

 returns to its normal character, only to lose it again as it approaches a second 

 fault. It seems, therefore, fairly clear that some of the alteration is due to 

 silicification along the fault-lines: though much of the alteration is charac- 

 teristic of the rocks as a whole, and is quite independent of any earth- 

 movements. 



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