670 MR. C. A. MATLEY ON THE [Aug. 1 899, 



99-6 per cent, of silica ; but purity is not a sine qua non of a qnartz- 

 knob, for Prof. Blake includes among them, on account of their 

 peculiar stratigraphy, some which ' might well pass for grits.' 



Leaving Prof. "Watts to deal with their microscopic structures, 

 we may notice the manner of occurrence of some of these rocks in 

 the field. The mass at Llanbadrig Point has already been dealt 

 with (p. 653 & fig. 5). It is cut up by thrust-planes, parts of it are 

 well bedded, and it contains abundance of clastic grains. Another 

 quartzite-mass is shown in the Ogof Gynfor section (fig. 3, p. 648). 

 This latter is continued inland, by way of Penterfyn, as a bold, 

 sometimes interrupted ridge, with a branch to Is-allt. Another 

 ridge, overlain by Ordovician conglomerate, occurs between Ogof 

 Gynfor and Perth Llanlliana. 



Prof. Blake figures another quartz-knob at Perth Llanlliana,^ but 

 its structure is not so simple as a general view leads one to suppose. 

 At the base of the cliff it is a bedded and markedly clastic quartzite, 

 the bedding being almost flat. It is separated from the more 

 inclined mass above by a nearly horizontal plane which I take to be 

 a shear-plane, as along it may be found some pieces of impure 

 limestone and crushed shale. This plane is continued as a steep 

 upward curve, and we find a mass of impure limestone forming a 

 kind of sharp anticline in the heart of the kno>). Climbing the 

 steep slope to investigate it from the top of the cliff, we find that 

 the quartzite is not a typical ' knob/ but a reef or ridge along which 

 are a number of small disused quarries. The rock is clearly a 

 quartzite with abundant clastic grains, and it is overlain by a purple 

 conglomerate. 



The quartzite-ridge crops out again at Hell's Mouth and can be 

 followed with the overlying purple conglomerate almost unintei-- 

 ruptedly to Craig Wen, * the greatest quartz-knob of the district,' ^ 

 which is being rapidly removed to make silica-brick. Here the 

 outcrop widens. The north side of this hill gives the impression of 

 a set of thick beds dipping steadily enough, though with a slight 

 undulation, below the conglomerate; but the bedding- planes are 

 full of slickensided surfaces. Along the road on the west side are 

 faint indications of what seem to be southward-tilted anticlines, and 

 small patches and pockets of a pale, highly- crushed, quartzose shale 

 lie in the quartzite. A quarry driven into the eastern side of the 

 hill, in the general direction of the strike, shows that a thick shale is 

 included in the quartzite, and moreover that the rock has undergone 

 such movement that the shale forms a tortuous band around masses 

 of quartzite and has been so crushed in places as to be mylonitic. 

 The north side of this quarry is represented in fig. 12 (p. 671). 

 Purple and green are the prevailing colours of the shale. 



Descending into Perth Wen, we find the beds to be much disturbed. 

 By the pier is a conspicuous detached rock of the same quartzite 

 containing some thin crushed shale, and overlain by the samepurplr 

 conglomerate. The broken and confused rocks south of the pier 

 seem to be a continuation of the Craig Wen mass, and Prof. Blake 



' Quart. Journ. Geol. Soc. vol. xliv (1888) p. 520, fig. 22. 

 2 lUd. p. 521. 



I 



