244 ME. C. A. MATLEY ON THE [May I9OO, 



(iii) Further Remarks on the Quartzites. — Some features 

 of the quartzites were discussed by Prof. Watts and myself in our 

 recent paper, but other points, hardly affecting the conclusions 

 arrived at, were deferred by me till the mapping had been completed. 

 These points are, their association with limestone, their variations 

 in thickness, and their relationship to the conglomerates. 



Association with Limestone. — In some localities the 

 quartzites are seen to include some limestone, not, however, of great 

 thickness. This may be seen east of Penterfyn, and at the eastern 

 end of the Llanlliana quartzite-ridge. At the latter locality the 

 limestone is composed of highly-contorted laminae. The limestone 

 enclosed in the quartzite at the western extremity of the same 

 ridge (see p. 670 of my former paper) would be perhaps more 

 accurately described as an interbedded limestone-band locally 

 crushed out. In Porth y Wylfa also a wedge of quartzite includes 

 a thin limestone. Sections taken from some of these limestones 

 show them to be of very fine grain, veined with calcite and some- 

 times showing siliciflcation [N. A. 123], and the rock between the 

 limestone and the quartzite is in one example [N. A. 130 ; cliffs 

 above Hell's Mouth] a sandstone with calcareous cement. The 

 limestone-zones seem to be merely inconstant developments, laid 

 down locally in clear water at intervals between the deposition 

 of the sandy material. Their inconstant occurrence may be also 

 partly due to removal by solution or siliceous replacement, and 

 in part to crushing-out by movement. Sediments of intermediate 

 composition, such as ordinarily form calcareous sandstones, would, 

 by the solution of the carbonate of lime and its replacement by 

 silica, become rocks like some of these quartzites whose clastic 

 grains are embedded in a mosaic of quartz. 



Rapid Variation in Thickness. — The map shows that the 

 principal quartzite-ridges increase in width of outcrop as they are 

 followed from west to east. This is especially noticeable with the 

 Llanlliana Head and Craig Wen quartzites, where the appearances 

 cannot be altogether explained by faulting. Of the two the Craig 

 Wen quartzite offers the more striking and simpler example. Near 

 Hell's Mouth it is perhaps 30 feet thick, the purple conglomerate 

 above it is certainly not separated from it by a fault, nor is a fault 

 visible at the base of the quartzite ; yet at Craig Wen, only 1 mile 

 distant, the quartzite has an apparent thickness of about 200 feet. 

 This point must be considered in connexion with the overlying 

 conglomerates. 



Relationship to the Conglomerates. — The Craig Wen 

 quartzite-ridge is overlain -and underlain by conglomeratic beds. 

 The overlying purple conglomerate is often bleached near its base 

 and converted into a quartzite which, though not very different in 

 appearance from the quartzite-ridge itself, shows its conglomeratic 

 origin plainly enough, so that there is as a rule no difficulty in de- 

 termining the junction. Near Craig Wen a lenticle of the quartzite 

 lies surrounded by the conglomerate, and, close by, a great tongue 

 of the latter runs into the former. Also, when the width of outcrop 



