Part of Carnarvonshire. 19 



variation is usually to be seen even in a single rock section. The 

 matrix is typically cryptocrystalline, but occasionally becomes 

 macrocrystalline in character. Microspherulitic textures are of 

 frequent occurrence, and on the whole this is more commonly the 

 case in the dark variety. 



In many cases the manner in which the various types of matrix 

 are intermingled one with another and drawn out in streaky fashion 

 shows that we are dealing with rhyolites having a well-defined flow- 

 structure. This structure is sometimes developed on a very small 

 scale (PI. I, Figs. 1, 3). In other cases, however, the original 

 character of the rock is by no means so clear, since in these the 

 matrix with its different types of recrystallization presents a patchy 

 rather than a streaky texture, suggesting at first sight a tuff. The 

 streaky appearance, however, still persists to a certain extent, and in 

 such cases the patches frequently end off rather abruptly along the 

 direction of the flow-structure. It is extremely difficult to determine 

 whether such rocks are lavas with flow brecciation or rhyolitic tuffs 

 containing rhyolitic lapilli. 



The presence of the very numerous and large felspar phenocrysts 

 and still more so of the quartz phenocrysts immediately suggests 

 that the rocks belong to a series of Pre-Cambrian age, since these 

 characters appear to be of universal occurrence in the Pre-Cambrian 

 rhyolites of Wales, whereas they are not found in the Ordovician 

 rhyolites. In this connexion it is also noteworthy that the micro- 

 spherulitic texture described above appears to be of frequent 

 occurrence in the Pre-Cambrian rhyolites of North Wales, whereas 

 it is not found to any extent in rhyolites of Ordovician age in North 

 Wales. 



C. Cambrian Sedimentary Sekies. (i) Conglomerate. («) Strati- 

 graphy. — Beyond the rhyolite we come to the crags which form the 

 highest ridge on the hill at its south-west end. These crags are due 

 to the presence of a hard conglomerate, the nature of which is well 

 shown on the weathered surfaces. The well-rounded pebbles in this 

 conglomerate vary considerably in size ; whilst the majority are from 

 1 to 2 inches in diameter, others may attain a diameter of 12 inches. 

 These pebbles are embedded in a fine gritty matrix. All the pebbles 

 consist of felsites with porphyritic crystals of quartz and felspar, and 

 they closely resemble the rocks of the rhyolitic series already 

 described. In fact, near the line of junction of the conglomerate and 

 the rhyolitic series, where the pebbles of the conglomerate have been 

 pressed into the rhyolitic rock, the resemblance between the two is 

 so close that it is only on careful examination that the outline of the 

 pebbles can be seen. 



This conglomerate strikes IS. 80° E. When followed across the 

 strike in a south-westerly direction the conglomerate passes into 

 a grit which is, however, still occasionally pebbly. This conglomerate 

 band continues until it is buried in the drift on the south-west slopes 

 of the hill. 



Proceeding along the strike in the opposite direction — north-east — 

 the pebbles decrease rapidly in size in the course of a few yards, but 

 the matrix retains its original character. In a newly opened quarry 



