382 EEPOET— 1888. 



been developed a lenticle composed of snfcural elements of quartz sur- 

 rounding the original fragment, and pushing asunder on either side the 

 lines of lamination. 



We next come to that large group of rocks which are at the same 

 time crystalline and foliated. Of these we do not actually know the 

 original condition, and it has to be deduced, if possible, from the pheno- 

 mena of the altered rock. The general composition of these rocks is 

 pretty much the same throughout ; the differences are only minor ones. 

 In all there is a large proportion of quartz, while felspar is more abun- 

 dant in the rocks of the Central district than in those of the Eastern ; 

 epidote is an occasional ingredient in the rocks of every district, and 

 mica is found in almost all. These minerals being authigenetic the 

 material from which they have been derived must have been in a different 

 form, and may have been, for all we know, of the same kind as elsewhere 

 forms the slates and grits. The i-eason that these rocks now differ so 

 greatly from the others must be either that they were different originally, 

 or have been subjected to different processes since their deposition. But 

 they do not differ in the nature of their ingredients, and it can be shown 

 to be probable that they did not differ in the state of its aggregation ; in 

 other words, they were probably fine-grained rocks like those which pro- 

 duced the mosaics. In those examples which contain unaltered fragments, 

 the groundmass differs in no respect from those from which fragments 

 are absent ; we may therefore restrict ourselves to the former. These 

 fragments are most commonly felspar, a mineral not likely to resist more 

 than others crushing or decomposition. Similar fragments of felspar are 

 met with among the mosaic rocks — in fact they are the last to disappear — 

 •while they are very rare among the coarse-grained quartzites. It is 

 probable therefore that the felspar fragments in both cases owe their 

 preservation to their larger size, in which cfise the matrix must have been 

 of fine material. It may have been laminated, and very probably was. 

 The fine-grained rocks also are more liable to change, as far as the experi- 

 ence of those hitherto examined goes, than the coarse-grained rocks. The 

 conclusion is, that the present size of the crystalline elements is no 

 criterion of the original texture, which was probably fine. Hence the 

 greater crystals must be due to later processes, which have affected 

 these rocks more than the others. Some light may possibly be thrown 

 on the reasons for these differences, by considering the distribution of 

 the rocks with larger and smaller crystals respectively. The elements 

 being throughout of proportionate size, we can judge by the size of the 

 mica. Now the largest mica crystals with linear orientation occur in the 

 neighbourhood of Gaerwen, northward to Penmynydd and southwards to 

 Llangaffo (150, 151, 152, 153, 156, 158), in the Eastern district ; also at 

 Porth-y-ly-wod (74) and Grwalchmai Turnpike (71) in the Central district. 

 If we are able at all to tell strati graphically the centres of change, these 

 would be in their neighbourhood ; whereas, when we approach the dis- 

 tricts of less altered rocks, as at Bodowyr (146), Bwlch, near Llanddona 

 (209), south of Hafodty (144), Porth-y-fawch (75), and Llangwyfen (17), 

 Mynydd Mechell (211), and the district south of Traeth Dulas (142), we 

 find the mica in smaller elements, and for the most part only quincun- 

 cially orientated. This seems to indicate that the greater crystals are 

 due to more intense metamorphism, as we might expect, since the 

 crystals being authigenetic, every large one must have been a small one 

 first. With regard to the other minerals, the present size seems to de- 



