MONIAN SYSTEM OF KOCKS. 469 



jiorth is described as " soft brown and felspathic schist with beds of 

 quartz-rock and conglomerate." It is obvious therefore that the 

 most important point here to determine is whether there are really 

 two distinct series of rocks in this area, or whether one class merges 

 gradually into another. For this purpose I have examined microsco- 

 pically a considerable number (24) of the rocks which are referred 

 to the chloritic schist, and have collected some forty other examples ; 

 but the country is so wide that even these seem hardly adequate. 

 Nor are matters so simple as Dr. Callaway's statement that " true 

 schists with a south-west strike occur everyAvhere to the south " 

 would lead one to suppose. 



In the first place a large number of the exposures show no strike 

 at all, but the lines of division run in almost any direction, and 

 where the strikes can apparently be determined, they are discordant 

 from place to place. Thus, on the south of the river Alaw near 

 Llangynghenell, it is N. 20° ^Y., at the church E. 8°]S"., at Bodedern 

 E., south of Bodedern E. 20° N., and a little further south N. 30° E. 

 It is plain, then, that the rocks have suffered so much local dis- 

 turbance as to destroy the value of the strike and to drive us 

 to consider surface-distribution only. 



The larger number of exposures inland show us fine chloritic 

 schists, which differ considerably from those of Holyhead in being 

 composed of much more minute particles, whether crystalline or 

 original, and thus possessing a much more slaty aspect, though often 

 actually foliated *. But in several places there are bands of grit 

 of irregular development, as near Llanfihangel and near Llanddeu- 

 sant. These can hardly be said to be foliated at all. At Abersant, 

 near Llanddeusant, the other extreme is met with in a regular mica- 

 schist. The constituents are almost entirely mica and quartz, the 

 former in large folia, the latter completely recrystallized, and all 

 thoroughly orientated. This is the only place in the western district 

 in which I have met with a rock of this description ; and the 

 cause of its appearance here, unless the granite of the central district 

 is continuous in this direction, is not easy to see. The occurrence, 

 however, is of some importance for the correlation with other 

 districts. 



Besides the ordinary metamorphism which has produced the 

 chlorite, we find in places proof of much change of dynamic origin, 

 bringing about a peculiar fibrous aspect, as near Bodedern, in the 

 valley west of Llanddeusant, and at Llanddeusant itself, where the 

 beds are also very slaty and a large proportion of the material 

 consists of unaltered dust. The roads between this village and 

 Llantrisant are mended with a purplish slate, which is mixed with 

 green in the small openings that have been made. 



These observations already prove that there are considerable 

 differences in the amount of metamorphism which the rocks have 

 undergone, and that in some cases there is scarcely any change. 



* To distinguish these from the coarser laminated rocks near Holyhead, I 

 have called them chloritoid schists, not meaning thereby that they contain 

 the mineral " chloritoid." 



