MONIAN SYSTEM OF ROCKS. 499 



ment of the rocks is here very complicated. Prom Llandrygarn to 

 Ynys Dodyn there are good diorites, with a band of granite in the 

 centre at the latter place ; but round Mynydd Mawr the diorites 

 are inextricably mixed with the quartzose gneiss, and can scarcely 

 be anything but intrusive. 



The fourth dioritic area is that long tongue which skirts the east 

 side of the granite, and figures truly as greenstone on the Survey 

 map. As seen on the east side of Plas Llanfihangel this is in every 

 way comparable to the rocks at Craig-yr-allor, where the foliated 

 hornblendic rock is spotted with lighter flakes of felspar. Between 

 this spot and the granite it is a very complex rock with well-marked 

 granitic segregation-veins, which may serve to throw light on the 

 origin of some of the smaller veins and flakes which spot the 

 diorite breccias near Llecheynfarwy and elsewhere. At Yr-ynys- 

 goed the " greenstone " is a broken mass, infiltrated with chlorite- 

 veins, of quartz- and mica-particles so arranged as to suggest that it 

 may be really a brecciated form of the grey gneiss. A little further 

 to the south we come on the true grey gneiss with the diorite lying 

 to the west, but just on the other side of the stream it is so broken 

 up as to suggest a fault. 



Besides the above four types of rock there is a granitic-looking 

 rock at Bryngolen, and a remarkable andesitic rock just north of 

 Lechej-nfarwy ; and Prof. Bouncy has indicated that some parts in 

 the hiilleflinta-band consist of quartz-felsite. 



These observations are not sufficient to give a complete idea of 

 the granitic area, which would require the microscopical examina- 

 tion of every single exposure ; but they are enough to show that no 

 stratigraphical sequence can be truly made out, as every one that 

 is suggested by one district will be contradicted by another. Hence, 

 considering the nature of the rocks, we may regard the whole as 

 showing the characteristic irregularity of an eruptive centre, in which 

 acid and basic intrusions take place in basic and acid ashes and mud, 

 and subsequent or consequent disturbances break up and mingle 

 them all in confusion. The order of events would seem to be, first 

 the protrusion of the diorite, under the influence of foliating forces ; 

 then the production of the halleflinta, perhaps from the fragments of 

 the grey gneiss, and of the pelite from the materials of the diorite, 

 and these perhaps took place more or less simultaneously and 

 repeatedly ; but after all this came the intrusion of the granite, 

 which is thus the most recent rock of the whole development. 



The relation in age of the granite and its associates to the grey 

 gneiss and other rocks of the eastern region cannot directly be 

 proved : but that the former are younger is a priori probable, 

 because in the Western District volcanic rocks form the upper part, 

 and here also they are followed by the Ordovician ; while it would 

 require very complicated stratigraphy to make the grey gneiss the 

 younger, to say nothing of the probable intrusion, in more places 

 than one, of the granite into that rock itself. 



d. Bodafoii Mountain to Llanerchymedd.- — The rocks which lie 

 beyond the granite to the north-east are, with the exception of 



