Rev. J. F. Blake on the Monian System. 441 



dee, Ceryg-defaid. Craig-y-allor, and Maen-wyn, or absorptive, as 

 near Llyn-faelog. This granite is of various kinds, often passing 

 into a felsite. 



3. The District west of Traeth Dolus shows granite intrusive into 

 grey gneiss, and also passing into a felsite ; it is correlated with the 

 western half of the central district, of which it appears to be a con- 

 tinuation. 



4. The Eastern District. — The lowest portion here is the grey 

 gneiss, which is very compact towards the west, especially near the 

 igneous rocks, but becomes more micaceous and chloritic towards 

 the east, passing through chloritoid rocks into others of the volcanic 

 facies, with the usual quartz knobs and sporadic limestones, but 

 here, on the whole, more slaty. The complete unity of the whole 

 system is here well seen. The most remarkable feature is the in- 

 trusive foliated diorites, which are coarse and non-foliated near 

 Holland Arms, but become finer towards the east, where also 

 glaucophane takes the place of hornblende. They are seen intruding 

 into and contorting the grey gneiss in the Llangaffo cutting. 



At the southern end at Careg-gwladys is a remarkable volcanic 

 group, with a spherulitic diabase breaking into and surrounding the 

 baked blocks of calcareous slate. There are associated great 

 masses of mixed agglomerate, and terminated masses of limestone 

 and quartzite filled with brecciated fragments. 



5. The Northern District. — This commences in the south with the 

 chloritic schists, but soon becomes slaty, and such rocks with grits 

 occupy the greater part of the area. But towards the north we 

 reach the volcanic facies, characterized as usual by ashes, agglome- 

 rates, quartz knobs, in one case, near Bull Bay, seen to cross the 

 bedding, and sporadic limestones at Llanbadrig, showing oolite 

 within oolite, and suggesting its origin by a petrifying spring. 

 Above the quartz is found a great conglomerate, apparently derived 

 from it, and immediately above this conglomerate occur the fossils 

 discovered by Prof. Hughes, and no line of separation can be dis- 

 covered between them and the rest of the rocks in the district. 

 These fossils have been referred to Bala species, and there are 

 three alternatives to choose : either (1) they are not Bala fossils, 

 but are characteristic of the Pre-Cambrian rocks ; or (2) they are 

 Bala fossils, and the dividing line has as yet been missed ; or (3) 

 there is no dividing line, and the whole series is of Bala age. 

 Against the latter is their similarity to the rocks of the eastern 

 district definitely overlain by Cambrian ; and against both the two 

 latter is the fact that the series is unconformably overlain in the 

 neighbourhood by other conglomerates succeeded by black shales in 

 which Llandeilo Graptolites have been recorded. 



6. The District north-east of Parys mountain is a volcanic com- 

 plex, in which granitic and felsitic rocks with others of a more 

 basic character are inextricably mixed with the debris of the same 

 materials, and both are altered so as to be, in most places, insepa- 

 rable. This is connected on the N.W. side with grey gneiss. 



In the Lleyn the rocks belong to the volcanic facies, in which 



