]MONIAN SYSTEM OF ROCKS. 520 



volcano ; and the confusion and apparent mixture of more and less 

 crystalline materials is incidental to such an origin. This con- 

 clusion, based on what is seen where the rocks are best exposed, 

 affords a confirmation of the interpretation put upon similar rocks 

 when their surroundings and relations are less clear. 



Exclusion of Parts Mountain from the Series. — Both Dr. 

 Callaway and Dr. Hicks insert this " mountain " as Pre-Cambrian 

 on their maps, and the former gives a description of its volcanic 

 rocks. He admits, however, that they " are quite distinguishable 

 from any other rocks in Anglesey," and has no better reason 

 to assign for their age than that they are not unlike some Pre- 

 Cambrian rocks in Shropshire and at St. David's. I cannot see the 

 slightest reason for their Pre-Ordovieian age. Dr. Roberts says 

 there is a gradual passage from the contorted rocks of Amlwch 

 into the slates of Parys Mountain ; but this has been shown to be in- 

 compatible with the stratigraphy, and the change seems to me to be 

 both sudden and marked. The admirable description of this 

 "mountain" in Phillips's ' Ore -deposits' shows that the higher 

 part is due to a long reef of quartzite and two parallel bands of 

 felsite, between which is the copper-bearing nest with black shale " 

 in which Prof. Hughes records Ordovician Graptolites. The quartzite 

 is full of crystals of iron-pyrites, and the felsites weather into 

 slate-like bands ; but I do not recognize any resemblance to any 

 Pre-Cambrian rock that I happen to know. Moreover, as Dr. Calla- 

 way has already shown, the northern end of the felsite at Pensarn 

 is brecciated, and the knob a little beyond is a rhyolite, facts which 

 indicate a contemporaneous origin. In these parts they most 

 resemble the intrusive felsites of Mynydd Mechell, and. if similarity 

 is to be any guide, might well be referred to the same age. But 

 here they are wholly surrounded by Ordovician rocks, and the 

 mere fact of their being volcanic is certainly no proof that they are 

 earlier in age. 



There is, however, beyond the Parys Mountain to the south-west, a 

 remarkable quartz-knob, not at all like the neighbouring quartzite. 

 Had this occurred in the neighbourhood of Bull Bay, I should have 

 unhesitatingly recognized it as the ordinary knob to which we are 

 by this time accustomed. It is one of the toughest, and the quartz 

 is very pure and shows throughout the polygonal structure. There 

 are, however, signs of banding in fragments which lie scattered in 

 all directions, and are tightly cemented together by more quartz. 

 This I should interpret, as before, as a kind of geyser-formation, 

 the banded portions being the earlier deposits on the pipe broken off 

 and carried up. Of course, there is no reason why such a formation 

 should be confined to Pre-Cambrian times, and it may be a pheno- 

 menon of the locality rather than of any definite epoch. Its occur- 

 rence amongst Ordovician shales here may even be made to throw 

 doubt on the earlier period of the formation of the others ; but con- 

 sidering that this is quite exceptional, it seems more probable that 

 we have here an isolated relic of an older rock, like that of 



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