MONIAN SYSTEM OF EOCKS. 533 



any signs of pressure, it is terribly squeezed and broken, the spectral 

 polarization being most beautifully developed. It is possibly, there- 

 fore, of greater age. 



In association with this is the only part of the district where any 

 portion of the older schists can be recognized. The mass of the 

 rounded elevation caUed Mynydd Ystum consists of dark foliated 

 mica-schists, which are terribly broken, but contain nothing but 

 quartz and white mica, and compare exactly with the more contorted 

 examples met with in the Eastern District, while in microscopic 

 appearance they resemble the rock near the Gualchmai turnpike. 

 This material, even more broken, continues as far as Llwydiartan, 

 where it is lost beneath the peat. 



Other Caeei^arvonshiee Disteicts. 



A considerable number of isolated areas scattered about Caernar- 

 vonshire have been in times past claimed by various authors as 

 Pre-Cambrian. The most important of these is that which lies 

 between Bangor and Caernarvon, and next to it is the quartz-felsite 

 of Llanberis and Moel Tryfaen. The examination of the true age 

 and character of these rocks has required so much detailed observa- 

 tion, and the resulting balance in favour of Pre-Cambrian rocks is 

 so small, that I have discussed these questions in a separate paper 

 (see Quart. Journ. Geol. Soc. vol. xliv. p. 271). In this I have of 

 necessity described the portions which I still consider may be Pre- 

 Cambrian, and to that description I have nothing to add. It is now, 

 however, possible to refer to the close resemblance which the granite 

 of Twt Hill bears to the Porth-Lygan granite, which latter we have 

 independent reasons for considering eruptive, and to remark on the 

 association in both places of quartz-felsite with the granite. But 

 in their character these felsites will not strictly compare with any in 

 Anglesey. The common characters of all quartz-porphyries may be 

 seen in some portions of the granite-mass in the Central District ; 

 but in their great development and in their association with breccias 

 they still stand alone. I have also shown that the felsites of Llan- 

 beris and Moel Tryfaen are of Cambrian age, and therefore have no 

 relation to Pre-Cambrian rocks. 



I have now to consider those other masses to the south which 

 have been claimed by Dr. Hicks as Pre-Cambrian *. These are the 

 volcanic rocks of Bwlch Mawr, Pen-llechog, Tr Eifl, Mynydd Nevin, 

 Carn Boduan, Pwllheli, and Llanfihangel Bachiellaeth, the last of 

 which I have not examined. It has been stated that these have 

 been so claimed on "purely theoretical grounds," and certainly 

 I can find no others, nor is the theory in these cases a possible one. 

 Of Mynydd-y-Cennin Dr. Hicks observes that Sir A. Ramsay describes 

 it as a quartz-porphyry, similar to that of Llyn Padarn, and that '^ this 

 is therefore undoubtedly .... like it, of Pre-Cambrian age." As the 

 Llyn Padarn porphyry has now been proved to be not Pre-Cambrian, 

 this theory now leads the other way. The only other evidence given 



* Loc. cit. 



