ON CERTAIN ERUPTIVE ROCKS OF NORTH WALES. 423 



20. On the Chemical and Mineralogical Changes which have taken 

 place in certain Eruptive Rocks of North Wales. By J. 

 Arthur Phillips, Esq., E.G.S., &c (Read February 7, 1877). 



[Plate XIX.] 



At Penmaenmawr, in the county of Caernarvon, six miles south- 

 west of Conway, a crystalline felspathic rock has been erupted 

 through Silurian strata, and rises to a height of 1553 feet above the 

 sea-level, forming a boss-like elevation two miles in length and one 

 in width. 



This stone, which is of a greenish-grey colour, is moderately 

 fine-grained, and to the unassisted eye, appears to consist of felspar 

 associated with minute crystals of some dark hornblendic or pyro- 

 xenic mineral. The general mass is divided into beds by a series 

 of predominating joints dipping north at an angle of about 45°; and 

 these are again divided by a system of double jointings which are 

 sometimes so developed as to the cause the rock to become distinctly 

 columnar. 



On the northern flank of the mountain, where four large quarries 

 have been opened upon it, this stone is extensively worked both for 

 general building-purposes and for road-making. Of these quarries 

 the two more easterly ones belong to Messrs. Kneeshaw, Lupton, 

 and Co., while those situated further towards the west are worked 

 by Messrs. Brundrit and Co. The crystalline rock at the eastern 

 extremity of the outcrop, near its point of contact with the slates, 

 sometimes encloses small schistose fragments, but is fine-grained 

 and frequently assumes a columnar structure. 



Here the stone is often so close in texture as to almost resemble 

 chert, and to break with a conchoidal fracture ; but particles of kao- 

 linized felspar show that it has, even here, been subjected to a 

 certain amount of alteration. 



The rock in the next two more westerly quarries is not only 

 coarser in structure, but its jointings are irregular, and the felspar 

 is less fresh than in the quarry first described ; it is nevertheless 

 still a somewhat fine-grained rock, since the largest crystals do not 

 exceed y 1 ^ of an inch in length. 



At the top of the most easterly of these quarries is a thin foliated 

 deposit, a specimen of which afforded on analysis a somewhat re- 

 markable amount of oxide of manganese. 



The stone in the most westerly quarry, which belongs to Messrs. 

 Brundrit and Co., and is situated nearly opposite Beaumaris, is 

 generally fresher in appearance than that from the two quarries 

 last referred to, besides which it is closer in grain and greener in 

 colour. 



A comparatively superficial examination of the different varieties 

 of stone obtained from the various quarries at Penmaenmawr is 

 sufficient to indicate that they are probably mere modifications of 



Q.J.G.S. No. 131. 2e 



