50 



PROF. T. G. BONNET ON THE SERPENTINE 



Proceeding a little further, we find, about fifty yards south of the 

 boathouse, a compact green rock with indubitable spheroidal struc- 

 ture. This rock now continues along the northern and western face 

 of the peninsula, becoming in places very distinctly spheroidal. The 

 face of a crag (a joint-plane) near the sea at the N.E. angle exhibits 

 one of the most conspicuous instances of the structure that I have 

 ever seen. The rock, where not spheroidal, is compact and sharply 

 jointed, in one or two cases a little platy in structure. Under the 

 microscope it is seen to be an altered basalt, the structure being 

 recognizable, though the felspar is replaced by earthy decomposition- 

 products, and the grains and crystals of augite are much altered. 

 I also noticed one or two small dykes, seemingly of late date. The 

 eastern side of the peninsula had occupied so much of my time that I 

 was unable to examine into the relations of the above group of rocks 

 with the schist of the district (which is not to be seen on that side), 

 and had to be content with satisfying myself that, though there are 

 igneous rocks, there is no true serpentine at Porthdinlleyn. 



Discussion. 



Mr. Bauerman had examined the steatite- deposit of the locality, 

 and had arrived at much the same conclusions as the author. 

 There were true serpentine rocks, and doubtful or " serpentinous 99 

 ones. He considered an examination of the composition of these 

 rocks desirable. 



Dr. Hicks congratulated the author on speaking of the intrusive 

 origin of the Anglesey serpentines with a certain degree of doubt. 

 He was himself inclined to regard these rocks not as intrusive, but 

 as altered representatives of certain ancient sediments. He repudi- 

 ated the notion of " selective metamorphism " being generally ap- 

 plicable to these rocks. The rocks at Porth Dunbar he regarded 

 as mainly indurated and altered ashes. 



Prof. Hughes pointed out that when the Survey map was made 

 the definition of "serpentine" was much wider than that which 

 was now adopted. He was not convinced of the intrusive cha- 

 racter of the serpentines in the section given by the author, but 

 thought it might be one of the numerous cases in which during 

 the crumpling of the gnarled series the harder masses were pro- 

 truded through the schists, producing small local faults. He believed 

 that calcareous beds in the gnarled series, and magnesian minerals 

 in the dykes, generally associated with the serpentinous rock and 

 ophicalcite, were sufficient to account for both. 



Mr. Hudleston had examined the rocks of the district, and 

 thought they differed so greatly from the Cornish serpentines as to 

 make it somewhat doubtful whether they could have been formed 

 by the alteration of olivine rocks of intrusive origin. He was in- 

 clined to think that the Anglesey serpentines, like those of the 

 Shetlands described by Dr. Heddle, might in part be altered acti- 

 nolitic rocks. 



