48 



PROF. T. G. BONNET ON THE SO-CALLED 



consider altered olivine rocks, such as those of Cornwall, Scotland, 

 Italy, &c. ; the general aspect of the rock in the field perfectly agrees 

 with the same ; and its relation to the schist would be most difficult 

 to explain, except on the theory that it was intrusive in the latter. 

 We must also remember that olivine rock partially altered has been 

 detected in the Lleyn peninsula*. The evidence in favour of the 

 serpentine being produced from the schist by some kind of selective 

 or local metamorphism is of the weakest possible description. We 

 therefore need not hesitate to add the Anglesey serpentines to the 

 list of altered peridotites. 



2. The so-called Serpentine of Porthdinlleyn. 



The northern and larger part of the promontory at Porthdinlleyn 

 (west of Nevin) is coloured as serpentine in the Survey map, and 

 thus noticed (by Mr. Selwyn)in the memoir f : — " The rock at Porth- 

 dinlleyn is a kind of coarse green and purple serpentine, with nests 

 of red jasper ; veins of the serpentine are observed to dash in among 

 the slaty series at Porthween." So far as true serpentine is con- 

 cerned, I might have modelled this note on the well-known chapter 

 concerning the snakes in Iceland ; for there is none at Porthdinlleyn. 

 The rocks, however, which I examined in my search for serpentine 

 are so interesting that I am tempted to add a few words concerning 

 them. 



Specimens of several of these rocks have been examined microsco- 

 pically ; and one or two exhibit structures of much interest. On the 

 present occasion, however, I shall not enter into details foreign to 

 the immediate, subject, but only endeavour to identify the specimens. 

 The first rock seen (by a landing-stage) as we approached Porth- 

 dinlleyn along the shore from Bwlchdinlleyn is a massive green rock 

 of serpentinous aspect. In the craglets and reefs near the hamlet 

 itself we find a similar rock which also shows in parts a distinctly 

 brecciated structure, pale-green fragments of various sizes (generally 

 up to 2 or 3 inches diameter, but in one case about 18 inches) 

 occurring in a. darker and more ashy-looking matrix. In other 

 parts the structure more resembles brecciation in situ ; sometimes 

 the rock is almost spheroidal, and reminded me of certain instances 

 which I have seen in old igneous rocks of a rather basic nature, where 

 the outer shells of the spheroid had become decomposed, simulating 

 a conglomerate with but little matrix. A reddish variety of the 

 compact rock also occurs ; and these rocks continue for some distance 

 to the north of Porthdinlleyn, the " nests of red jasper " (haematite 

 with quartz, calcite, &c.) being in places very conspicuous. In the 

 field it is very difficult to pronounce upon the nature of the above 

 compact rock. It is hard, with a subconchoidal to rather argil- 

 laceous fracture, traversed by well-marked divisional planes, but 



* Geol. Mag. dec. ii. vol. vii. p. 208. 

 t The Geology of North Wales, p. 170. 



