40 



PEOE. T. G. BONNEY ON THE SEEPENTINE 



4. On the Serpentine and associated Roce:s of Anglesey ; ivith a 

 Note on the so-called Seepentine of Poetildinlleyn (Caeenae- 

 vonshiee). By Prof. T. G. Bonney, M.A., E.R.S., Sec. G.S. 

 (Read November 3, 1880.) 



1. Anglesey with Holyhead Island. 



The serpentine from the vicinity of Rhoscolyn, though known within 

 a limited area as an ornamental stone, does not appear to have 

 received much attention from geologists. It is dismissed in Pro- 

 fessor Ramsay's memoir on North Wales in a few brief sentences, 

 and has not, so far as I know, met with fuller notice in any other 

 quarter. On the Geological-Survey map it is delineated as forming 

 an elongated lozenge-shaped patch about a mile and three quarters 

 in length from E. to W., extending inland north of Rhoscolyn from 

 the west shore of Holyhead Island, and occurring in several smaller 

 patches on or near the opposite coast of Anglesey. In each of these 

 localities I have studied the rock in the field, and have examined 

 microscopically the specimens there collected. The results, I hope, 

 may be of some use, though, after three separate visits, I have not 

 been able to visit every one of the outcrops or examine minutely 

 every part of the district. Owing to the intricacy of the coast in 

 certain places, a map on a considerably larger scale than 1 inch to 

 the mile would be required before an elaborate study could be 

 undertaken. 



The questions which I have attempted to solve (as in all former 

 studies of serpentine), were (1) its relations to the other associated 

 rocks, and (2) its nature and origin. 



The ordinary rock in the vicinity of these masses of serpentine is 

 a dull bluish or greenish schist, composed chiefly of minutely 

 crystallized micaceous or chloritic minerals with some quartz, the 

 foliation being parallel to the bedding. The latter is generally very 

 distinct and sometimes exhibits very remarkable crumplings. The 

 serpentine occurs in low rugged knolls, cropping out here and- there 

 from the fields over which in the map the colour denoting its presence 

 is extended — the general aspect of the rock masses reminding us of 

 the serpentines of Cornwall, Scotland, and Italy. It differs markedly 

 in all its characters from the ordinary schists of the district, and, 

 where associated with the latter, occurs in such a manner as to 

 suggest either intrusion or sporadic metamorphism of a very singular 

 character. This is also suggested by the mapping ; but it is even 

 more conspicuous in the field. 



My examination, however, showed me that under one name two 

 distinct rocks had in some cases been confounded, viz. a true serpen- 

 tine and a gabbro, especially in the Anglesey group. For instance, 

 if we alight at the Valley Station on the Bangor-Holyhead railway, 

 and follow the road to Pour-mile bridge, we observe in the fields on 

 our left, near Ty Newycld, a long craggy mass of serpentinous 



