^'*^^- 55-] SERPENTINE AND ASSOCIATED KOCKS IN ANGLESEY. 277 



evidence favourable to the hypothesis of a fault. The green schists 

 have a squeezed appearance, but the brecciation and crushing of 

 the rock adjoining them are still more marked. In it are angular 

 fragments coloured by haematite, the whole being permeated by- 

 carbonates^; hence the rock is one of the so-called ' ophicalcites.' 

 jN'ear by are foliated masses, including a squeezed diallage-enstatite- 

 rock associated with serpentine and gabbro. Lenticles of the rock 

 have escaped, while surrounding parts have been crushed. 



Bosses of gabbro occur in the fields between here and Tyddyn-y-cob^ 

 Inlet, where the rock becomes schistose. The gabbro reappears inland 

 beyond the sluice, and extends as far as the outcrops of schist near 

 Glan-rhyd-uchaf and east of the railway, but is interrupted south 

 of Crugias by serpentine and ' ophicalcite,' ^ the latter rock being 

 brecciated and in places reddened by haematite. 



(2) Mainland of Anglesey — near the Lakes. 



Large bosses or mounds of gabbro occur in the fields along the 

 northern border of Llyn Dinam,^ two of them rising abruptly from 

 the water. Between these are bosses of serpentine, one of which is 

 quarried, and others occur close to Dinam Fawr. The serpentine is 

 mostly schistose, occasional ' eyes ' only being left, but a small 

 boss of uncrushed rock shows an interesting and rapid change in an 

 enstaiite-serpentine. 



Numerous boulders' of various rocks occur in some of the fields 

 north-east of the lake, as well as bosses of schist ; while, farther 

 south, between the two lakes, many of the latter are conspicuously 

 ice-worn. Gabbro extends from south of Llyn Dinam and some 

 islet-like masses in the marsh to the island in Llyn Penrhyn : it 

 is cut by dykes varying from diabase to not very fresh basalt. 



The serpentine, which here again borders the gabbro and separates 

 it from schists on the west and south, is interesting from the way in 

 which it weathers. It is generally much crushed, and the surface 

 of the bosses exhibits thin laminae, often with crumpled edges, 

 varying in colour from dark pistachio-green to a greyish-blue. 

 Frequently ' eyes ' of serpentine have escaped the crushing ; one 

 of these, a dull slaty-blue compact rock, measuring roughly 3 feet 

 in length, contrasted so strikingly with the crushed serpentine 

 around, that it might have been taken for an intrusion or an 

 included lenticle. In some places angular fragments of white 

 dolomitic veins,*^ often schistose, are conspicuous in the dark serpen- 

 tine and resemble enclosures. This seems to show brecciation in a 

 rock already veined by a carbonate, and the much-crushed rock 

 adjoining was probably similar. In one mound (mainly of ser- 



^ As proved by its efferyescence when treated with hydrochloric acid. 

 2 ' Tyddyn Grob ' on the 1-inch Survey map. 

 ^ Described in Quart. Journ. Geol. Soc. vol. xxxvii (1881) p. 42. 

 * ' Llyn Treflas on the 1-inch Survey map. 



' One is a picrite, many are dolerite. Boulders have been only incidentally 

 studied. 



" Hardness about 4 ; effervesces with hydroc-hloric acid somewhat slowly. 



