AXD ASSOCIATED ROCKS OF ANGLESEY. 



47 



cleaved fibrous mineral of an altered aspect. This may be only 

 the result of pseudomorphic action on the diallage ; but it may have 

 been from the first a separate mineral. I incline to the latter view, 

 as the normal constituent is sometimes altered, but seems then to 

 produce a different mineral. 



A specimen from the islet near the south shore closely resembles 

 the last. Here the pyroxenic mineral sometimes contains microliths, 

 apparently of augite, sometimes exhibits lamellar twinning ; it is 

 occasionally altered as in the last case. A third specimen, from a 

 knoll on the shore between an outcrop of schist and another of ser- 

 pentine, corresponds generally with the above. 



Specimens of the normal schist from near the chapel on the Holy- 

 head island, from near Tycoch. and within about 5 feet of the ser- 

 pentine at the presumed intrusive junction (see above, fig. 1, p. 41) 

 have been examined. They are representatives of a class of schists 

 of which I have seen and received numerous examples from Anglesey. 

 Under the microscope they are seen to be composed mainly of two 

 minerals, both of minute size : — one quartz ; the other a greenish 

 mineral in little scales *, fairly dichroic, changing from a greenish 

 yellow to a strong green, occurring also in veins in little tufts : 

 it resembles a chlorite more than a mica. There is an occasional 

 granule of epidote and a good deal of a sort of grey earthy dust, 

 sometimes in clots, probably in part decomposed magnetite or ilme- 

 nite. There are also some vein-like bands of clear quartz, probably 

 segregation-products. The third specimen has a rather coarser 

 structure, and contains some scattered quartz-grains which may 

 indicate original constituents. These rocks show a banded structure, 

 probably due to original bedding ; but they are very highly altered. 

 The schistose rock at Tyddyn Gob, as might be supposed from its 

 aspect, is very different. It has a marked foliated structure, being 

 chiefly composed of wavy bands of an earthy mineral, almost opaque, 

 and of a nearly colourless fibrous mineral, probably a variety of 

 actinolite. Among the latter, in the lower and coarser rock, are 

 the remains of a mineral with well marked parallel cleavage, from 

 which the other may have been produced by secondary change. In 

 appearance the mineral more resembles augite, and it is practically 

 not dichroic ; but in other optical properties it agrees better with 

 hornblende : a little epidote is present. I cannot help suspecting 

 the possibility of this rock being an altered gabbro with pressure 

 foliation ; but if so, it is a most exceptional case. 



Prom the above observations it will appear that the whole of this 

 district needs remapping ; and this task will not be an easy one. 

 That the gabbro is an ordinary igneous rock cannot, I think, be 

 doubted. As regards the serpentine, the evidence is a little less 

 clear. The microscopic structure, owing probably to the great age 

 of the rock, is rather obscure and anomalous ; its relations to the 

 schist are not clearly displayed. Still the structure in several cases 

 coincides with that of serpentines which we need not hesitate to 



* 2fot more than -001 inch diameter. 



