AND ASSOCIATED ROCKS OP ANGLESEY. 



43 



of ophicalcite "which is bordered by true serpentine on either side. 

 The annexed diagram (fig. 2) roughly represents the relations of the 



Pig. 2. — Quarry at Ceryg Moelion. 



A. Serpentine, crushed and slickensided. 



B. Ophicalcite. C. Debris. 



two rocks. The boundary between them is generally quite sharp ; and 

 the serpentine is shattered, greatly slickensided, and in parts almost 

 flaky, as if it had undergone great pressure. The ophicalcite varies 

 much : in parts nearer the edge it is evidently a breccia of angular frag- 

 ments of dark serpentine united by crystallized calcite ; in other parts 

 the latter predominates and is mottled by a green serpentinous 

 mineral without distinct evidence of fragments. The section is per- 

 plexing ; but, after careful examination aided by the microscope, I 

 think that we have here a fissure which has been connected with 

 some faulting and has subsequently been filled up. As the serpen- 

 tine itself contains little or no lime, and as the schists of the island 

 are not markedly calcareous, one is disposed to attribute the calcite 

 to infiltration from overlying masses of Carboniferous Limestone 

 which have been subsequently removed by denudation. 



Following the road for perhaps a quarter of a mile further south, 

 we pass some more serpentine and several craggy outcrops of schist, 

 and then in a field on the left find another interesting pit * which 

 has been opened in order to work a band of steatitic rock. The 

 annexed diagram (fig. 3) expresses the relations of these rocks. 



A is a green schist with a rather high N.IOY. dip. B is the 

 band quarried. It is about 4 feet thick, but is rather irregular, 

 seemingly conformable with A, and distinctly schistose in structure. 



* I believe it is called Plas Groch ; but it is most difficult to obtain informa- 

 tion as to names in these wild parts of Wales ; the spot must be very near the 

 division between the two quarter sheets of the Ordnance map, 



