ROCK-METAMORPHISM. 



CHAPTEE III, 



STRUCTURAL CHARACTERS OF OPHITES AND RELATED 



ROCKS, 



As already stated, serpentine is the most important mineral 

 constituent of the rocks under consideration. It is, moreover, 

 of especial interest in being represented, besides its typically 

 amorphous form, by some remarkable allomorphs. 



The best-known allomorph is chrysotile. This in its typical 

 state is fibrous or asbestiform ; and it occurs as layers of a silky 

 or metallic lustre, varying in colour from silver-white to bronze 

 or golden. The colour and lustre are doubtless due to the 

 peculiar divisional structure of the allomorph. The layers are 

 included in, and frequently alternate with, amorphous ser- 

 pentine, so as to affect a parallel arrangement ; the parallelism, 

 however, is often interfered with by the layers of chrysotile 

 coming together, or coalescing at irregular distances, or by 

 others intersecting them obliquely. Often two or more layers 

 are in close contact. 



The layers vary much in thickness, running from less than 

 one eighth of an inch to one or more inches. The fibres cross 

 the layers at a right angle, occasionally obliquely or slightly 

 undulating. 



Chrysotile occurs beautifully developed in a number of loca- 

 lities the original one being Reichenstein, in Silesia, which 

 yielded the specimens first analyzed by Kobell. The analysis 

 proved it to be identical in composition with serpentine. Typical 

 specimens also abound in Canada, New Jersey, and a great many 

 other places. The specimen represented in fig. 1, Plate I., is 

 from Colafirth, one of the Shetland Isles*. A description of the 

 rock from which it was taken has been given by Dr. Heddle in 

 the ' Mineralogical Magazine/ vol. ii. p. 183. 



Although usually taken to be a fibrous mineral, chrysotile 

 occurs under certain modifications, each representing a stage in 

 the process of development from incipiency to completeness. 



* The specimen was presented to us by our late respected colleague 

 Professor Harkness, F.R.S, &c. 



