94 CROCIDOLITE 



mineral which forms compact, heavy and exceedingly 

 tough layers, and also fibrous layers, usually parallel to 

 the enclosing siliceous beds. The compact rock consists 

 of minute crystalline fibres of crocidolite matted together 

 and not arranged in any uniform way ; but in the fibrous 

 rock each crocidolite individual has a length which may 

 be more than 100 times longer than its width, and they 

 are arranged parallel to one another and perpendicular 

 or at a high angle to the surfaces of the enclosing rock. 

 This fibrous form is flexible and is mined and sold under 

 the name of blue asbestos. The two forms of the 

 mineral are alike in their optical properties and composi- 

 tion. 1 There can be no doubt that the compact form is 

 the result of the alteration in place of sediments deposited 

 under water ; it is found, just as any other variety of sedi- 

 mentary rock, in layers between rocks, which differ from 

 it in containing a smaller proportion of crocidolite grains 

 or short fibres. The fibrous layers, on the other hand, 

 may vary much in thickness within a very short distance, 

 especially where the rocks happen to be bent. In these 

 folds the fibrous crocidolite thickens out considerably in 

 the arches and troughs, and when the enclosing rock is 

 broken the cracks are often found to be filled with it. 

 The fibrous variety thus behaves more like minerals 

 filling veins than the compact sort, but it chiefly occurs 

 in layers which maintain their width over at any rate 

 many square yards, as proved by mining operations, and 

 which lie conformably between two other beds without 

 breaking across them. 



1 Analyses are quoted in G. C., x., 158, from Hintze's Miner alogie, 

 vol. ii., p. 1265, and other works. 



