VEIN CAVITIES 



133 



fibers of the true asbestos (a variety of amphibole), which are polygonal. 

 In diameter they vary down to .002 of a millimeter, and even less. 



Origin of the Vein Cavities 



The origin of these veins and the fibrous structure of the filling mate- 

 rial are involved in some obscurity. The fact that the same mineral 

 occurs in both massive and fibrous forms, so closely and intimately as- 

 sociated, is certainly striking. That the vein material is of a later origin 

 than the massive rock is self-evident, but why the later formed material 

 should be always fibrous and of a distinct crystalline structure, while 



Figure 2.— Asbestiform Veins in massive Serpentine. 



the older is massive and nearly or quite amorphous, it is difficult to say, 

 and existing literature is discreetly silent on the subject. The massive 

 serpentine rock is itself described as an altered diorite or doleritic rock, 

 rich in olivine.* The time that the present writer was on the ground 

 did not permit a thorough investigation on this point, but blocks were 

 noticed which, though highly altered, showed beneath the microscope 

 structures more nearly that of massive enstatite rocks or pyroxenites. 

 There is, however, apparently no doubt that they are altered highly 

 magnesian igneous rock. Now, the process of hydration (serpentiniza- 

 tion) in rock of this class must, provided there is no loss of material by 



♦Asbestos and asbestic, by R. H. Jones, 1897, p. 

 Engs., vol. xviii,1889, p. 322. 



; also R. W. Ellis, Trans. Am. Inst. Min. 



