136 G. P. MERRILL — ORIGIN OF VEINS IN ASBESTIFORM SERPENTINE 



and more solid form, and perhaps also to loss of silica, as suggested by- 

 Professor Kemp. In other words, he would compare them with the 

 shrinkage cracks which appear in clay on drying, or, better yet, those 

 which result from the shrinkage of a gelatinous mass of iron carbonate, 

 as in the so-called septarian nodules of clay-iron stone (see plate 34). 

 Such cracks resemble those of the serpentine in a striking manner, their 

 disposition to widen toward the center and pinch out toward the margin 

 being, in the case of the septarian, due to the spherical or oval form of 

 the body — a condition of affairs that does not exist in the serpentine. 



In putting forward this idea the writer realizes that he is complicat- 

 ing the commonly accepted ideas regarding serpentinization, since, as 

 here outlined, it involves a process of hydration and swelling sufficient 

 to produce the jointing and slickensiding and a shrinkage sufficient to 

 produce the cracks, and this latter, too, while the rock contained a suffi- 

 cient amount of moisture to carry the new material into the veins, where 

 it crystallized. 



Filling of the Vein Cavities 



It is perhaps a question if both the formation and refilling of the fis- 

 sures were not contemporaneous with and incidental to this process of 

 dehydration and shrinkage. The assumption of a fibrous structure under 

 quite similar conditions is sometimes seen in gypsum and more rarely 

 in calcite. In the first named the crystallization apparently takes place 

 by a process of growth from one of the walls, considerable force being 

 manifested — sufficient, it may be, to rupture the rock mass in which it 

 is taking place* Whether or no such a condition of affairs exists in the 

 case of the asbestos veins is perhaps yet to be proved. It is noted, how- 

 ever, that veins of any considerable width rarely show continuous fibers 

 extending from side to side. In most cases the continuity is interrupted 

 by small fragments of the wall rock ; or, again, where this is lacking 

 there exists at some intermediate point between the walls a break or 

 line of separation as though the crystal fibers had been pushed outward 

 from either wall until their extremities met. In many such cases the 

 growth has continued until the fibers are pushed past one another to a 

 slight extent, the line of contact thus becoming jagged or saw-like. 

 Again, there are other indications of pressure from the direction of the 

 walls, manifesting itself most frequently in a crimping of the fibers. 



In brief, the views of the present writer are to the effect that the vein 

 cavities are due to shrinkage and the vein filling to processes of crystal- 

 lization, extending from either wall inward. 



* G. P. Merrill : The formation of gypsum in caves. Proc. U. S. Nat. Mus., vol. xvii, 1894, p. 81. 



