VEIN CAVITIES 135 



Dr J. H. Pratt, on the other hand,* regards the vein cavities as pri- 

 mary. Noting that such occur only in those portions of the serpentine 

 in close contact with the country rock, it is argued that they were pro- 

 duced by the more rapid cooling of these portions of the igneous magma, 

 and have been " filled with serpentine deposited from aqueous solutions 

 from their walls,"' the filling and crystallization in fibrous form having 

 taken place " some time before the complete alteration of the primary 

 rock into serpentine." With this view the present writer does not agree, 

 since not only does he know of no instance in which an igneous rock 

 has become cracked under the conditions described, but, further, these 

 cracks and the fibrous serpentine are to be found among rocks which 

 are not of igneous origin at all. In the case of the Montville, New 

 Jersey, serpentine, for instance, the serpentine has been here shown f to 

 originate through the hydration of a lime-magnesian pyroxene occurring 

 in disseminated granules and nodules throughout a white crystalline 

 granular dolomite. In this are reproduced all the characteristic phe- 

 nomena of jointing, slickensided surfaces, platy and fibrous structures, 

 but, as well, narrow veins of fibrous material (chrysotile), the veins being 

 in a general way parallel to the surface of the original nodule of pyrox- 

 ene. It is self-evident that in this case no cooling of an igneous mass 

 can be considered as in any way effective, and almost equally evident 

 that strains due to the movement of the material in mass also need no 

 consideration. 



The writer's own opinion, founded on the facts at present available, 

 is that the crevices are due to shrinkage such as is incidental to the 

 change of a highly hydrated, colloidal substance into a less hydrated 



* Mineral Resources of the United States, 1903. The exact wording is as follows : 



" It can be conclusively shown in nearly all cases that the serpentine in which the chrysotile 

 asbestos is found is of igneous origin. . . . The original rock in cooling would solidify first 

 along it3 contact with the rocks through which it had penetrated and where it was in contact with 

 any included masses of the country rock that had been broken off during the intrusion of the 

 molten magma. The outer portions of the molten rock would thus cool much more suddenly than 

 the interior portion, and there would be a tendency for them to develop cracks and parting planes. 

 In the alteration of these primary rocks to serpentine, through the agency of aqueous solution, 

 vapors, etcetera, there would be perhaps, to some extent at least, a widening of these cracks; but 

 in the end they would be filled with serpentine deposited from aqueous solutions from their walls, 

 and the resulting fibrous structure of the serpentine filling these seams represents the nearest 

 approach to a true crystallization that the mineral serpentine assumes except when it is found as 

 a pseudomorph after another mineral. It is probable that this chrysotile asbestos may have been 

 formed some time before the complete alteration of the primary rock into serpentine. . . . 



"A study of the occurrence of the chrysotile asbestos in the field shows that wherever commer- 

 cial quantities of this asbestos are found they are in the serpentine and close to its contact with 

 the surrounding country rock or adjacent to masses of the country rock that have been included 

 in the serpentine. Farther away from these masses of gneiss or other country rock the chrysotile 

 variety begins to give out. With very few exceptions, all the fibers of the asbestos are standing at 

 nearly right angles to the sides of the seams, which would conclusively show that they were not 

 formed by any shearing movements of the rocks." 



fG. P. Merrill ; Proc. U. S. Nat. Mus., vol. xi, 1888, pp. 105-112. 



