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



solution, result in expansion. T. S. Hunt showed* that the passage of 

 olivine into serpentine under such conditions would result in an increase 

 in bulk amounting to 33 per cent.f That some material is almost in- 

 variably lost we have abundant proof. Nevertheless, expansion at some 

 stage in the serpentinizing process usually results, and it is to the inci- 

 dental readjustment of the rock mass that is commonly attributed the 

 characteristic jointed condition and the slickensided surfaces.]; These 

 joint faces are often coated with platy and fibrous material, and bear out 

 the author's opinion, expressed elsewhere,§ to the effect that ordinary 

 asbestiform structure among amphibolic and pyroxenic minerals is due 

 to shearing stresses, the elongation taking place along the line of least 

 resistance and being in some cases but an exaggeration of the normal 

 cleavage property. Such structures are, however, quite different and 

 independent of the veins we are now discussing. For the production of 

 these last quite different causes need be evoked. 



Fritz Cirkel, in a paper on the occurrence and mining of asbestos in 

 Canada,|| mentions the two prevailing theories. The one, that the fis- 

 sures were formed in the serpentine magma as a result of cooling and 

 contraction and subsequently slowly filled through a process of lateral 

 secretion, and the other, seemingly ignoring the fissures, deriving the 

 asbestos from the alteration of olivine and serpentine at high tempera- 

 tures. No preference is indicated for either theory, nor does he express 

 independent views of his own. 



In the course of a discussion with Professors T. N. Dale and J. F. Kemp 

 the suggestion was advanced by the one that the fissures might be the 

 result of tension or a stretching movement, and the other that they are 

 due either to dynamic causes or produced by a shrinkage due to a loss 

 in silica in the process of alteration (serpentinization). If due to ten- 

 sion, it would seem to the writer that they should conform to one or 

 more definite and determinable directions. Such, however, is apparently 

 not the case, the veins being, as a rule, even more irregular in their direc- 

 tion and distribution than shown in our plates and figures, and the opin- 

 ion held by the present writer is more in harmony with the hypothesis 

 of shrinkage.^! 



* Mineral Physiology and Physiography, p. 506. 



f According to Professor Sollas but 30 per cent. See Geol. Mag., vol. ii, 1895, p. 259. 



X See " On the serpentines of Montville, New Jersey." Proc. U. S. Nat. Mus., vol. xi, 1888, p. 105. 



§The Non-Metallic Minerals, p. 183. 



|| Zeit. fur praktische Geologie, vol. ii, 1903, p. 123. 



fi The possibility of the vein cavities being produced by a torsional stress could be definitely 

 decided could it be shown that the veins were themselves of more recent origin than the joints, 

 since, under these conditions, any probable stress would find relief along the old lines of weakness 

 rather than in the production of new rifts. Proof, if such shall be found, that the veins are older 

 than the joints, while not in itself conclusive, would render the torsional theory less objection- 

 able, though even then the lack of any definite direction in their trend would be wellnigh fatal 

 to any such conclusion. 



