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University of California Publications in Geology [Vol. 13 



was stated to be fan-shaped, the richest ore being at the center of the 

 fan where the midpoint of the vein rested upon a strong shear joint 

 or flat fault. Briefly, all these and similar phenomena appear to find 

 their explanation in the stresses existing in the vein walls during vein 

 deposition, the richness of the ore deposited being inversely propor- 

 tional to the later compressive stress in the walls. In the description 

 of the veins I attempted to show evidence indicating that they were 

 due to the metasomatic replacement of the wall rocks. I regard it as 

 obvious that the replacement was grain-for-grain in the sense of 

 volume-for-volume and not molecule-for-molecule, since the veins in 

 mineral character, structure, and all other respects are the same in 

 diabase, Keewatin, and Cobalt Series, and the pebbles in the Cobalt 

 conglomerates consist of virtually every sort of igneous and sedi- 

 mentary rock, and yet are replaced indiscriminately and in the same 

 manner by smaltite and its associates. 



The even tabular dimensions of many veins, and the rare and 

 anomalous occurrence of crustifieation as against the general massive 

 character of the vein matter must be regarded as a priori evidence 

 that the veins do not represent chamber fillings, since such chambers 

 could not have existed under the conditions of origin of these fissures. 

 In other cases the gradual transition between solid vein, vein with 

 band-like inclusions of wall rock, unchanged in orientation, walls with 

 parallel veinlets, and slightly altered walls, would seem to constitute 

 equivocal evidence of replacement. 



It seems clear, largely upon the basis of the vein characteristics 

 already described, that a vein grew outward from the original narrow 

 fissure as a starting point. It makes no difference, however, if one 

 prefers to consider that the fissure, tight as it must always have been, 

 remained open until the last, and then was sealed in such a manner 

 as to show a total absence of comb structure. In either case the vein 

 minerals were deposited in the midst of firm rock, and must have 

 traversed either firm ore or firm rock in order to be deposited on the 

 vein margins. 



At this point, lest it might be thought that the last step in the 

 delivery of the ore to its destination was accomplished by passage 

 through colloidal vein matter, I must object that although the pres- 

 ence of colloids is often demonstrable or very plausible at or near the 

 earth's surface, or in the vadose region, I believe the opinion of chem- 

 ists will support me in denying such possibilities in the depths, where 

 there is considerable heat for long periods of time, and particularly 



