1914] 



Lawson: Ore Deposition by Meteoric Waters 



223 



understand why lie adopts the hypothesis of magmatic waters as the 

 only one explanatory of the origin of such contact deposits. It 

 behooves me therefore to demonstrate this possibility. I will first, 

 however, disclaim the belief, which I fear w r ill be imputed to me if I 

 do not anticipate the imputation, that the sedimentary rocks of the 

 earth's crust everywhere contain similarly large quantities of water. 

 The water in rocks is very unequally distributed and is in general 

 more abundant in the siiperficial portions of the crust than in the 

 deeper, though under certain conditions this relation may be reversed 

 as, for example, in certain artesian basins. The inequality of dis- 

 tribution is, however, due to the variation in size or in prevalence, or 

 both, of the voids in the rocks. In some rocks such as shale, the voids 

 may be so small and the movement of the water through them may 

 be so slow that in an excavation they may appear to be dry. Thus, 

 in certain coal-mines beneath the sea, where the coal-seams lie between 

 beds of shale, the workings may be so dry as to be dusty. The escape 

 of water from the shale is less than can be evaporated. Nevertheless 

 all sedimentary rocks below the water-level are saturated to an un- 

 known but great depth. But saturation does not imply an abundant 

 flow to a conduit or fissure. The rate of flow from rocks in the 

 direction of negative pressure is a function, not of the percentage of 

 volume occupied by water, but of rock texture as expressed in the size 

 of the voids in which the water is held. "With the increasing pressure 

 that comes with depth and as a result of cementation, these voids 

 diminish in size, thereby decreasing the proportion of water which 

 the rock is capable of holding as well as the rate of flow. But making 

 all allowance for this diminution of storage capacity there is a vast 

 quantity of water contained in the sedimentary rocks in their unaltered 

 state at all depths ; and at high temperatures, which are an essential 

 condition of our problem, the rate of flow and therefore the rate of 

 escape to fissures, etc., is undoubtedly greatly accelerated. Moreover, 

 brittle rocks such as quartzite, sandstone, and limestone, which have 

 been folded and otherwise disturbed, are usually traversed by frac- 

 tures, faults, and joints, and these, together with the partings of 

 stratification, afford comparatively free access of surface waters to the 

 limit of the zone of fracture for such rocks. That limit for strong 

 rocks is very much deeper than Van Hise 6 would have us believe, as 

 we know from the experimental work of Adams. 7 



6 U. S. G. S. 16th An. Rpt., pt. I, p. 593. 

 i Journ. Geo]., vol. 20, pp. 97-118, 1912. 



