652 RECORD OF MEETINGS OF THE 



from one sixth to one hundredth of the contained metallic 

 mineral might be leached out. 1 When, therefore, as is often 

 the case in monographs upon the geology of a mining district, 

 inferences are drawn as to the possibility of deriving the ore 

 of a vein by the leaching of wall-rocks whose metallic contents 

 have been proved by assay, the total available contents ought 

 to be divided by a number from six to one hundred if the above 

 reasoning is correct. This diminution will tend to modify in 

 an important manner our belief in the probability of such 

 processes as have been hitherto advocated. We may justly 

 raise the following questions : How closely set, as a matter 

 of fact, are the cracks which are large enough to furnish solution 

 water-ways in the above rocks, and can we reach any definite 

 conception regarding their distribution? Some quantitative 

 idea of the relations may be obtained from the tests of the 

 recorded absorptive capacity of the igneous rocks which are 

 employed as building stone. G. P. Merrill in his valuable work 

 on Stones for Building and Decoration, pp. 459, has given 

 these values for thirty-three granites and four diabases and 

 gabbros. They vary for the granites from a maximum of 

 one twentieth to a minimum of one seven hundred and fourth. 

 I have averaged them all and have obtained one two hundred 

 and thirty-seventh as the result. That is, if we take a cubic inch 

 of granite and thoroughly dry it, it will absorb water up to 

 one two hundred and thirty-seventh of its weight. The volume 

 of this water indicates the open spaces or voids in the stone. 

 The average of the specific gravities of the thirty-three granites 

 is 2.647. If . by the aid of this value, we turn our weight of water 

 into volume we find that its volume is one ninetieth that of 

 the rock. For the four diabases and gabbros, similarly treated, 

 the ratio of absorption is one three hundred and tenth; the 

 specific gravity is 2.776 and the ratio of volume one one hundred 

 and tenth. We can express all this more intelligibly by saying 



1 With regard to the flow of waters through crevices and the relation 

 of the flow to varying diameters or widths, a very lucid statement will 

 be found in President C. R. Van Hise's valuable paper in the Transactions 

 of the American Institute of Mining Engineers, XXX, 41, and in his 

 monograph on Metamorphism. 



