26 



SCIENCE. 



[N. S. Vol. XXIII. No. 575. 



■a o o 



whose width bears the same relation to 

 the width of the rock mass as is borne 

 by the diameter of the particle of ore, 

 the chance of crossing a particle varies 

 from one in six to one in one hundred. 

 Or we may say that with cracks of this 

 spacing from one sixth to one one-hun- 

 dredth of the contained metallic mineral 

 might be leached out.^^ When, therefore, 

 as is often the case in monographs upon 

 the geology of a mining district, infer- 

 ences 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 rea- 

 soning is correct. This diminution will 

 tend to modify in an important manner our 

 belief in the pro,bability 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 



"'With regard to the flow of waters through 

 crevices and the relation of the flow to varying 

 diameters or widths a very lueid statement will be 

 found in President C. R. Van Hise's valuable 

 paper in the Transactions of the American Insti- 

 tute of Mining Engineers, XXX".', 41, and in his 

 Monograph on Metamorphism. 



cracks which are large enough to furnish 

 solution waterways in the above rocks, 

 and can we reach any definite conception 

 regarding their distribution? Some quan- 

 titative idea of the relations may be ob- 

 tained from the tests of the recorded ab- 

 sorptive 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 33 granites and 4 

 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 hun- 

 dred 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 33 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 ab- 

 sorption 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 that, if we assume a cube of granite 

 and if we combine all its cavities into one 

 crack passing through it, parallel to one 

 of its sides, the width of the crack will be 

 to the edge of the cube, as 1 to 90. In 

 the diabases and gabbros, similarly treated, 

 the ratio will be 1 to 110. These values 

 are very nearly the same as the average 

 of the ratios of the edges of the cubes of 

 rock and ore given in the table above, 

 it being 1 to 104. We may conclude, there- 

 fore, that in so far as we can check the 

 previous conclusion by experimental data, 

 it is not far from the truth. 



