NEW YORK ACADEMY OF SCIENCES 653 



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 no. 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 on p. 226, it being 1 to 104. 

 We may conclude, therefore, that in so far as we can check 

 the previous conclusion by experimental data it is not far from 

 the truth. 



It maybe stated that the porphyritic igneous rocks which have 

 furnished nearly all the samples for the above analyses are as 

 a rule extremely dense, and that their absorptive capacity 

 is more nearly that of the compact granites than the open- 

 textured ones. It is highly improbable that underground water 

 circulates through these rocks to any appreciable degree except 

 along cracks which have been produced in the mechanical way 

 either by contraction in cooling and crystallizing, or by faulting 

 and earth movements. The cracks from faulting are very 

 limited in extent and in the greater number of our mining 

 districts they affect but narrow belts, small fractions of the 

 total. Of the cracks from cooling and crystallizing those of 

 us who have seen rock faces in cross-cuts and drifts under- 

 ground, where excavations have been driven away from the 

 veins proper, can form some idea, if we eliminate the shattering 

 due to blasting. My own impression is that in rocks a thousand 

 feet or so below the surface such cracks are rather widely spaced, 

 and that, when checked in a general way by the ratios just 

 given, these rocks are decidedly unfavorable materials from 

 which the slowly moving meteoric ground-waters (if such exist) 

 may extract such limited and finely distributed contents of the 

 metals. 



I have also endeavored to check the conclusions by the 

 recorded experience in cyaniding gold ores in which fine crushing 

 is so important, and I can not resist the conviction that we 

 have been inclined to believe the leaching of compact and 

 subterranean masses of rock a much easier and more probable 

 process than the attainable data warrant. 



