24 



SCIENCE. 



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



volcanic action, they have passed through 

 the ground-water, and if this is carefully 

 impounded in thp upper levels of the mines 

 and not allowed to follow the workings 

 downward, it is found that there is not 

 only less and less water, but that the deep 

 levels are often dry and dusty. Along 

 this line of investigation, Mr. John W. 

 Finch, recently the State Geologist of Colo- 

 rado, has reached the conclusion, after wide 

 experience with deep mines, that the 

 ground-waters are limited, in the usual ex- 

 perience, to about 1,000 feet from the sur- 

 face and that only the upper layer of this 

 is in motion and available for springs. 



Artesian wells do extend in many cases 

 to depths much greater than this and bring 

 supplies of water to the surface, but their 

 very existence implies waters impounded 

 and in a state of rest. 



To this objection that the ground-waters 

 are shallow it has been replied that when 

 the veins were being formed the rocks were 

 open-textured and admitted of circulation, 

 but subsequently the cavities and water- 

 ways became plugged by the deposition of 

 minerals by a process technically called 

 cementation, and the supply being cut off, 

 they now appear dry. There must, how- 

 ever, in order to make 'the head' effective 

 have once been a continuous column of 

 water which introduced the materials for 

 cementation. It is at least difficult to un- 

 derfstand how a process, which could only 

 progress by the introduction of material in 

 very dilute solution, should by the agency 

 of crystallization drive out the only means 

 of its production. Some residue of water 

 must necessarily remain locked up in the 

 partially cemented rock. This residue we, 

 of coiirse, do not find where rocks are dry 

 and drifts are dusty. In many cases also 

 where deep cross-cuts have penetrated the 

 fresh wall-rock of mines, cementation if 

 present has been so slight as to escape de- 

 tection. 



If we once admit that this conclusion is 

 well based, it removes the very foundation 

 from beneath the conception of the meteoric 

 waters and tumbles the whole structure in 

 a heap of ruins. 



While I would not wish to positively 

 make so sweeping a statement as this about 

 a question involving so many uncertainties, 

 there is nevertheless a growing conviction 

 among a not inconsiderable group of geol- 

 ogists that the rocky crust of the earth is 

 much tighter and less open to the passage 

 of descending waters than has been gen- 

 erally believed ; and that the phenomena of 

 springs which have so miich influenced con- 

 clusions in the past, affect only a compara- 

 tively shallow overlying section. Such 

 phenomena of cementation as we see are 

 probably in large part due to the action of 

 water stored up by the sediments when 

 originally deposited and carried down by 

 them with burial. Under pressure a rela- 

 tively small amount of water may be an 

 important vehicle for recrystallization. 



It has been assumed in the above pre- 

 sentation of the case of the meteoric waters 

 that they are able to leach out of the deep- 

 seated wall rocks the finely disseminated 

 particles of the metallic minerals, but the 

 conviction has been growing in my own 

 mind that we have been inclined to over- 

 rate the probability of this action in our dis- 

 cussions. In the first place our knowledge 

 of the presence of the metals in the rocks 

 themselves is based upon the assay of 

 samples almost always gathered from ex- 

 posures in mining districts. The rock has 

 been sought in as fresh and unaltered a 

 condition as possible and endeavors have 

 been made to guard against the possible in- 

 troduction of the metallic contents by those 

 same waters which have filled the neighbor- 

 ing veins. But if we admit or assume that 

 the assay values are original in the rock: 

 and, in case the latter is igneous, if we be- 

 lieve that the metallic minerals have crys- 



