746 



SCIENCE. 



[N. S. Vol. XIV. No. 359. 



that one rock changes into another rock ; 

 that rocks may have different textures, 

 structures and compositions from those 

 they originally had. The chief immediate 

 agent producing these changes is under- 

 ground water. Everywhere underground 

 water permeates the rocks. Everywhere 

 it is the medium of exchange by means of 

 which the mineral particles are changed 

 from one form to another. 



In order, therefore, to understand this 

 matter of the alterations of rocks, it be- 

 came necessary to consider the flowage of 

 the underground water ; to attempt to as- 

 certain, if possible, how it moves through 

 the rocks, in what directions, to what 

 depth it penetrates, whence it comes, 

 whither it goes. After a certain number 

 of principles and conclusions had been 

 reached upon the alterations of all rocks, 

 by all processes — and especially by the 

 work of underground water with the help 

 of heat derived from the igneous rocks, 

 from dynamic action, and the increment 

 due to depth — it seemed to me that many 

 of the principles of ore deposition followed 

 as corollaries from these general principles. 

 Therefore, it is from this general point of 

 view that the question is discussed to- 

 night ; not from that of a particular ore 

 deposit or particular district. 



For many years ore deposits have been 

 classified into those which are produced (1) 

 by the direct action of igneous agencies, (2) 

 by the process of sedimentation and (3) by 

 the action of underground water. There is 

 no difference of opinion as to the existence 

 of all these three classes ; but there is a 

 difference of opinion as to their relative 

 importance. Some hold that deposits of 

 igneous origin are of very great impor- 

 tance. By this I mean ore deposits which 

 have been directly formed by some of the 

 strange processes of vulcanism. Also it is 

 certain that large quantities of ore, and es- 

 pecially the placer gold deposits, are largely 



the result of the process of sedimentation. 

 Bat I hold that the greater number of ore 

 deposits, those which contribute most to 

 the wealth of Colorado, to the entire Cor- 

 dilleran region, to the Lake Superior re- 

 gion, to the Mississippi Valley, to the 

 Appalachian region, are those deposited by 

 underground water ; that is, they are the 

 direct result of the work of the permeating 

 solutions, which go for at least a considerable 

 depth below the surface of the earth ; and 

 are taking material into solution all the time, 

 depositing material from solution all the 

 time. During thejourney these underground 

 waters take up from igneous, aqueous 

 and metamorphic rocks the sparsely dis- 

 seminated metallic material which is of 

 consequence to man. This material is 

 deposited in the openings of the rocks and 

 within the more easily replaced rocks in 

 sufficient abundance to form ore deposits. 

 With the exception of iron, the quantity 

 of metal which is contained in an ore is 

 ordinarily small ; in the case of gold, usually 

 an exceedingly small fraction of one per 

 cent.; in the case of silver, usually less than 

 one per cent.; in the cases of copper, lead 

 and zinc, from one per cent, to a rather high 

 percentage, but in the great majority of in- 

 stances less than 20 per cent. It therefore 

 appears that the majority of so-called ores 

 consist mainly of deposited materials other 

 than the metals which are extracted from 

 them. This predominant material is known 

 as gangue, and plainly was deposited simul- 

 taneously with the ores, in the openings of 

 the rocks, or else replacing rock material. 

 In a given instance to attempt to answer 

 the question as to the source of the gold or 

 the silver or any other metal, without at 

 the same time considering the minerals 

 with which it is associated, is futile. If we 

 can answer the question as to where the 

 gangue minerals came from, and how they 

 got into the positions they now occupy, the 

 question is in large measure answered as to 



