Wadsworth.j 202 [May 7, 



be fresh, unaltered, and free from any foreign inclusions that 

 would influence the result. Such materials could only be ob- 

 tained from recent lava flows, recently formed limestones, etc., for 

 no rock that has been exposed for a considerable length of time to 

 the earth's meteoric agencies can, in the writer's opinion, be said 

 to be in its pristine condition. Most analyses of such rocks 

 have dealt too little in tests for minute quantities of such mater- 

 ials as comprise the more valuable ore deposits, to permit as yet 

 any general conclusion to be drawn. The nearest approach we 

 have to such analyses is in the meteorites, which are unaltered and 

 which in composition and structure are closely allied to certain 

 classes of terrestrial basic rocks. 1 These meteorites are found 

 to contain copper, tin, nickel, cobalt, arsenic, zinc, manganese, 

 chromium, and graphite. 



While it would appear probable that the elements of the useful 

 ores were often originally disseminated through the rocks with 

 which they are associated and subsequently concentrated, by the 

 agency of percolating waters, proofs that this theory is correct are 

 yet wanting, the theory resting mainly on the observed structure 

 of the ore deposits, their association and the alteration of the ad- 

 jacent rocks. 



Of ail theories that have been proposed to account for ore de- 

 posits, there are few, which are not correct for some form of 

 ore deposits in certain localities, while the practical use of these 

 theories is to aid us in understanding the nature of the deposits, 

 as a guide in their exploitation. The difficulty in the use of these 

 theories lies in their abuse, through their indiscriminate application 

 to all deposits. Our rule ought to be to study every deposit 

 thorough^, and after that study, not before, apply that theory which 

 best answers to the observed conditions, since all theories ought 

 to be generalizations or expositions of observed facts, with a 

 prophecy for the future. 



It is not doubted here that all ore deposits, not of a mechani- 

 cal or eruptive origin, can be attributed to the general alteration 

 and change in rocks, resulting from the general dissipation and 

 degradation of the potential energy of the constituents of the 

 earth's crust, in the universal passage of matter from an active state 

 towards a passive and inert condition. 



This general alteration manifests itself in a universal chemical 



1 Science, 1883, 1, 127-130. 



