THEORIES OF ORE DISPOSITION HISTORICALLY CONSIDERED. 327 



proved the metals he found to be necessarily original constituents of 

 the rocks in which they were supposed to occur. Whatever opinion 

 may be held as to the merits of Sandberger's theory, as such, it un- 

 doubtedly contributed to the advance of the study of ore deposits in 

 stinudating- what may be called " verification ;" that is, the practical 

 testing of theory in its ap})lication to concrete instances in nature. 



In general it may be said of the period ihat was now closing that, 

 though facts of obsei'vatiou and exj)eriment had been accumulating, 

 the advances in the study of ore deposits during that time were much 

 less than those that had been made in other branches of geological 

 science. 



THE VERIFU'ATrON PEKKn). 



The third period, covering in a general way the last quarter of the 

 past century, may be called the period of verification. So fertile 

 had been the imagination of previous thinkers on this subject that at 

 this time it was practically impossible to conceive a theory of origin 

 for a given ore deposit that had not already been j^roposed or at least 

 suggested. The investigations now to be carried on with more perfect 

 methods, or in the light of recent advances in the science, would seem 

 more properly verifications of old theories than the propounding of 

 new ones. 



Method and the microscope have been the two great agents of 

 progress. The greatest impro\'ement in method has resulted from 

 government aid, under which it has been possible for organized bodies 

 oi scientific workers to make special examinations of entire mining 

 districts, and thus determine all the facts bearing upon ore deposi- 

 tion in those districts with an exhaustiveness that was impracticable 

 for the unaided individual observer. The newly created science of 

 microscopical petrography, through the intimate knowledge it has 

 afforded of the internal structure of rocks and ores, has admitted so 

 accurate a determination of the processes by which they have been 

 formed that much that was formerly mere conjecture has become 

 established on basis of fact. America, which hitherto had occupied 

 a very subordinate position, had come to the front, not only in the 

 production of metallic ores. I)iit in its correct understanding of the 

 processes by which they wei'e foi'uied. 



In order to pro])erly appreciate the i)i()gress which has Ijeen made 

 during this period, one nnist endeaxor to realize the mental stand- 

 point of the average student at the close of the preceding period. 



To the miner and [)rospector, whose opinions carry weight because 

 of their wide j)ractical experience, a typical ore dei)osit was a vein 

 which, once an open crack extending to an indefinite de})th, had been 

 filled by material introduced in one way or another from below, and 

 the more nearly a deposit approached this typical form the greater 



