THKOKTKS OF OKE DISPOSITION HISTORICALLY CONSIDERED. 327 



provod the inotals ho found to he necessarily original constitnents of 

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

 may he liehl as to the nii'rits of San(lher<2:ei-"s theory, as such, it un- 

 douhtedly contrihuted to the advance of the study of ore deposits in 

 stinudatin<>- what may he calknl "'verification;" that is, the practical 

 testin<2: of theory in its ajjplication to concrete instances in nature. 



Tn iioiici-al it may he said of the period that was now closing; that, 

 tiiouoh Tads of ohservation and experiment had been accunudating, 

 the advances in the sludy of ore deposits (hn'in<>' that time were much 

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

 i^'cience. 



THE VEKn'TCATION PKKIOU. 



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

 past century, may l)e 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 proposed 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 improvement in method has resulted from 

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

 of 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 b}^ which the}' have been 

 formed that much that was formerly mere conjecture has become 

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

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

 production of metallic ores, but in its correct understanding of the 

 ])r()cesses by which they were formed. 



In order to properly appreciate the progress which has been made 

 during this period, one must endeavor to realize the mental stand- 

 l)oint of the average student at the close of the preceding jieriod. 



To the miner and prospector, -whose opinions carry weight because 

 of their wide practical experience, a typical ore deposit was a vein 

 which, once an open crack extending to an indefinite depth, 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 



