20 S. F. EMMONS — THEORIES OF ORE DEPOSITION 



to make special examinations of entire mining districts, and thus deter- 

 mine all the facts bearing upon ore deposition in those districts with an 

 exhaustiveness that was impracticable for the unaided individual ob- 

 server. 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 con- 

 jecture 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, but in its correct understanding 

 of the processes by which they were formed. 



In order to properly appreciate the progress which has been made dur- 

 ing this period one must endeavor to realize the mental standpoint of 

 the average student at the close of the preceding period. 



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 its value. In- 

 deed, for a time, some of the most valuable deposits in the West were 

 entirely neglected by the prospector because they did not possess the 

 physical characteristics of the "true fissure vein." This misconception 

 arose from the fact that this, being^the most clearly defined form of de- 

 posit, had been the only one mentioned in early speculations, and that 

 hitherto the classification of text-books, based as they were on the almost 

 accidental characteristic of form, relegated other types of deposit to a dis- 

 tinct and relatively subordinate class, disregarding the fact that this 

 class includes many of the largest and most productive ore bodies which 

 may not only have the same origin, but often be associated in the same 

 deposit with a typical fissure vein. 



Von Groddeck (on the other hand), who represents the most advanced 

 scientific opinions of his time (1879), divides ore deposits into two classes : 



1. Those formed contemporaneously with the enclosing rock, whether 

 (a) sedimentary or (b) eruptive. 



2. Those of later formation classed under two heads : 



a. Those filling preexisting open spaces ; 



b. Metamorphic deposits formed by alteration of rock in place. 

 His two main divisions corresponded to a certain extent with those 



made in 1854 by J. D. Whitney (Metallic Wealth of the United States), 

 namely, stratified and unstratified. One difference is that metamorphic 

 deposits were included by Whitney in the first division and by Von 

 Groddeck in his second. Neither recognized their true importance, and 

 the latter, while admitting that he included in this class those that 



