374 GEOLOGY AND MIXING INDUSTRY OF LEADVILLE. 



Pumpelly's classification ignores the division of stratified or contem- 

 poraneous ore deposits, and in his text he states his belief that the greater 

 number of ore deposits have been formed later than the inclosing rock ; 

 he also says that all metalliferous aggregations are the result of a proces.8 

 or series of processes of concentration. 



Posepny states his opinion on contemporaneous deposits even more 

 strongly in the following words : l 



In the course of my nearly twenty-years studies of ore deposits I have yet met 

 with no deposits (carrying sulphides) which answer to Werner's definition that is, 

 whose ores are contemporaneous with the country rock and which form a regular 

 interstratifled bed between other rock strata. 



Like Posepny", Pumpelly recognizes the importance of deposits which 

 do not fill pre-existing cavities, devoting to these his subdivisions I and II. 

 These he says fall under two heads, as regards the manner in which the 

 space occupied by them was obtained : (1) by mechanical displacement of 

 the inclosing material ; (2) by a chemical replacement similar to that to 

 which pseudomorphs owe their origin. His use of the form as a basis of 

 subdivision for deposits tilling pre-existing cavities seems more legitimate 

 than in the case of those which his title seems to imply are merely concen- 

 trations of metallic minerals already existing in the rock, and the use of 

 the word "concentration," as applied exclusively to the latter classes, seems 

 unfortunate, as implying that the others are not concentrations also. 



Geikie's classification has the merit of conciseness and his principal 

 divisions are based on genetic principles, but his subdivisions, like those of 

 von Cotta, recognize only differences of outward form. 



In view of the difficulty, or even, in many cases, the apparent impossi- 

 bility, of determining definitely the genesis of a given deposit, it may well 

 be questioned how far it is advisable to adopt genetic relations as the basis 

 of a classification, since it will frequently happen that an observer will be 

 at a loss to determine under which subdivision the deposit he is studying 

 should be placed. It seems to the writer, however, that in such a case, 

 although his determination may not be final and may give rise to discussion 

 and difference of opinion on the part of other observers in the same field, 



'Op. cit., p. 423. 



