CLASSIFICATION OF ORE DEPOSITS. 



373 



All of the above are an ativance upon von Cotta in that form is not in all 

 cases the exclusive basis of classification. Whitney's first two subdivisions 

 are distinctly genetic, but the third, which embraces the majority of metal- 

 liferous deposits, is an unsystematic grouping of a variety of forms having 

 only one common quality, that of not being stratified. Whitney recog- 

 nizes a genetic quality in his division a, that of being of eruptive origin, but 

 few geologists of the present day agree with his wide application of this 

 quality — for instance, to the great deposits of magnetic iron of Missouri and 

 Lake Superior. In his "segregated veins" he recognizes the possibility 

 of an unstratified deposit which is not the filling of a pre-existing cavity, 

 while no such recognition is found in Geikie's classification. Geikie's term 

 "subsequently introduced ores," on the other hand, is to be preferred to 

 " unstratified deposits," as being based on a more essential characteristic 

 of the deposit. This would involve, however, a definite statement as to 

 the age of Whitney's Classes III, a and b, which his general term avoids. 



