100 IOWA ACADEMY OF SCIENCES. 



monly followed in attempting to determine the amounts 

 of ore materials that rock masses are supposed to contain. 

 The endeavor is further to show that in the usual analysis 

 of rock materials for the metallic content, as an explana- 

 tion of the immediate source of the ores, there is a serious 

 fundamental eiTor, both in manner of procedure and in the 

 premises of the logical inference. In many cases the 

 results obtained have been not only indecisive in their 

 nature, and misleading in fact, but they have, in reality, 

 millitated directly against the very propositions they were 

 intended to prove. 



Stelzner and Posepy, in their attempts to show that 

 the results of Sandberger, Becker and others are in error 

 regarding the mineralogical source of ore minerals, have 

 manifestly allowed their zeal to swing the pendulum as 

 much too far in the opposite direction as they believed 

 their opponents had in the other. In particular cases, 

 Sandberger may be wrong in his conclusions, and his 

 methods may be indecisive in character, as Posepy claims. 

 But some of the assumptions of the latter regarding the 

 primary nature of certain metallic rock-forming minerals 

 cannot only not be proved, but the exact contrary has been 

 thoroughly demonstrated. There is an element of prob- 

 able error in the methods of these rivals; and it only differs 

 somewhat in kind. 



Some of these features may be pointed out later in con- 

 nection with the references to the modern microscopical 

 examinations of rocks. While it may be shown that the 

 metallic content of igneous rocks is a far more important 

 factor than is often supposed, the attainment of definite 

 data is an effort much more complex than has been gen- 

 erally believed. 



At the present time, it is generally conceded that an 

 important primitive or ultimate source of the metals is the 

 igneous rocks, either those already at the surface or deep- 

 seated bodies, solid or molten, but it is not considered that 

 the concentration of the metallic substances into ore-bod- 

 ies is in any way always connected necessarily with vol- 

 canic activity. 



