436 H. C. BIDDLE 



under which the oxidation of ferrous salts may result in the 

 deposition of copper are those which obtain in the circulation of 

 underground waters. The theory of Pumpelly and others based 

 on paragenetic relations is thus fully sustained by chemical evi- 

 dence. 



H. C. BiDDLE. 



University of Chicago, 

 June 1901. 



