LETTER OF TRANSMITTAL. 
Department of the Interior, 
United States Geological Survey, 
Division op Chemical and Physical Research, 
Washington, I). C, July 5, 1901. 
Sir: The subject of the secondary enrichment of ore bodies is one 
of the most important in chemical geology, and probably also one of 
the most complex, because nothing Likechemical homogeneity is to be 
anticipated at <>r beneath the croppings of ore deposits. A long time 
must elapse before the processes involved are fairly clear. 
The memoir by Dr. II. N. Stokes which I transmil herewith, for 
publication in the Bulletin series, is an important contribution to this 
subject, describing a method Cor the quantitative determination of 
pyrite and marcasite when in mixture, and discussing some of the 
relations of these most abundant sulphides to those of copper. This 
memoir represents only a portion of Dr. Stokes's work in this field, 
and it is to be expected that other papers will follow as soon as 
practicable. 
Very respectfully, your obedient servant, 
George l*\ Becker, 
(1< ologisi in ( 'hargt . 
Hon. Charles I). Walcott, 
Director of I 'nihil States Geological Survey. 
