1886.] NATURAL SCIENCES OF PHILADELPHIA. 283 



crystals. The analj-ses of the mineral from these places do not 

 give the ratio of 1 : 1 for Ag and Cu either, but a little closer than 

 the present locality of Zacatecas. Since Ag.^S and Cu.^S may 

 replace each other isomorphically in all proportions without 

 question, we may look upon such a ratio as Stromeyerite demands, 

 1 : 1 , as quite an exception, owing no doubt to peculiar condi- 

 tions present at the time of crystallization. 



The material for this investigation was obtained from Dr. A. E. 

 Foote, who collected it during his recent visit to Zacatecas and 

 other mineral localities in Mexico. 



Christian E. Metzler and Charles H. Marot were elected 

 members. 



The following was ordered to be published : 



