402 PROCEEDINGS OF THE CALIFORNIA MEETING 



mediately to the north. The following groups and zones are represented : 

 The Franciscan, Knoxville, Chico(?), Tejon, Vaqueros (Turrltella inezana 

 zone), Temblor (Turritella ocoyana zone), Monterey shales, Fernando, and 

 Pleistocene terrace deposits. Two thrust-faults are the main structural fea- 

 tures north of the Santa Ynez Mountains. An anticline which shows more 

 intense deformation in its eastern part forms the Santa Ynez Mountains. 



Kead in full from maniiscript. Discussion was deferred. 



INTERESTING CHANGES IN THE COMPOSITION OF THE SALTON SEA 



BY A. E. VINSON ^ 



{Abstract) 



This paper discussed the change in composition of the waters of the Salton 

 Sea developed by progressive evaporation. 



Presented in full extemporaneously. Discussion was deferred. 



EXAMPLES OF SUCCESSIVE REPLACEMENT OF EARLIER SULPHIDE MINERALS 

 BY LATER SULPHIDES AT BUTTE, MONTANA 



BY J. C. bay" 



(Abstract) 



The ore deposits at Butte, Montana, belong to the replacement type. They 

 include three distinct sets of ore minerals arranged zonally. These zones 

 grade into each other and the divisions must be made arbitrarily. They are ; 



First. Silver zone. Silver, lead, and zinc minerals predominating. 



Second. Zinc zone. Sphalerite predominating. 



Third. Central or copper zone. A complex series of copper minerals. 



This paper deals more particularly with successive and selective replace- 

 ment of earlier sulphides by later sulphides in the copper zone and the inner 

 border of the zinc zone. 



The importance of this type of replacement has, perhaps, not been fully 

 appreciated by students of ore deposits, and the present paper presents a few 

 examples of this process which the writer has termed progressive enrichment. 



The mineral sequence as determined for the Butte district is as follows : 



1. Period. Quartz-pyrite. Replacement of quartz-monzite 



Pyrite-quartz. along fractures. 



2. Period. Sphalerite. Chalcopyrite. 



Galena. 



Silver sulphides. 



3. Period. Tetrahedrite. Chalcopyrite. 



Enargite. Bornite ? 



Tennantite. 



13 Introduced by C. F. Tolman, Jr. 

 " Introduced bv C. F. Tolman, Jr. 



