ABSTRACTS AND DISCUSSIONS OF PAPERS 403 



4. Period. Bornite. Chalcopyrite. 



Covellite. Bornite. 



Chalcoeite. 



5. Period. Chalcoeite. 



Those ill the second column are the minerals of any one period, while the 

 minerals in the third column are of secondary importance as ores, but of great 

 interest from a scientific point of view. 



Presented in full extemporaneously. 



Discussion 



Then followed general discussion of all papers bearing on ore deposition. 



Prof. A. F. Rogers gave a full summary of the evidence on the temperature 

 of the formation of sericite. 



Mr. Sidney Paige questioned whether sericite is not a low-temperature min- 

 eral and the result of weathering. 



Professor Rogers gave reasons for believing that sericite is the result of 

 hydrothermal alteration and not of weathering. 



Dr. E. S. Bastin gave an appreciation of the work being done at Stanford 

 University on ore deposits. 



Prof. C. F. ToLMAN, Jr., called attention to the complex relation discovered 

 by the metallographic examinations of ores and the care that should be used 

 in interpreting these. 



STRUCTURE OF THE SOUTHERN SIERRA NEVADA 

 BY JOHN P. bulwada" 



(Abstract) 



The rocks of the southern Sierra Nevada may be grouped into two divisions, 

 as are those of the central and northern Sierra: a basement complex consist- 

 ing of pre-Cretaceous intrusive rocks and metamorphosed stratified forma- 

 tions intensely deformed, and a superjacent series, made up of igneous and 

 sedimentary rocks but little deformed and lying with marked unconformity 

 upon the basement complex. 



The structure of the few remnants of stratified rocks in the basement com- 

 plex of the southern Sierra corresponds to that of the pre-Cretaceous rocks in 

 the more northerly portions of the range; the strata dip steeply and usually 

 strike approximately north-south. The structure of the rocks of the super- 

 jacent series in the southern Sierra differs markedly from that in the central 

 and northern Sierra. Instead of lying nearly flat, they have been folded along 

 both the northwestern or San .loaquin Valley side and the southeastern or Mo- 

 jave Desert liordcr of the range, as well as in the summit region. The struc- 

 ture of the Siena thus comes to resemble that of the Coast Ranges somewhat 

 as the Sierran range approjiches the coast;il mountains. 



Tiie great fault zone which l)ord(>rs the central and northern Sierra on the 

 east continut's along the southeastern face of the southern Sierra to the Coast 

 Ranges. 



"* Introduced by A. C. I.awson. 



