CONTENTS. 5 



CHAPTER I. GENERAL GEOLOGY Continued. Page. 

 Description of the rock formations Continued. 

 Tertiary lavas Continued. 



Basalt 55 



Location 55 



Relations and composition of basalt of Siebert Mountain 56 



Age 56 



Chemical composition of lavas 56 



Transitions in silica content 56 



Analyses of Tonopah lavas 57 



Chemical composition of the dacite-rhyolite series 57 



Differences and relations 57 



Comparison with Eureka and Washoe dacites and rhyolites 58 



Retention of the term dacite 58 



Rhyolitic nature of both dacites and rhyolites 59 



Determination according to a quantitative classification 59 



Varying composition in different vents 60 



Theory of differentiation of Tonopah lavas from a uniform type. 61 



Pseudomorphs in rhyolite 61 



Character of pseudomorphs 61 



Magmatic origin of pseudomorphs 62 



Hornblende in Tonopah lavas 62 



Derivation of rhyolite and basalt from intermediate magma 63 



Statement of theory 63 



Rhyolite-basult differentiation theory tested by analyses 63 



Complementary nature of dacites and later andesites 64 



Statement of differentiation theory 66 



Summary of geological history 66 



Age of the rocks at Tonopah 68 



Place of Tonopah lavas in (ireat Basin volcanic history 68 



Probable Neocene age .". . . 69 



Infusoria in the Siebert tuffs 69 



Comparison of Siebert tuffs with Miocene Pah-Ute lake deposits 70 



Conclusion 71 



Principles of faulting 72 



Criteria of faulting 72 



Siebert tuff boundaries 73 



Dikes along fault-zones 73 



Boundaries of lavas 74 



Erosion fault-scarps 74 



Scarp phenomena west of Brougher Mountain 75 



Description of zigzag scarps 75 



Zigzag scarps explained by faulting 77 



Consequences of explanation 77 



Zigzag fault-scarp on Tonopah-Sodaville road 77 



Origin of zigzag fault-scarps 78 



Origin of zigzag faults 79 



