4 CONTENTS. 



CHAPTER I. GENERAL GEOLOGY Continued. Page. 

 Description of the rock formations Continued. 

 Tertiary lavas Continued. 



Rhyolites and dacites Continued. 



Fraction dacite breccia Continued. 



Relative age 40 



Microscopic characters 40 



Tonopah rh yolite-dacite 41 



Appearance 41 



Microscopic characters 41 



Alteration near contacts 41 



Distinction between the northern and the southern areas 42 



Age and origin 43 



Brougher dacite 44 



Location 44 



Volcanic necks 44 



Contact phenomena 44 



Dikes from main masses 44 



Included basalt : 45 



Vestiges of cinder cones 45 



Formation of the present Brougher dacite 46 



Flow structure and other phenomena 46 



Faulting due to Brougher dacite eruptions 47 



Tuff dikes near contacts 47 



Mineral composition 48 



Oddie rhyolite 4!) 



Location 49 



Contact phenomena of Oddie-Rushton neck 49 



Contact phenomena of Ararat neck 49 



Smaller necks 50 



Relative age of Oddie rhyolite 60 



Mineral composition 50 



Latest rhyolite or dacite 51 



Location 51 



Age and origin 51 



Mineral composition 51 



Siebert tuff (lake beds) 51 



lacustrine origin 51 



Size of the lake 52 



< )rigin of lake basin 52 



Thickness of sedinu-nti 53 



Conditions during deposition 53 



Explosive eruptions of the lake period 54 



Uplift terminating lake period 54 



Basaltic eruptions 55 



Regional tilting accompanying uplift 55 



