THE POST-JURASSIC IGNEOUS ROCKS OF SOUTH- 

 WESTERN NEVADA^ 



SYDNEY H. BALL 



INTRODUCTION 



Three mighty periods of igneous activity mark the geological 

 history of southwestern Nevada and eastern California. The latest 

 of these, the Tertiary, has been ably treated by Spurr^ while con- 

 cerning the earliest, the pre- Cambrian, we know little. It is the 

 purpose of this article to describe briefly the period of igneous activity 

 beginning in post- Jurassic time and ceasing long prior to the Eocene. 



During the field season of 1905 the writer made a reconnaissance 

 of some 8,500 square miles in southwestern Nevada and eastern 

 CaHfornia for the United States Geological Survey.^ This area lies 

 between 36° 30' and 38° North latitude and 117° 30' and 116° West 

 longitude (see Fig. i). 



GENERAL STATEMENT 



The earhest of the post-Jurassic igneous rocks recognized in the 

 area are the granular siliceous intrusive rocks; next came the intru- 

 sion of quartz-monzonite porphyry, poor in ferromagnesian minerals, 

 while the youngest of the pre-Tertiary rocks are diorite and diorite- 

 porphyry. 



DISTRIBUTION 



The post- Jurassic igneous rocks are widely distributed over the 

 area under consideration although they cover larger expanses in 

 its western portion, and are the predominant formation from longi- 

 tude 117° 30' westward to the Sierra Nevada. The Tertiary lavas 

 and sedimentary rocks and the Pleistocene and Recent terrestrial 

 deposits cover vast stretches so that a cursory examination of a 

 geological map of Nevada gives but little idea of the importance of 



1 Published with the permission of the Director of the U. S. Geological Survey. 



2 J. E. Spurr, Journal of Geology, Vol. VIII, pp. 621-46. 



3 See Bull. U. S. Geological Survey No. 308. 



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