68 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



Mountain there was an explosive outburst of basaltic material, followed by a 

 thin basalt flow. Subsequently columns of liquid lava welled up and stood in 

 the vents of the volcanoes, but did not outflow. Some of these were composed of 

 dacite, some of rhyolite. As these columns cooled, heated waters rose along 

 their contacts and deposited chalcedony and other minerals, and mud dikes were 

 injected into the soft intruded rocks. The explosive outbreaks and the intrusion 

 of these large necks must have broken the rocks into blocks and displaced the 

 blocks, for at this time many faults were formed. 



On the cessation of this dacite- rhyolite period of volcanic activity there was 

 a collapse or depression around the vents. This sinking took place largely along 

 the fault planes, and was especially prominent around the volcanic necks, which 

 as they sagged dragged down blocks of the intruded older rocks with them. 



Since this time, which was probably somewhere in the Pliocene, erosion has 

 been active, stripping away the debris covering from the dacite-rhyolite necks, 

 and leaving them as hills, and in general removing the surface layers from the 

 hills to the desert valleys. 



AGE OF THE ROCKS AT TONOPAH. 



It is known that all these volcanic rocks are of Tertiary age. They belong 

 to a series of lavas which occupy a large part of the Great Basin and whose 

 Tertiary age has been established. 



Place of Tonopah lavas in Great Basin volcanic history. Some years ago" 

 the writer attempted to classify the known facts concerning the nature and 

 succession of the lavas in this region. He found that in many places the same 

 lavas occur in much the same relative quantity, have nearly the same mineralogical 

 composition, and give evidence of about the same relative age. Moreover, where 

 two or more of these lavas are found close together, their order of succession is 

 in general much the same, although at any given place certain members of the 

 series may be lacking. In no one locality has the complete succession, as 

 indicated by the correlation of all the sections, been observed; but in order to 

 find it, gaps in one place may be filled from observations in another. 



The result of this comparison was the separation of the Tertiary lavas into 

 five successively erupted groups, as follows: 



1. Rhyolites. 



2. Hornblende-biotite-pyroxene-andesites, followed by dacites. 



3. Rhyolites, Homi-tinies accompanied by basalts. 



4. Pyroxene-andantes. 



5. Basalts, sometimes acc'omi>amed by rliyolites. 



aSuccemlon and relation of lava In the Ureat Basin region: Jour. Geol., vol. 8. No. 7. pp. 621-646. 



