LA VAS IN THE GREA T BASIN REGION 625 



4. Earlier diabase. 



5. Later diabase. 



6. Earlier hornblende andesite. 



7. Augite andesite. 



8. Later hornblende andesite. 



9. Basalt 



Messrs. Hague and Iddings, 1 after a careful microscopic 

 study of the collections made by Mr. Becker in the Comstock 

 region, arrived at somewhat different conclusions, although 

 agreeing with Mr. Becker as to the identity of propylite with 

 andesite. They concluded that the granular diorite and diabase, 

 and the augite andesite, were variations of the same body, the 

 granular rocks representing textural differences brought on by 

 slowly cooling in the deeper parts of the extruded mass, while 

 the finer-grained porphyritic rocks represented the periphery of 

 the same. To substantiate their conclusion they show the exist- 

 ence of all possible gradations between the extreme types . 

 They also conclude that the porphyritic diorite is identical with 

 the hornblende andesite, and the mica diorite with the later 

 hornblende andesite, the difference in each case being due to 

 variations of texture. The quartz-porphyry of Mr. Becker they 

 regard as partly dacite, and partly rhyolite ; while the later 

 diabase, or " black dike," they regard as identical with the effu- 

 sive basalt. They find, also, that the pyroxene and hornblende 

 andesites are difficult to separate, but that these are cut through 

 by hornblende andesites, dacites, rhyolites, and basalts. The 

 succession of lavas in this district is, according to Mr. Hague : 



1. Pyroxene-hornblende andesite (inner portions pyroxene-hornblende 



diorite porphyry, and pyroxene-hornblende diorite). 

 Period of volcanic rest and denudation. 



2. Hornblende-mica andesite. 



3. Dacite. 



4. Rhyolite. 



5. Pyroxene andesite. 2 



6. Basalt. 



1 " On the Development of Crystallization in the Igneous Rocks of Washoe," 

 Bull. 17, U. S. Geol. Surv. 



2 See Mon. XX, U. S. Geol. Surv., p. 281. Compare the succession in the adja- 

 cent Pine Nut Range, p. 628. 



