ABSENCE OF PROPYLITE. 279 







His views, which can not well be abridged here, will be found admira- 

 bly stated in his chapter devoted to a discussion of the genesis of volcanic 

 species, in which he treats of geological causes leading to the formation 

 of local lakes of lava. 



Later observations. Since the publication of King's memoir the study of 

 volcanic rocks has progressed with rapid strides, and nowhere have they 

 been investigated with more untiring energy than in the Cordillera of 

 North America. Notwithstanding our knowledge of the rocks of the 

 Washoe District and the Coinstock Lode, derived from the works of Rich- 

 thofen and King, later study of them, aided by methods of microscopical 

 research, has developed fresh points of interest bearing upon their order of 

 succession and mutual relations. After a thorough examination of the 

 propylites, Mr. George F. Becker 1 has shown that they can not be separated 

 from the andesites as an independent rock species based upon any mineral- 

 ogical distinctions, since the peculiar habitus of the propylite is due to 

 chemical change and decomposition of the constituent minerals. Moreover, 

 the propylites and andesites are found to pass into each other by gradual 

 transitions. 



Hague and Iddings, 2 in the course of their examination of the Washoe 

 rocks, confirmed the results of Mr. Becker so far as the identity of the 

 propylite and andesite is concerned, and also failed to see any geological 

 evidences of a preandesitic eruption. 



Similar views as regards the independence of propylite are now main- 

 tained by nearly all petrographers who have given much thought to the 

 subject or who attempt to classify volcanic rocks upon either a structural or 

 mineralogical basis. 3 



1 Geology of the Comstock Lode and the Washoe District, Washington, 1882. 



*On the development of crystallization in the igneous rocks of Washoe, Nevada; with notes on 

 geology of the district. Bull. U. S. Geol. Survey, No. 17, Washington, 1885. 



3 Since this chapter was written Prof. J. W. Judd has published an admirable paper on " The 

 Propylites of the Western Isles of Scotland, and their Relation to the Andesites and Diorites of the 

 District." He revives the use of the term propylite, but in the strict sense suggested by Rosenbusch, 

 regarding it simply as a "pathological variety" of andesite. His detailed descriptions identify the 

 Scottish rocks with similar rocks found iu Hungary. From his description it would be difficult to 

 distinguish thorn in any particular from the altered andesites of the Washoe District in the Virginia 

 Range. They even show the development of metallic sulphides. Quart. Journ. Geol. Soc., vol. XLVI, 

 pp. 341-382. London, 1890. 



