Wadsworth.] 418 [October 3, 



quartz-propylite, dacite, and its felsitic modification rhyolite." 

 (I. c. pp. 294, 299.) Dr. Becker also states that the predominat- 

 ing feldspar is orthoclase in this rock. He further remarks that 

 the porphyritic diorites, the diabases, the hornblende and augite 

 andesites have all, when decomposed, been taken for propylites ! 

 {1. c. p. 300.) 



My observations had led me to believe that many of the rocks 

 described as trachyte by Zirkel were not properly so classed. Dr. 

 Becker states that the rock of Mt. Rose which Zirkel regarded as 

 trachyte, but which I had taken to be an andesite, is really an 

 andesite. Further, he says that Dr. Hawes after separating the 

 feldspars and having them analyzed found none " corresponding 

 either physically or chemically to orthoclase. There is much rea- 

 son to believe that trachyte occurs less often than had been sup- 

 posed in the Great Basin area." (I. c. p. 300.) 



Mr. S. F. Emmons, in like confirmation of my paper, states 

 that the earlier classification of the western rocks as andesitic 

 and trachytic is doubtful, and that "many facts already observed 

 by us suggest a doubt whether von Richthofen's classification of 

 volcanic rocks will be found to hold good everywhere in Colo- 

 rado." 1 Nevadite I had considered to be a glassy rock, not crys- 

 talline as claimed by King, and I had assigned it to the trachytes 

 (W. 269 ; Science, i, 128) ; Messrs. Hague and Iddings have now 

 discoA r ered that it is glassy, but they place it in dacite — the so- 

 called quartz bearing andesite. They also state that their basalt, 

 hypersthene (augite), and hornblende-andesites, and dacite show 

 every intermediate grade between the types. (Am. Journ. Sci. 

 1883, xxvi, 222-235.) Since the publication of my paper, Profes- 

 sor Whitney has informed me that the mode of occurrence of the 

 propylite about Aurora and elsewhere, which he visited in com- 

 pany with Richthofen, indicates that it is but a local modification 

 of andesite, and that the results of his field studies are in accord 

 in all essential points with those independently obtained by my- 

 self from a different mode of study. 



It would appear to be now clearly shown that my previous 

 papers have been well sustained by the later published studies (so 

 far as they cover the same ground) of the United States Geolog- 

 ical Survey, and as great mistakes pointed out by others in Zir- 

 kel's work as any that I have asserted existed in it. 



1 Bull. U. S. Geol. Survey, 1883, p. 17. 



