286 BULLETIN OF THE 



The analyses of the trachytes seem best to bear out Mr. King's idea ; 

 but on looking over the list of analyses, and omitting those of rocks 

 which were not described since they lay outside of his district, only two 

 or three remain that can be considered as trachytes, viz. Nos. 154 and 

 155, and possibly 153. The rest belong to quartz-bearing basalts and to 

 andesites. The basalts that have been described as trachytes are not 

 doubtful ones, that might naturally have been classed with the trachytes, 

 but are well-marked specimens, identical with some that have been de- 

 scribed under the head of basalt in the same report. Attention should 

 also be called to the fact that, contrary to Mr. King's theory, the quartz- 

 bearing trachytes of Professor Zirkel's report are basic rocks (true basalts), 

 and his typical augitic trachytes are acidic ones. 



Regarding the union of the basalts and rhyolites, it should be remem- 

 bered that they have not a single character in common, and also that the 

 highest analysis of basalt published by Mr. King differs by fourteen per 

 cent of silica from the lowest analysis of rhyolite. Professor Zirkel's 

 method leaves the presence of nephelite very doubtful, certainly not 

 proved. My examination shows that part of his nephelite-bearing rocks 

 are andesites and the remainder basalts, in all of which the presence of 

 nephelite is very doubtful. His supposed hatiyne seems to be nothing 

 but some grains of ultramarine rubbed off the boxes and enclosed in the 

 balsam used in mounting. After careful study of the reports and col- 

 lections of the Fortieth Parallel Survey, and of collections of the same 

 rocks in the same and adjacent regions, I am unable to find any basis for 

 Mr. King's classification, or for his theory of the genesis of volcanic rocks. 

 In studying propylite I have not only examined the collections mentioned 

 above, but have also had constantly at hand Baron Richthofen's collec- 

 tion made in California and Washoe. Placing propylite under andesite, 

 as an altered form of the latter, Richthofen's law of succession seems 

 then to be an observed order of occurrence in a comparatively brief 

 geological period, and as such is hardly at present entitled to be 

 considered a law. "V\Tien it will express the order of succession during 

 all time, and the reasons for that succession, then, and not till then, 

 will it rank as one of nature's great laws. 



We need more accurate field, microscopic, and chemical work on 

 unaltered rocks, and a more careful study of our rock species in all 

 their relations, since the best descriptions are at present so imperfect 

 as to be of but very little value. 



It is to be noticed that only so many minerals form in a lava as 

 can crystallize during the time of cooling, and the interchange of chemi- 



