364 F. L. Bansome — Lava Flows of California. 



pitchstone-like facies, which on petrographical grounds was 

 correlated with the biotite-augite-latite of the second flow. 

 There are certain relations which suggest, however, that 

 it may really have preceded the Table Mountain flow instead 

 of following it. The chemical analysis lends additional 

 color to this suggestion, and it is possible that in the two 

 small masses, which alone are known of this rock, there may 

 survive the remnants of an earlier flow than any of the three 

 typical latite considered in this paper. 



The most noticeable feature of the analyses as a whole, 

 taken in connection with the mineral ogical composition of the 

 rocks, is the amount of potash present. By far the greater 

 number of the thin-sections (excepting of course the biotitic 

 facies, which however are no richer in potash than the others) 

 contain no recognizable potash-bearing mineral. Several of 

 the analyses contain as much as 5 per cent of potash, which, if 

 it were present in the form of orthoclase, would make up 

 nearly 30 per cent of the rock. This amount is entirely too 

 large to be accounted for on the supposition that it is present 

 in the form of orthoclase microlites which have escaped detec- 

 tion among the plagioclases of the groundmass, and it is fairly 

 certain that nearly all this potash, in those facies which do not 

 contain biotite, is present in the residual glass of the ground- 

 mass. 



It is frequently observed* that in rocks of intermediate 

 composition, like the latites, orthoclase forms a thin envelope 

 around the basic plagioclases, while in the plutonic mon- 

 zonites the orthoclase was the last to crystallize, forming a 

 mesostasis for the earlier minerals. In the Sierra Nevada 

 latites the conditions under which orthoclase (or some 

 other potash alumina silicate) could separate out were never 

 reached. This would indicate that the method, not infre- 

 quently resorted to, of assuming the presence of orthoclase in 

 the groundmass of an effusive rock from the chemical compo- 

 sition of the latter, requires caution in its use. 



Classification of the Sierra Nevada latites. — With the advance 

 made in methods of petrographical research during recent years,, 

 there has come inevitably greater difficulty in the proper classi- 

 fication of the rocks themselves. The increased refinement in 

 chemical analyses, and their more critical interpretation, to- 

 gether with the strides made in the optical investigation of the 

 feldspars by Becke, Federow, Michel-Levy, and others, is lead- 

 ing to the gradual splitting up of some of the older, more 

 comprehensive groups. A notable example of this tendency 

 was Brogger's differentiation of the monzonite family from 

 the syenites, while a similar process has long been reducing the 

 once large family of the trachytes to a size more commensu- 



* See Washington, Jour. Geol., vol. iv, p. 551, for several references. 



