368 F. L. Eansome — Lava Flows of California. 



rocks, or granites and syenites, on the one hand, and the plagio- 

 clase rocks, or diorites and gabbros, on the other, the miner- 

 alogical distinction being the visible sign of difference in 

 chemical composition. In his tabular classification* he has 

 indicated the effusive equivalent of monzonite as trachyte- 

 andesite, and it is here that the latites of the present paper 

 belong, as will now be more fully shown. 



It is plain from the descriptions of the latites in the pre- 

 ceding pages, that in spite of their superficial basaltic char- 

 acter, they are in no sense basalts, being far too high in silica 

 and alkalies, and too low in magnesia and lime, and differing 

 from the latter rock in microscopic structure, and in the pro- 

 portion of ferromagnesian constituents to the feldspars. Their 

 specific gravity, too, is lower than that of basalts of equally 

 compact texture. 



Their relation to the andesites is much closer, and were it 

 advisable to crowd them into one of the well-established older 

 groups they would be placed with little hesitation in this 

 family. Leaving out of consideration the glassy base, the 

 latites described in the present paper possess practically 

 the mineralogical composition of andesites. But they differ 

 from normal andesites in chemical composition, being gener- 

 ally slightly lower in lime and magnesia and always higher in 

 alkalies. In the andesites the total alkalies are as a rule less 

 than six per cent, with soda in excess of potash. In the 

 latites the total alkalies range from six to ten per cent, with 

 the potash, reckoned in percentages, equal to or slightly in 

 excess of the soda. 



Rocks of this type can hardly be classed as trachytes if the 

 latter name is to retain any precision of meaning. ISTot only 

 are they decidedly untrachytic in color and texture, but differ 

 widely mineralogically from typical trachytes. Sanidine, 

 instead of being the dominant feldspar, is entirely absent and 

 the conspicuous phenocrysts are labradorite. Mr. Turnerf 

 has provisionally classed some of the latites with the tra- 

 chytes on the basis of their chemical composition. The 

 analysis upon which the comparison is based was quoted from 

 Zirkel's Petrographies and as Mr. Turner remarks " contains 

 more lime and less silica than most of the analyses given by 

 Zirkel." The analyses in question is one by Ricciardi§ of the 

 so-called trachyte of Bolsena, and is quoted in column Y of 



*Die Eruptivgesteine des Kristianiagebietes; II Die Eruptionsfolge der tria- 

 dischen Eruptivgesteine bel Predazzo in Sudtyrol, Kristiania, 1895, p. 60. 



f Further Contributions to the Geology of the Sierra Nevada, l7lh Annual 

 Kept. U. S. Geol. Survey, p. 729. 



£ Vol. ii, p. 378 (1893). 



§ Klein, Petrographische Untersuchung einer Suite von Gesteinen aus der 

 Umgebung des Bolsener Sees; Neues Jahrb. fur Min., etc., B.B. vi, 1889, p. 8. 



