372 F. L. Ransome—Lava Flows of California. 



The sanidine-bearing andesiteof the Rosita Hills is described 

 by Cross* as containing plagioclase, orthoclase, quartz, biotite, 

 aiigite, and accessory minerals. The plagioclase is referred 

 to oligoclase. This rock is said to have its chief develop- 

 ment as dykes, although occurring also in effusive masses. 



The foregoing citations are sufficient to show how great 

 mineralogical variety may be presented by the effusive rocks 

 having the peculiar intermediate chemical composition of the 

 latites. It is possible, and perhaps desirable, to assign the 

 more conspicuous and well-marked of these mineralogical 

 combinations separate names, but the number of the latter is 

 apt to be large, and they are of a distinctly lower grade of 

 importance than those rock names that express not only a cer- 

 tain mineral constitution but a corresponding and characteristic 

 chemical composition as well. 



It accordingly seems advisable to attempt to bring into use 

 some more general name to embrace all the effusive rocks 

 standing chemically about midway between the typical tra- 

 chytes and the typical andesites, — a name that can be used 

 as the general effusive equivalent of the increasingly important 

 group of the plutonic monzonites. I should gladly have 

 avoided the necessity of introducing a new name into a rapidly 

 growing nomenclature, the more so, as Washington, by his 

 admirable petrographical studies in the Italian volcanic regions, 

 has added so greatly to an accurate knowledge of intermediate 

 rock types which must be necessarily embraced by such a new 

 term. But the Sierra Nevada lavas cannot be classed with the 

 typical toscanites, vulsinites, or ciminites, as defined by Wash- 

 ington, although, like the vulsinites and some of the ciminites, 

 they are to be regarded as the effusive equivalents of the mon- 

 zonites. Thus there is an imperative demand for some more 

 comprehensive name to cover the mineralogically diverse forms 

 which monzonitic magmas, cooling under effusive conditions, 

 may assume. For this reason, and in recognition of the 

 importance and interest of the Italian types described by 

 Washington, the name latite, derived from the Italian province 

 of Latium, is proposed as a broader term, comparable in its 

 scope with monzonite, to designate the effusive forms of the 

 monzonite magma. It is to be noted, that the particular facies 

 of latite represented by the Sierra Nevada lavas might also be 

 given varietal names of the same order of importance as vul- 

 sinite or ciminite ; but this has not been done in the present 

 instance. 



Abich and Hartung have used tracky-dolerite for rocks which 

 in part belong with the latites; Fouque and Michel Levy, 

 trachy-andesites ; Zirkel and Rosenbusch, andesitic-t?xtohyte ; 



* Proc. Colorado Scientific Soc, 1887, p. 248. 



