370 F L. Eansome — Lava Flows of California. 



they occur in nature may traverse existing schemes of classifi- 

 cation at practically any angle, but mainly, in this case, that 

 the name of a rock is generally determined more by its min- 

 eralogical than by its chemical composition. 



In the case of most igneous rocks, and of nearly all plu- 

 tonic rocks, the principal mineralogical constituents can be 

 deduced with a fair degree of certainty from an accurate bulk- 

 analysis, and vice versa, the chemical constitution can be pre- 

 dicted from a microscopical investigation of a thin-section with 

 sufficient accuracy to leave little doubt of the general position 

 of the rock in an ordinary scheme of classification. But with 

 the effusive latites this appears to be true only in a slight 

 degree. In effusive rock of intermediate chemical com- 

 position the circumstances which determine the crystallization 

 of the different possible mineralogical combinations, appear to 

 depend upon a very delicate adjustment of physical and chem- 

 ical conditions. This is already noticeable in the andesites, 

 where a given magma cooling under certain conditions may 

 give rise to a hornblende or biotite andesite, and under other 

 conditions, by the magmatic resorption of these two minerals, 

 may solidify as a pyroxene andesite. The latites, being 

 intermediate rocks par excellence, and containing more or 

 less unindividualized glass in reserve, exhibit this apparent 

 capriciousness of crystallization to an unusual degree. 



Thus, the vulsinites of Washington " are characterized min- 

 eralogically by the presence of alkali feldspar with a large 

 amount of basic plagioclase (labradorite to anorthite), together 

 with augite and diopside. Hornblende and biotite are not 

 abundant in the type specimens, though they may be present 



in large amounts in other varieties Olivine is wanting, 



or if present is so in only accessory amounts."* 



The biotite-vulsinite from Monte Santa Croce (column II) is 

 particularly interesting with reference to the point under dis- 

 cussion, and the dilemma in which one is placed in attempting 

 to adhere to a purely mineralogical classification of these rocks 

 is well expressed in Washington's own words. He says : " We 

 have then a rock which is chemically a ciminite and minera- 

 logically a biotite-vulsinite. 



" In regard to the name by which they should be called 

 there may be some doubt. From a mineralogical standpoint 

 they are obviously not ciminite, nor chemically can they strictly 

 be called vulsinite. Since, however, in the schemes of classifi- 

 cation in general use at the present time, the mineralogical 

 composition takes precedence over the chemical, and bearing in 

 mind the unadvisability of adding new names to the already 

 overburdened nomenclature, I shall designate these^ rocks as 

 biotite-vulsinite."f 



* Journal of Geology, vol. iv, p. 553. 



f Journal of Geology, vol. v, pp. 250-251. 



