CLASSIFH'ATION OF LAVAS. 233 



Classification of Lavas.— ( )ii till' geological uiiii) :ill volciiuic vocks liave beeii 

 classed under tile tuUowiug heads: Horuhleiide-audesite, dacite, rhyolite, 

 pumice and tuti', augite-audesite/ and basalt. All the more important bodies 

 of lava belong to some one of these sharply defined mineralogical groups. 

 It must be borne in mind, however, that several of these types of igneous 

 lavas 2>ass by insensible gradations from one into another and it is uot 

 always easy to decide to which group an isolated exposure in the field or a 

 hand specimen in the laboratorv properly belongs. All di\ision lines are 

 more or less arbitrary. They are necessary for purposes of classification, 

 although they may not exist in nature. 



To infonn those readers who have not kept up with recent ad- 

 vances in the classification of igneous rocks and at the same time to pre- 

 vent all niisuiiderstaiidings as to the use of terms, a brief description 

 will be given of the physical characteristics of each group of lavas 

 which have been recognized as occurring in the district. Those who 

 desire a more detailed description of the special petrographical features 

 of the rocks are referred to the report of Mr. J. P. Iddings, which accom- 

 panies this Avork. As he has presented the results of a most thorough 

 microscopical investigation of the material collected in the field, in order 

 to prevent a duplication of facts much that might properly find place in 

 these pages is omitted, and only such data are employed here as are deemed 

 necessary to make this chapter complete in itself and to bring out more 

 clearlv the important facts beaiing upon the geology of the region. 



Hornbiende-andesite.— Under horiibleude-andesites are classed those vol- 

 canic rocks which are com2:)osed mainly of triclinic feldspars and liorii- 

 blendes as essential constituents. Here at Eureka the fresh unaltered 



' Since the priuting of the atla.s (1883) accompanyiug this Monograph, the pyroxcnic minerals 

 of the andesite of Richmond Mountain, which at that time were (considered to lie angites, have been 

 shown to consist of both hypersthcno and angite, the former mineral being in most instances largely in 

 excess. The rock, therefore, should more properly be designated as pyroxene-andesite, a designation 

 more in accord with the nomenclature now generally adopted by lithologists for similar rocks else- 

 where. 



Throughout this volume the term "pyroxene-andesite" will be used to designate tlie rocks of 

 Richmond Mountain and Cliff Hills, the two localities colored as angite-andesite on the atlas sheets. 



See Notes on the volcanic rocks of the Great Hasin, Hague and Iddings: Am. Jour. Sci., June, 

 1884, p. 457. 



