Classification of Rocks. — Sp7irr. 2 1 7 
signified in the nomenclature by having tiie word aplyte* ap- 
pended to the name of the granular rock, thus, alaskyte aplyte, 
augite syenyte aplyte. etc. Third, the structure characterized 
by phenocrysts. or the ]Xjrphyritic structure, with a ground- 
mass made up entirely of comparatively coarse holocrystalline 
material. This is signified in the rock nomenclature by ap- 
pending the word porphyry to the name of the granular rock, 
the word implying the characteristic structure"^. Fourth, the 
structure which, as before, is porphyritic, being marked by 
phenocrysts, but \\hose groundmass is fine, with the con- 
stituent minerals often not large enough to be identifiable, or, 
finally, with a glassy groundmass. Following the established 
custom, this variation is given a different name from rocks 
having the three preceding structures which have been de- 
scribed. Most of the already established names for this type 
have been applied chiefly to lavas, but they are here considered 
as representing simply a certain structural variation of the 
mineralogic group, and may represent lavas, dikes, or any 
other habit of igneous rock: thus, — tordrillyte. rhyolyte, 
trachyte, and dacyte. Fifth, where a rock is wholly glassy, 
but W'here its association or chemical analysis shows it to be- 
long to a certain mineralogic combination, the name of the 
preceding structural division, for example, rhyolyte, dacyte, 
andesyte, etc., is applied, with the appendix glass, this ap- 
pendix indicating sufficiently the structure. Thus, basalt 
glass. 
Age as a factor in rock nomenclature and classification is 
entirely omitted in this system, the writer's experience in the 
Alaskan rocks showing it to be without tangible application 
here, or indeed in any other district that he is acquainted with. 
The same is true of the mode of occurrence. Geologically, it 
is important to know the age of an igneous rock with rela- 
tion to the surrounding rocks and to know and consider the 
form of its occurrence, but it seems very plain that the rock 
*Following and extending the structural use of the term by petro- 
graphers of late. (Brogger, Eruptivgesteine des Kristianiagebietes, 
Kristiania, 1894, p. 95: Rosenbusch, Mikroskopische Fhysiographie der 
.Massigen Gesteine, I, 1896, p. 463, et. see}. 
fAbondoning the term porphyrite, and using porjihyry as a structur- 
al term, applicable to all rocks, as used for some time by many petro- 
graphers and set forth in a circular of the U. S. Geological Survey, Mav 
24th, 1898. 
