2i8 Tlie American Geologist. April, isoo 
must be classified, first, as a rock, and subsequently its age and 
form of occurrence must be specially stated. The very con- 
ception of a rock is expressed thoroughly by the accepted defi- 
nition. — an aggregate of minerals; and an igneous rock is one 
th:;t has evidently consolidated from a molten condition and 
which bears in its composition and structure the undoubted 
marks of this origin. Chemically, many of these igneous 
rocks may be identical with certain sedimentary rocks or met- 
amorphic schists, but there are distinguishing features which 
separate the igneous rocks into a distinct class, and these 
features depend upon the character and habit of the minerals 
and upon the structure of the rock. 
Just as the chemical composition of an igneous rock may 
be shared by it in common with other igneous rocks, as well 
as with sedimentaries and metamorphic rocks, m tne same way 
a given igneous rock may occur in almost all conceivable ways 
(as a buried or plutonic mass, a dike, a stock, a laccolith, a 
flow, etc.), and the period of its consolidation may belong 
within a vast range of geologic time, without changing its 
nature. The element of mode of occurrence as a principle in 
the classification of rocks, made of prime importance by 
Rosenbusch, has already been abandoned in the classifications 
of Zirkel, Michel-Levy, and Teall. The element of time, or 
period of consolidation, also held important by Rosenbusch. 
has been reduced in importance in the classification of Zirkel,* 
and A'Iichel-Levy,t and has been entirely abandoned by Teall,$ 
and Brogger.§ 
Although the classification proposed takes cognizance, so 
far as nomenclature goes, only of mineralogic composition 
and structure, the chemical element will be seen tO' enter very 
largely into the establishment of the mineralogic families, 
groups, and classes. As regards the four other possible prin- 
ciples of classification, enumerated by Teall j;, namely, mode of 
occurrence, origin, geologic age, and locality, these can be 
*Zirkel. Lehrbuch der Petrographie, Leipzig, i, 1894, p. 834. 
tMichel-L6vy. Structures et Classification des Roches Eruptives, 
Paris, 1889, p. 42. 
J British Petrography, Birmingham, 1889, p. 70. 
§Brogger. Triadische Eruptionsfolge bei Predazzo, p. 60. 
liOp. cit., p. 64. 
