THE CLASSIFICATION OF THE IGNEOUS ROCKS 5 
The definition of a rock as an object rather than as an integral 
part of the earth’s crust—The Wernerian conception of a rock asa 
geologica] unit or integral part of the earth’s crust, still held by 
German petrologists, was adequate enough so long as rock 
masses were regarded as essentially homogeneous. With the 
discovery that such masses are usually quite heterogeneous and 
frequently represent not only several rock species but sometimes 
include almost the whole gamut of rock families, it became 
necessary to adopt some other definition. No other course 
seems open under these circumstances than to consider the indi- 
vidual rock specimen as the unit of classification and describe it 
primarily as an object, as is done with the units in the systems 
of other sciences.* If this is done it should be possible to name 
a rock from study of the specimen only though the full descrip- 
tion would involve no less of field study than is undertaken 
when rocks are classified on the basis of their geological occur- 
rence. 
The importance of texture as a basis of classification.— All sys- 
tems of classification of the igneous rocks emphasize more or 
less strongly rock texture as a basis of classification, for the 
reason that the texture is one of the’properties of a rock most 
easily examined; and, further, because it is dependent so largely 
upon the peculiar conditions of rock consolidation or subsequent 
metamorphism. If rocks are described as objects this property 
of texture becomes inevitably of the very first importance. 
The two main groups of the igneous rocks which are now 
generally recognized as distinguishable on the basis of texture 
are: first, those having a texture designated by Rosenbusch as 
hypidiomorphic granular, but which may in simpler language be 
referred to as granitic, the essential characteristic of which is 
that the mineral constituents by their manner of interlocking 
indicate for the rock in which they occur practically an uninter- 
rupted period of crystallization; and, second, the porphyritic 
™Cf. WHITMAN Cross, The Geological vs. the Petrological Classification of 
Igneous Rocks. JourN. GEOL. VI, p. 79, 1898. See also TEALL, British Petrography, 
p. 65. 
