Classification of Rocks. — Spurr. 221 
later error, but it is not much to be wondered at that the 
somewhat more puzzling problems which igneous rocks pres- 
ent should lag in their solution a little behind the rest, and that 
geologic age should still be held by a few (though less and less 
firmly) as entailing characteristic textures or mineralogic com- 
binations. 
Still later, when the study of comparatively lithology was 
begun, but as yet without the aid of the microscope, miner- 
alogy could be determined only roughly, and in the case of 
dense or fine-grained rocks not at all. At this stage of the de- 
velopment of the science, therefore, the most striking differ- 
ences which the unaided eye perceived in igneous rocks were 
those of structure, and the greatest difference, that between 
the coarse granular and the porphyritic structures, was seized 
upon by these truly advanced reasoners (as compared with 
those who had only the first two principles as criteria) as a 
prime basis for rock classification. 
Finally, chemical investigation was applied to the study of 
igneous rocks, and still later came accurate microscopic min- 
eralogy, and each has opened up and cleared the way to a 
better understanding of the true nature and relations of these 
rocks. 
The system of Rosenbusch*, therefore, (a pioneer work, 
with the burden of old inheritances) was based on (i) manner 
of occurrence, (2) age, (3) mineralogic composition. In this 
classification there are three chief modes of occurrence used 
as bases of classification, — plutonic rocks, dike rocks, and ef- 
fusive rocks. As to the question of age, there are only two 
chief divisions, and these are confined to the effusive rocks, 
namely, the paleovolcanics and the neovolcanics. 
Besides Rosenbusch 's classification, many partial and com- 
plete classifications have been offered, in which one or the 
other of the possible principles of classification have been 
given first place. To some it has seemed that a systematic 
classification could be based most securely upon chemical com- 
position. Thus H. O. Lang f has offered a classification based 
*Mikroskopische Physiographic der Massigen Gesteine. 
See also Michel-Lgvy's tabulation of Rosenbusch's classification, in 
Structures et Classification des Roches Eruptives, Paris, 1889, p. 42. 
tMin.und petr. Mitth. XII, 189I, p. 199. 
