CLASSIFICATION OF IGNEOUS ROCKS 87 



are divided into three great classes: "Tiefengesteine," " Gang- 

 gesteine," and " Ergussgesteine." These terms are commonly 

 translated into English as Deep-seated rocks, Dike rocks, and 

 Effusive rocks. Let us examine this classification from the dif- 

 ferent standpoints of the systematic petrographer and the penol- 

 ogist. 



To begin with it is self-evident that these class names express 

 geological occurrence. They represent natural divisions of the 

 geologist, they were used by him long ago and must be used in 

 future, to express natural relationships. The geologist has a 

 logical and practical claim upon these terms which cannot be 

 denied. The question then is, can this geological classification 

 be applied to the uses of systematic petrography, producing 

 natural classes of rocks, a result which would be of great benefit 

 to all concerned. I believe that it cannot be so used. 



The system of Professor Rosenbusch is avowedly intended to 

 meet the legitimate demands of geology upon his systematic 

 science, as formulated by Lossen, to the effect that geological 

 relations of rocks must be recognized as petrographical relations. 1 

 But while aspiring to meet the conceded demands thus expressed 

 Rosenbusch has so redefined each of these grand divisions that 

 it does not include all rocks belonging in it upon the criterion 

 most clearly expressed in the name, the criterion the geologist 

 must apply, and does include rocks that cannot logically be 

 placed there. To illustrate by the most striking instance, the 

 Dike rocks of Rosenbusch are not rocks occurring in dikes, which 

 must be the geologist's definition, but rocks of as yet hypothet- 

 ical derivative relation to certain other rocks. This class 

 includes a small part of the rocks actually occurring in dikes 

 and many not so occurring. Similarly the Deep-seated rocks of 

 Rosenbusch are not necessarily abysmal. They appear in dikes 

 and other intrusive bodies near the surface and even in some 

 effusive masses. The Effusive rocks of this system occur in 

 many intrusive masses and in the peripheries of deep-seated 



1 tjber die Anforderungen der Geologie an die petrographische Systematik. Jahr- 

 buch der K. pr. geol. Landesanstalt, 1883, p. 486. 



