SYSTEMATIC PETROGRAPHY 495 



That some factors have been introduced into classification in 

 a manner that is quite unscientific seems plain. The age dis- 

 tinction is one of the factors thus abused. It has long been 

 known that no general distinction separated pre-Tertiary and 

 Tertiary igneous rocks. It may be that the average chemical 

 composition of magmas erupted in successive ages has undergone 

 some change ; but neither the character of the change, nor, least 

 of all, any special connection with the particular time limit in 

 question, has been established. The assumption that igneous 

 rock textures, such as the granular, porphyritic, or vitreous, are 

 functions of geological form or place of occurrence, is known to 

 be contrary to the facts displayed by the rocks. Both of these 

 assumptions have been and are now used in rock classifications. 



Stability of system is certainly desirable, within the bounds 

 of reason. But it is also self-evident that a system of artificial 

 character, in which the subjective element is dominant, can be 

 permanent only by universal consent of petrographers, and such 

 consent is not to be expected. It is a matter of experience that 

 genetic theories have made systems into which they have been 

 introduced very unstable and impossible of general adoption. 

 The danger of using hypotheses in classification has been well 

 characterized by von Cotta, somewhat as follows : Geology is 

 a particularly alluring field for premature attempts at the expla- 

 nation of imperfectly understood facts ; indeed, such attempts 

 are almost unavoidable in the study of this science. When one 

 considers hypotheses simply as such, i. e., as stimulants toward 

 their possible demonstration, then they are not harmful ; the 

 danger lies therein that one may believe them already proven 

 and rest contented. 1 



The danger pointed out by von Cotta has been illustrated in 

 the classification of igneous rocks by such able men as von 

 Richthofen, King, and Rosenbusch. As regards the interior of 

 the earth, whence the molten magmas come, we cannot as yet 

 be sure that what we regard as a law today may not be relegated 

 to the status of a theory or even of an hypothesis tomorrow. 



1 B. von Cotta, Gesteinslehre, 2d ed., 1862, p. vi. 



