THE 



JOURNAL OF GEOLOGY 



JULY-AUGUST, 1902 



THE DEVELOPMENT OF SYSTEMATIC PETROGRAPHY 

 IN THE NINETEENTH CENTURY. 



Part II. 1 



Beginning of the microscopical era. — Having presented the state 

 of systematic petrography up to the time when the polarizing 

 microscope became the instrument of prime importance in the 

 investigation of rocks, we now proceed to the study of the more 

 recent schemes of classification, based upon the larger knowl- 

 edge. For all systems thus far reviewed, it must be recognized 

 that ignorance of the characters and relationships of many rocks 

 rendered a comprehensive and logical scheme impossible. The 

 frame work of system was necessarily constructed without full 

 knowledge of the applicability of some of the factors employed. 

 With the polarizing microscope and improved methods of chemi- 

 cal analysis this condition has now disappeared, and while the 

 time may not be ripe for unbiased criticism it is plain that 

 modern systems of classification must ultimately be judged with 

 regard to the greater and almost perfect knowledge of the actual 

 characters of rocks enjoyed by the authors of those systems. 

 The responsibility for the choice of factors suitable for the con- 

 struction of a comprehensive system and for a logical, conse- 

 quent, and consistent application of those factors clearly increases 

 with knowledge of the objects to be classified. 



The microscopical study of rocks, continuing for nearly four 



'Continued from p. 376. 

 Vol. X, No. 5. 451 



