SYSTEMATIC PETROGRAPHY 469 



graphie der massigen Gesteine, "massig" being used in the sense 

 explained by Lossen, and not in that of textural condition. In 

 the second edition of this widely-used handbook, which appeared 

 in 1887, Rosenbusch repeated his approval of Lossen's proposi- 

 tion, and in the revision of his petrographic system further 

 emphasized geological occurrence as a factor of prime classifi- 

 catory value for eruptive rocks. The great changes in system 

 found in this volume make it practically a new work, but a 

 detailed statement of its scheme seems unnecessary since all 

 petrographers may be assumed to be familiar with it. 



In the decade since the appearance of the first edition, Rosen- 

 busch effected a complete change of systematic base in some 

 important respects, the reasons for which are given in the preface. 

 He now considers rocks as the documents in which the history 

 of the earth is written, and petrography, as the science which 

 teaches us how to decipher those documents, becomes to him a 

 historical and not a descriptive scietice. To quote his declaration : 



The recognition of these relations made the new edition of this book 

 practically a new work. Its end will have been achieved if I have succeeded 

 in procuring for this fundamental conception a general acceptance, and have 

 demonstrated that rock structure affords the safest and most productive means 

 for the construction of a natural system of rocks. 1 



A natural system of rocks must therefore be historical, i. e., 

 genetic. Plainly the logical analysis of the broad science of 

 rocks presented by Naumann was either forgotten or ignored by 

 Rosenbusch. 



With this statement of controlling principles in mind, one 

 involuntarily recalls that in the first edition of the Mikroskop- 

 ische Physiographic the rocks to be discussed were designated 

 massive in preference to eruptive because the former term was 

 considered free from expression of any genetic idea. In this 

 second edition, while advocating the genetic basis of classifica- 



1 " Die Erkenntniss dieses Verhaltnisses machte die neue Auflage dieses Buches 

 zu einer Neubearbeitung. Der Zweck derselben wird erreicht sein, wenn es mir 

 gelungen ist, dieser Grundanschauung eine allgemeinere Anerkennung zu verschaffen 

 und darzuthun, dass die Gesteinsstructur das sicherste und ausgiebigste Mittel zum 

 Aufbau eines natiirlichen Systems der Gesteine an die Hand giebt." 



