SYSTEMATIC PETROGRAPHY 489 



from eruptive rocks, and para to those derived from sediments. 

 The enumerated divisions of Crystalline Schists are not defined 

 in a systematic manner, and even the terms gjieiss and schist are 

 given no definite meaning. 



The fourth class of rocks advocated by Rosenbusch, on 

 genetic grounds, the origi?ial crnstal rocks, is considered by him as 

 not certainly represented by any known rocks. But it appears 

 to him probable that they possess the habit of the crystalline 

 schists. 



Johannes Walther, i8gy. — An outline of a general classifica- 

 tion of rocks upon a logical and consequent basis was presented 

 to the Seventh International Congress of Geologists in St. Peters- 

 burg, in 1897, by Johannes Walther. 1 Although but an outline 

 of a system this proposition deserves attention as the most con- 

 sistent effort yet made to formulate a system of petrography 

 co-ordinate in method for different classes of rocks. 



Walther starts from the consideration that the growth of 

 petrographic system in recent years has been very one-sided, a 

 fact recognized by all. He believes that a natural arrangement 

 of igneous rocks has been provided by petrographers of the 

 modern school, while sedimentary and metamorphic rocks are 

 still arranged upon old and partly incorrect bases. Aiming to 

 secure a logical system, Walther formulates the following prin- 

 ciples which he thinks should be observed in the classification 

 of rocks : 



I. The petrogenesis of recent deposits and the direct observation of actual 

 processes are the fundamental principles of classification. 



II. Every older rock has primary characters given it at its formation, and 

 secondary ones derived by diagenesis or metamorphosis. 



III. The derived characters may so change the type of the rock as to 

 become " essential," while the primary characters become "accessory," 



IV. In spite of this last condition only the primary characters should 

 determine the principal groups of petrographic system. 



V. Next to the primary lilhologic characters the primary form of occur- 

 rence has a classificatory value. There must be distinguished, therefore, 

 Unstratified, Stratified, and Dike rocks. 



J " (Jongres geologique international," Compte- Rendu de la VII session, St. Pelers- 

 bourg, i8q"j, p. 9 (issued in 1899). 



