PETROGRAPHY. 339 



and pressure, and the saturation of the magma with gases and 

 heated vapours. The latter play the chief role in the acid 

 rocks, producing pegmatitic, micro-pegmatitic, and other 

 structural types, and also determining a definite sequence of 

 eruption. On the other hand, the structure of the basic rocks 

 depends almost exclusively on temperature, i.e., on the greater 

 or less rapidity of the process of cooling. 



After this adverse criticism of the classification advanced by 

 Rosenbusch, Michel-Levy proceeds to discuss the varieties 

 of rock-structure, and shows the frequent agreement between 

 the views of Rosenbusch and his own; he also points out 

 that the differences of nomenclature are more apparent than 

 real, and tries to bring the French and German terminology 

 into harmony by means of a list of synonyms. In most cases, 

 Michel-Levy claims the priority for his own terms. 



Only a few minerals come into question in the composi- 

 tion of eruptive rocks. Fouque and Michel- Levy had classed 

 these minerals as original and secondary, sub-dividing the 

 secondary minerals in groups corresponding with the order of 

 formation. According to Rosenbusch, there are just two 

 fundamental laws controlling the order of formation the one, 

 that the magma is always more acid than the sum of the 

 mineral constituents already solidified in it; and the other, that 

 the separation of the elements which occur in less profusion 

 has generally been concluded before the separation of the 

 more richly distributed elements takes place. Michel-Levy 

 questions the correctness of these laws, and makes an elaborate 

 inquiry into the order of separation of the mineral con- 

 stituents. He devises a code of symbols by which the 

 structure, composition, and genesis of the massive rocks may 

 be represented by a short formula ; and finally arrives at the 

 conclusion that the classification and the nomenclature of 

 eruptive rocks must be kept free from any hypothesis re- 

 garding their origin, and consequently that structure and 

 mineralogical composition form the only basis of a rational 

 classification. 



Zirkel assumed a similar standpoint in the second edition of 

 his Lehrbuch der Petrographie (1893-94). This large three- 

 volume work is the only complete handbook of petrography. 

 All varieties of eruptive, schistose, and sedimentary rocks are 

 treated according to their macroscopic, microscopic, and 

 chemical constitution, their structure, and their geological 



