334 HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



posed by Klein, Breon, Rohrbach, and others, and have been 

 used for the purposes of separation. 



The important results of microscopical and micro-chemical 

 search were incorporated in the German text-books of Lasaulx 

 (1875) an d O. Lang (1877); while the admirable work of 

 Rosenbusch more especially gave an impulse to the study of 

 petrography in other countries. In France, two illustrious 

 petrographers, Fouque and Michel-Levy, adopted the improved 

 methods and advanced scientific research by many valuable 

 contributions. From the year 1873, both devoted themselves 

 to the artificial preparation of silicates, and made a comparison 

 of the artificial products with the natural occurrences in rocks; 

 while Fouque developed principally the crystallographical 

 aspects of microscopic investigations, Michel-LeVy devoted 

 himself more to the microscopic study of the petrographical 

 relations. In 1879, their conjoint work on the French Eruptive 

 Rocks appeared in the form of an explanatory text to the 

 detailed geological map of France. 



In this work MM. Fouque and Michel Levy followed 

 the general arrangement of the Microscopic Physiography of 

 Rosenbusch. The French authors distinguished original and 

 secondary minerals in rocks ; the former are said to be present 

 sometimes as essential, sometimes as accessory constituents ; 

 the secondary are sub-divided according to the time of their 

 generation into two main groups, and these are again divided 

 into sub-groups. The rocks are classified with respect to their 

 origin, their geological age, their mineralogical composition, 

 and their structure. The massive rocks of pre-Tertiary epochs 

 are held distinct from those of Tertiary and recent ages, and 

 certain differences are indicated between them. MM. Fouque 

 and Michel-Levy recognise two leading types of structure among 

 the massive crystalline rocks, the granitoid and trachytoid ; 

 these terms almost correspond to the use of the terms 

 granular-crystalline, and porphyritic in the works of the 

 German petrographers. 



The French authors bring into pre-eminence the mutual 

 development attained by the several elements in the rocks. 

 Their special study of this feature has led them to believe 

 that many massive rocks give evidence of the generation of 

 crystals or crystalline material in successive phases of consolida- 

 tion. In both the granitoid and trachytoid types, the larger 

 crystals are generated during the first phase of consolidation. 



