124 GEOLOGY 



PETROLOGY 



Rocks are composed of minerals; therefore a knowl- 

 edge of minerals is essential to an understanding of rocks, 

 and the science of mineralogy was necessarily developed 

 before that of petrology. In rocks, minerals are usually 

 present in very small crystals; therefore rocks are studied 

 chiefly by microscopic methods. FOUQUE and MICHEL- 

 LEVY introduced in France these methods, which are 

 based on optical properties first deduced by FRESNEL. 

 DES CLOIZEAUX applied the methods to the study of 

 minerals as such, and thus supplied the fundamental 

 data necessary for petrographic work. MICHEL-LEVY 

 and LACROIX continued the determination of data, de- 

 veloping at the same time additional methods of using 

 optical properties in identifying minerals. 



FOUQUE and MICHEL-LEVY proposed a classification 

 of igneous rocks, based on mineral composition and on 

 texture, which is the foundation of the classification now 

 in use in France, and has contributed much to classifi- 

 cations in use in other countries. MICHEL-LEVY empha- 

 sized the importance of mineralizing agents in processes 

 of differentiation as well as in those of contact metamor- 

 phism. 



LACROLX has shown that contact exomorphism con- 

 sists not only in physical changes, but also includes 

 chemical transformations due to introduction of material 

 of magmatic origin. He has also described evidence to 

 show that granitic magmas may be changed to diorites, 

 etc., by contact endomorphism. LACROIX has also 

 written a monographic work on the "Mineralogy of 

 France," in which he has emphasized the varying modes of 

 occurrence and of alteration of minerals in order to fix 

 the mode of origin and conditions of stability. In a similar 



