MEMOIR OF AUGUSTE MICHEL-LEVY 33 



of thin sections in the last century was due more largely to Michel-Levy 

 than to any other single individual. His contribution to this advance 

 consisted in the study and elucidation of the fundamental laws of light 

 and their relation to crystal structure, the determination of the optical 

 properties and constants of many mineral species, and the invention and 

 description of numerous devices and methods for the accurate measure- 

 ment of these optical constants. Some of these (for example, the "Tab- 

 leau des Birefringences," or colored chart, showing the relation between 

 the interference color, the thickness of the section, and the birefringence 

 of the mineral) are so important that they are in constant use in every 

 petrographic laboratory. Others, such as the stereographic projections of 

 the optical properties of the feldspars, are not so widely used, but are 

 nevertheless extremely valuable for precise determination. 



It was in collaboration with the late Professor Fouque that Michel- 

 Levy published the monograph on microscopic mineralogy (Mineralogie 

 Micrographique, Paris, 1879), which introduced modern petrography in 

 France ; and it was with the same savant that he performed and described 

 numerous remarkable successful experiments in mineral and rock S3^nthe- 

 sis (Synthese des Mineraux et des Eoches, Paris, 1882). In these experi- 

 ments the authors produced artificially a large number of minerals and 

 rocks, many of which had never before been S3'nthesized, and established 

 the fact that acid igneous rocks in the presence of water may be fused at 

 a temperature of about 1,000 degrees centigrade, and that basic igneous 

 rocks may be crystallized from dry fusion at about the same temperature. 



It was through his wide field experience, his remarkably skillful micro- 

 scopic work, and his elaborate experiments in rock synthesis that Michel- 

 Levy developed those theories^ of petrogenesis that made him a leader of 

 the French school of petrography. It is an irreparable loss to the science 

 of petrology that of the brilliant triumvirate (Fouque, Michel-Levy, and 

 Lacroix) who stood as the chief representatives of the French school at 

 the close of the last centurj^ only Lacroix is still living to carry forward 

 the work. 



The special combination of qualities which rendered ]\Iichel-Le^^^ 

 famous as a man of science included not only his keen and dispassionate 

 judgment and his wide and varied knowledge of geology, but also his 

 remarkable ability as a mathematician and geometer, which made possible 



1 See especially Structures et Classification des Roches firuptives, Paris, 1889 ; Classi- 

 fication des Magmas des Roches firuptives, Bull. Soc. Geol. Fr., xxv, 1897, pp. 326-377, 

 and xxvi, 1898. pp. 3-19, and numerous articles dealing with special types of rocks, or 

 rocks of special districts, in the Bulletin de la Soci^te G^ologique de France, Bulletin 

 des Services de la Carte G^logique de France, and elsewhere. 



Ill— Bull. Geol. Soc. Am., Vol. 23, 1011 



