PETROGRAPHY. 335 



A second phase of consolidation is marked by the generation 

 of smaller crystals of microlites or a microlitic ground-mass. 

 The development of crystallites and ground-mass at this phase 

 is limited to trachytoid rocks. 



In the case of granitoid rocks the consolidation is complete 

 at the close of the second stage, but in the case of trachytoid 

 rocks there follows still a third phase characterised by processes 

 of alteration in the crystals and matrix already formed, and by 

 the constitution of a micro-felsitic, microlitic or glassy ground- 

 mass. 



For the identification of the individual rock-varieties MM. 

 Fouque and Levy regard the felspars of primary import- 

 ance ; subordinate means of identification are afforded by the 

 magnesia - iron silicates (mica, hornblende, augite, diallage, 

 hypersthene, peridote). The work concludes with a detailed 

 description of the rock-forming minerals. In France, the 

 Fouque-Levy system has held an authoritative place in the 

 teaching of petrography. 



A second edition of his Mikroskopische Physiographic der 

 petrographisch wichttgen Mineralien was produced by Rosen- 

 busch in 1885. Rosenbusch had practically re-written this 

 work, and made it an exhaustive compendium of all the results 

 obtained by microscopical, crystallographical, and micro- 

 chemical methods. The optical phenomena of crystallography 

 were discussed with the utmost care. In the first edition 

 Rosenbusch had advanced microscopical research by the intro- 

 duction of new apparatus, in the second he was able to add 

 many valuable mineralogical results of the improved means of 

 research. He also gave full and precise instructions regarding 

 the use of the microscopic methods, so that by following the 

 directions given in this work any earnest student might become 

 a proficient crystallographer and mineralogist. 



In 1888, Michel-Levy and Lacroix published Les Mineraux 

 des Roches^ a work which provides an excellent general account 

 of all the physical and optical properties of rock-forming 

 minerals, and, like that of Rosenbusch, gives full directions 

 for the optical examination of thin sections, and for all micro- 

 chemical means of identifying mineral fragments. The French 

 authors relied in many cases on the crystallographical investi- 

 gations of Descloiseaux, and also incorporated many of the 

 methods and results of Rosenbusch. 



Although Sorby had been the great pioneer of modern 



