PETROGRAPHY. 331 



which is devoted to the mineral constitution and structural 

 features of rock-varieties. The work is fully illustrated by 

 woodcuts. 



The other important work was that of Rosenbusch, en- 

 titled, Die mikroskopische Physiographic der petrographisch wich- 

 tigen Mineralien (Stuttgart, 1873). It contains an exhaustive 

 statement of the practical methods according to which rocks 

 may be identified by means of the morphological, physical, 

 and chemical properties of their component minerals ; this is 

 followed by a full and methodical discussion of the microscopic 

 characters of rock-forming minerals. The optical consideration 

 of the phenomena of polarisation was elucidated so admir- 

 ably by Rosenbusch, that his work created a secure basis for 

 future petrographical researches. By the improvement of the 

 microscope and the polarising apparatus, by the introduction 

 of a rotating stage, and by other mechanical aids, it was now 

 rendered possible to distinguish not only singly or doubly 

 refracting bodies and uniaxial or biaxial minerals, but also to 

 determine more accurately the specific optical properties of 

 minerals belonging to the different systems of crystallisation. 

 After the publication of this great work, Rosenbusch took 

 rank along with Zirkel as one of the great pioneers in the 

 microscopical investigation of rocks. In 1877, Rosenbusch 

 published a second volume entitled Die mikroskopische 

 Physiographic der massigen Gesteine. 



Rosenbusch distinguished the massive rocks according to 

 the felspathic modifications: T, Orthoclase rocks; 2, Ortho- 

 clase, nepheline, leucite rocks; 3, Plagioclase rocks; 4, Plagio- 

 clase, nepheline, leucite rocks; 5, Nepheline rocks; 6, Leucite 

 rocks ; 7, Non-felspathic rocks or peridotites. Each of these 

 groups was subject to further sub-division according to the 

 particular rock-structure, or in the case of the felspathic rocks 

 according to the presence or absence of quartz. Like Zirkel, 

 Rosenbusch gave due consideration to the geological age of 

 the rocks, as the older and the younger representatives of each 

 group were handled separately. 



The optical method brought to such a high point by 

 Rosenbusch was still further elaborated by Bertrand, Klein, 

 and Lasaulx in memoirs which appeared in 1878. Schuster 

 proved in the following year that the felspars which had 

 been recognised in such a masterly way by Tschermak from 

 their composition to be isomorphous mixtures, represented a 



