PETROGRAPHY. 329 



mass of rocks. Sorby's paper was no less than epochal in its 

 effect, it appealed both to field geologists and to mineralogists, 

 for it revealed the community of interest in the results which 

 could be obtained by accurate microscopic examination of 

 rocks. 



Sorby's method was applied by Websky, who examined thin 

 sections of minerals by polarised light, and attained brilliant 

 results. A happy circumstance brought Sorby's influence 

 directly to bear upon Ferdinand Zirkel. In the year 1862, 

 while at Bonn, Sorby personally initiated Zirkel into his 

 methods of investigation, and inspired him with enthusiasm 

 for the new field of research. 



Specimens of crystalline rocks from all parts of the world 

 were secured by Zirkel, who submitted them to microscopic 

 examination by transmitted and polarised light, and arrived 

 at ever sharper definitions of the various inclusions, and the 

 appearances displayed in polarised light. By his compre- 

 hensive researches Zirkel established Sorby's methods upon 

 a broader empirical basis, and he at the same time introduced 

 the new methods in his teaching of petrography at Bonn 

 University. 



There were still some incredulous voices : Vogelsang in 

 1864 doubted the existence of glassy inclusions in the com- 

 ponent ingredients of porphyry, and other rocks of non-glassy 

 structure ; Laspeyres in the same year also disputed the 

 glassy inclusions in porphyritic rocks of Halle, and even 

 doubted the distinction between glass and water vesicles. 



The publication of Zirkel's Lehrbuch der Petrographie 

 (Bonn, 1866) may be said to mark the culmination of the 

 older methods, and the academical initiation of the new. In 

 his text-book Zirkel embraced all that was known about the 

 mineralogical and chemical composition, the structure, system- 

 atic arrangement, the mode of occurrence and origin of the 

 various rocks; he also described the crystallographical results 

 which had already accrued from microscopic investigation, and 

 indicated the far-reaching advantages opened up by the new 

 direction of research. Zirkel's work removed all doubt 

 regarding the value of the microscopic results for systematic 

 petrography. 



Vogelsang, in his Philosophic der Geologic und Mikro- 

 skopische Gesieins-Studien (Bonn, 1867), accepted the new 

 teaching in full, and added much to the knowledge of the 



