330 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



porphyritic series by his careful microscopic investigation of 

 the larger mineral constituents and the ground-mass char- 

 acteristic of different varieties. Vogelsang's observations on 

 the processes of the consolidation of rock-magma, on the 

 microscopical structure of slags, on "fluidal" structure, on 

 microlites, and on conditions of devitrification, are clear and 

 accurate. His illustrations are throughout of high excellence; 

 and his proof, given in collaboration with Geissler, that certain 

 liquid inclusions in minerals and rocks consist of liquid car- 

 bonic acid, is a discovery that will ever remain associated with 

 the name of this promising scientist, who unfortunately died 

 before he reached his prime. 



Special memoirs were contributed by Zirkel on phonolite, 

 on glassy and partially glassy rocks, and on leucite rocks. A 

 very important work was his Untersuchung iiber die mikro- 

 skopische Zusammensetzung und Struktur der Basaltgesteine 

 (Bonn, 1870). In this work, Zirkel showed for the first time 

 that the basalts and the lavas corresponding to them may be 

 classified in three groups (felspar, nepheline, and leucite 

 basalts), and that each of these three modifications can be 

 identified by its constitution and structure, as well as by the 

 ground-mass. 



A few months before the appearance of Zirkel's work 

 on basalt, Tschermak had published a short but valuable 

 paper on the microscopic differentiation of the minerals 

 belonging to the augite, hornblende, and biotite group, and 

 thus removed one of the chief difficulties in the identification 

 of rock-forming minerals. 



The year 1873 was signalised by the almost simultaneous 

 appearance of two works, in which the two most distinguished 

 masters in the domain of microscopical research comprised 

 the quintessence of their investigations. Under the title, Die 

 mikroskopische Beschaffenheit der Mineralien imd Felsarten 

 (Leipzig, 1873), Zirkel gives an introductory code of instruc- 

 tions as to the use of the microscope, examination by means 

 of polarised light, and the methods of producing faithful 

 illustrations. He then describes the microscopical structure of 

 rock-forming minerals with special respect to the various kinds 

 of inclusions and the products of decomposition. The 

 optical and physical characteristics of mineral sections are 

 next described ; and the results obtained in the earlier 

 chapters on minerals are applied in the latter half of the work, 



