340 HISTORY OF GEOLOGY AND PAL/EONTOLOGY. 



occurrence. The diction is clear, the previous literature of 

 petrography has been completely mastered, and its results are 

 fully incorporated, the historical development of the different 

 branches of the study being carefully indicated throughout the 

 work. The lack of illustrations has been deplored by many, 

 but the addition of plates would have rendered the work much 

 more expensive. 



The first volume begins with a detailed account of all 

 methods of investigation applied in modern petrography. 

 Rock-forming minerals are then described according to their 

 morphological, optical, physical, and chemical properties so 

 far as these are important for petrography. In discussing the 

 structure of rocks, Zirkel frequently dissents from the 

 terminology of Rosenbusch ; at the same time he endeavours 

 to establish the terms which had been applied in his own 

 text-book. 



The special petrographical part of the work starts with the 

 treatment of the massive rocks formed by the cooling and 

 consolidation of molten magmas. The geological occurrence, 

 the composition, the macroscopical and microscopical features 

 of their structure, are elucidated. The difficult questions 

 concerning ground-masses are then brought forward, and 

 finally the laboratory experiments are described by means of 

 which chemists and geologists have tried to produce different 

 kinds of massive rocks artificially. 



Zirkel contests the principle of classification adopted by 

 Rosenbusch, and adduces weighty arguments to show that the 

 group of "intrusive" or "dyke" rocks is intenable. He 

 adheres to the principle of mineralogical composition as the 

 true basis of classification, and draws up a Classification Table 

 on the same lines as he had followed in the first edition of 

 his text-book. Zirkel's sub divisions agree in many respects 

 with those of Fouque and Michel-LeVy. Taking the felspathic 

 constituents as the chief standard, Zirkel distinguishes two 

 felspar-bearing groups, a potash-felspar group, and a soda-lime 

 felspar group; also a third group, free from felspar, and 

 comprising the nepheline, leucite, melilite rocks. 



Like Michel-Levy, Zirkel distinguishes two leading types of 

 structure: i, uniformly granular; 2, porphyritic and glassy 

 rocks. Deep-seated rocks of various geological ages belong to 

 the granular or granitic type ; while eruptive flows may be 

 either porphyritic or glassy, and they may be sub-divided 



