PETROGRAPHY. 341 



according to age as pre-Tertiary ( Pal aeo volcanic), and Tertiary 

 and Post-Tertiary (Neovolcanic). 



The second and third volumes of the text-book are devoted 

 to an exhaustive description of the individual varieties of 

 massive and schistose crystalline rocks and the sedimentary 

 rocks. 



Zirkel's text-book will always remain a fundamental work 

 in petrography. While the macroscopic methods of the 

 older teaching are still predominant in the first edition of the 

 work, the second edition is at once a frank and full 

 acknowledgment of the petrographical reform necessitated by 

 microscopic and micro-chemical methods, and a convincing 

 witness of the rapid and remarkable success which had 

 crowned the labours of petrographers in the new field of 

 research. 



During the last few years the discrepancy between the views 

 of Zirkel and Rosenbusch has increased. Rosenbusch, in the 

 third edition of his Physiographic der massigen Gesteine (1896), 

 and also in his Elementender Gesteinslehre(\%$], has adhered 

 to the standpoint which he assumed in 1888, and has rejected 

 Zirkel's objections. The differences between the two leading 

 German petrographers refer in no sense, however, to the 

 methods of investigation, but expressly concern the inductive 

 conclusions at which they have arrived regarding the genesis 

 of the eruptive rocks, and the best system of classification. 

 The rapid progress of petrography is one of the greatest 

 acquisitions made to science during the latter half of the 

 nineteenth century, and has elevated petrography to the rank 

 of a thoroughly established branch of natural philosophy. 



As the microscope revealed more and more fully the fine 

 structure and microscopic elements, of rocks, the traditional 

 conceptions of geologists regarding the origin of the rocks 

 were gradually undermined. The old strife between Plutonists 

 and Neptunists had collapsed when the Neptunists admitted 

 the volcanic origin of basalt and the "trap" series of rocks. 

 The handsome monograph published by C. C. von Leon- 

 hard in 1832 had conclusively proved the agreement of 

 basalt with true volcanic rocks, both in the geological 

 occurrence of the basalt and in the contact phenomena 

 produced at its margins. Thanks to the observations of 

 Humboldt, Buch, Poulett-Scrope and others, not only was 

 the volcanic origin of basalt, trachyte, trap, porphyry, mela- 



