Notices of Books and Papers. 57 



where so many points come up for discussion this perhaps would 

 not always be possible. 



It is beyond the scope of the present notice even to mention the 

 many excellencies of the book. The present volume will be found 

 especially useful as presenting a resumi of our present knowledge of 

 various parts of the science in which rapid advances are now being 

 made, as for instance the question of the chemical relations of the 

 eruptive rocks, now so widely investigated and discussed — the 

 artificial reproduction of rocks, etc- It may be noted however that 

 even in the time which has elapssd since the writing of the earlier 

 parts of this volume some of the incorporated material has 

 already become somewhat antiquated. 



It is a matter of much satisfaction and one which will afford 

 much relief to geologists in general to find that in his classification 

 of the eruptive rocks Prof. Zirkel agrees in the main with 

 Prof. Rosenbusch, whose scheme is now in general use. It thus 

 seems that at least in its principal features the petrographical 

 classification has been generally agreed upon. Many modifications 

 will undoubtedly be found to be necessary with the advance of 

 the science, but we now have at least a good working classifi- 

 cation. 



Prof. Zirkel rejects Rosenbusch's division of Dyke Rocks, to 

 which many objections have been raised by others as well, and 

 although refusing to admit that the geological occurrence of an 

 igneous rock is a proper basis for classification, he substitutes for this 

 its structure, which in the great majority of cases depends on its 

 geological occurrence, and this substitution does not therefore 

 materially affect the form of the classification. 



The separation of the " old " from the " new " volcanic rocks is 

 still retained and justified on the ground that although their differ- 

 ences may be due merely to alteration, nevertheless since the dis- 

 tinction can be made in most cases the double nomenclature should 

 be retained, it being quite as convenient to use the terms Rhyolite 

 and Quartz-Porphyry as the terms Tertrary Rhyolite, Carbonifer- 

 ous Rhyolite, &c. 



This argument has especial weight in the case of the German 

 rocks, but since any classification universally adopted must be one 

 which will be suitable and convenient for all countries, it remains 

 to be seen whether this dual nomenclature for the eruptive rocks, 

 so long opposed by English petrographers and fast losing its hold 

 in all directions will not eventually be disgarded, being replaced 

 perhaps by some simple method of distinction such as that 

 recently proposed by Dr. Williams and Miss Bascomb, which 

 consists in placing the prefix apo before the name of any rock which 



