90 Reviews — A. M. Levi/ — On Eruptive Bocks. 



rocks, lie has recourse to the agents mineralisateurs to which French 

 petrologists from the time of Elie de Beaumont have attached so 

 much importance. 1 He ascribes all the differences of structure that 

 can arise from a magma of given composition to variations in the 

 three factors, temperature, pressure, and 'mineralisers,' and considers 

 that in the structure of the basic rocks the first of these factors has 

 had a dominant influence. Indeed he seems to imply that the order 

 of crystallization of the constituents of these rocks depends in 

 general on their relative fusibility, the initial temperature of the 

 magma, and its successive more or less sudden variations. In this 

 connection he cites the synthetic experiments of M. Fouque and 

 himself, which perhaps have not received due appreciation at the 

 hands of geologists in general. 



Our author strenuously denies the generality of the rule which 

 associates the granitoid types of structure with deep-seated igneous 

 rocks and the porphyritic and allied types with extravasated 

 products, and quotes a number of instances in opposition to it. The 

 citation of the Tertiary gabbro 'domes ' of the Hebrides as analogous 

 to the puys of Auvergne seems to rest on a misunderstanding of 

 Dr. Geikie's description, and in any case the exceptional occurrences 

 mentioned are scarcely sufficient to overthrow so general a law. 



The second chapter is devoted mainly to a discussion of the minute 

 structures of rocks and a comparison of the French and German 

 views of the nature of ' petrosilex,' and the various centric and 

 spherulitic structures of the acid series. From the concluding 

 remarks we gather that the author definitively abandons the age, or 

 presumed age, of igneous rocks as a principle of classification, and 

 his observations in this connection will certainly commend them- 

 selves to English petrologists. 



In the next chapter M. Michel Levy considers the mineralogical 

 constitution of igneous rocks, especially as an exponent of their 

 chemical composition. He shows that by selecting as a basis of 

 classification of the porphyritic rocks the elements of the first period 

 of consolidation Eosenbusch has been led into a scheme of arrange- 

 ment which accords but very imperfectly with the bulk-analyses of 

 the rocks. He prefers to take account more particularly of the 

 dominant ' white ' (i.e. non-ferriferous) constituents of the second 

 period of consolidation, which afford a more accurate index of the 

 chemical composition of the rock. To elucidate his views he intro- 

 duces a system of formulas designed to express at once the structure 

 of a rock, its mineral constituents, their order of consolidation, and 

 their division into two periods of consolidation. 



In the fourth chapter we have a detailed study of Eosenbusch's 

 classification of eruptive rocks, and the artificial character of some 

 of his divisions is clearly exhibited. Our author further taxes the 

 leader of the German school with altering the meaning of well- 

 established descriptive terms and often overlooking the prior claims 

 of the nomenclature proposed by French petrologists, and it is 

 impossible to deny that this protest is in many instances a just one. 



While all geologists will sympathize with M. Michel Levy's desire 

 1 Cf. de Lapparcnt, Bui. Soc. Geo!. Fr. (3) xvii. p. 282 ; 1889. 



