458 WHITMAN CROSS 



and A. Michel-Levy, which was published in 1879. While 

 presented as an "Introduction a l'etude des roches eruptives 

 francaises," there is in this work some discussion of principles of 

 classification, and a tabular view of the scheme in use by the 

 authors. Although the system in question has not had much 

 influence except in France, it is of interest from certain new and 

 peculiar conceptions which are given classificatory value in it. 



It is first to be noted that Fouque and Michel-Levy abandon 

 almost entirely the system of Cordier. Although affirming that 

 rocks are simply the most abundant natural associations of min- 

 erals (" les roches ne sont autre chose que les associations miner- 

 ales naturelles le plus frequentes "), they proceed to their 

 arrangement under the stated principle that a rational classifica- 

 tion of rocks in general must be based upon the following three 

 fundamental characters, namely : (1) The mode of formation; 

 (2) the geological age; (3) the specific mineral properties. 

 The last named character comprises: (a) the nature of the 

 integrant minerals; {b) the structure of the rock. 1 By the 

 application of the first factor they separate eruptive rocks from 

 those deposited as sediments or as vein filling. By the factor 

 of geological age (the applicability of which is not discussed) 

 they distinguish clearly — "nettement" — between pre-Tertiary 

 and Tertiary or post-Tertiary rocks. It is believed that the 

 same types occur in both groups, following approximately the 

 same order of eruption, with predominance of basic rocks and a 

 tendency to the vitreous condition in the most recent occur- 

 rences. In these respects the new French system is practically 

 like others which have been reviewed, but in the use of structure 

 and mi?ieral composition for the main framework of their system, 

 Messrs. Fouque and Michel-Levy apply certain peculiar con- 

 ceptions requiring some analysis. 



Considering that in practically all eruptive rocks there have 



been two (or more) distinct periods of formation of the primary 



1 " Un classement rationel des roches, en general, doit s'appuyer sur les trois 

 caracteres fondamentaux suivants : I ° le mode deformation; 2 ° l'age geologique ; 

 3 la specification mineralogique. Ce dernier caractere comprend : (a) la nature 

 des mine'raux integrants; (U) leur structure d'association (structure de la roche.)" 



