472 WHITMAN CROSS 



thetical considerations in giving form to this system, there is 

 little of note to comment upon in this place. In discussing the 

 essential characters of the three groups of Dike rocks, Rosen- 

 busch brings out more forcibly than before the genetic idea 

 really lying at the base of the distinction of the Dike rock class. 

 In connection with the discussion of the Dike rocks it is sug- 

 gested that a further class may be necessary to include the 

 intrusive rocks of sheets and laccoliths which seem to him to 

 possess distinctive structures. 



That the system of Rosenbusch does not result in a consistent 

 and logical classification of igneous rocks is abundantly illus- 

 trated by numerous instances, many of them freely acknowl- 

 edged by the author. The family of the diabase rocks furnishes 

 one of the most notable cases. In 1887 these rocks were classi- 

 fied with the deep-seated rocks, although many of them were 

 known to be effusive; in 1896 the same rocks are placed with 

 the effusives, with the statement that many are intrusive. 

 Placed in the effusive class, they are acknowledged to be partly 

 of older and partly of younger age, but no age distinction is 

 thought to be practicable. 



Justus Roth, 1883. — Shortly before Rosenbusch issued the 

 second edition of his Physiographie, there appeared a complete 

 systematic discussion of rocks by Justus Roth, in the second 

 volume of his Ge?ieral and Chemical Geology.* Petrography is 

 defined by Roth as the science of the mode of origin, constitu- 

 tion, and alteration of rocks ; i. e., the petrology of many 

 English and American writers. In introducing the systematic 

 descriptive part of the subject, Roth remarks : " The difficulty 

 in constructing a system of rocks is completely expressed in the 

 term aggregate, and thereby all recourse to genera and species is 

 prohibited." 2 From the nature of rocks and the conditions of 



1 Allgemeine und chemische Geologie; Zweiter Band, " Petrographie " (Berlin, 

 1883-1885), pp. X + 695. 



2 "Die Schwierigkeit der Systematik der Gesteine ist durch die Bezeichnung 

 Aggregat vollstandig ausgedriickt und damit alle Anlehnung an Gattungen und 

 Species ausgeschlossen," loc. cit., p. 41. 



