482 SCIENCE PROGRESS. 



Rosenbusch (22), perhaps for convenience of descriptive 

 treatment rather than as a strictly logical arrangement, 

 divides the igneous rocks in the first place into three classes 

 according to their mode of occurrence, a factor with which 

 the micro-structure of the rocks is closely connected. These 

 three classes, " Tiefengesteine," " Ganggesteine," and 

 " Ero-usso-esteine," are then divided into families defined 

 by mineralogical (and to some extent implicitly by chemical) 

 characters. It is noticeable that while there is a fair 

 correspondence between the families of the first and third 

 classes, the "Ganggesteine" or "dyke-rocks" include some, 

 such as the aplites and the lamprophyres, which are not 

 easily paralleled among either the plutonic or the volcanic 

 rocks. 



Brogger, on the other hand, takes as his primary 

 divisions families defined by chemical characters. These 

 families may be divided into subfamilies, also on chemical 

 grounds, and each includes abyssal, hypabyssal, and super- 

 ficial rocks. The granite family, for example, embraces all 

 igneous rocks having the composition of the granites ; such 

 as normal granites, granite-porphyries, granophyres, quartz- 

 porphyries, pitchstones, rhyolites, etc. But it also finds 

 places for rocks which, while having close relations with these, 

 differ from them in chemical, as well as in mineralogical 

 and structural characters. The hypabyssal members and 

 marginal modifications (Grenzfaciesbildungen) fall indeed 

 under two heads ; non-differentiated (" aschiste ") and dif- 

 ferentiated (" diaschiste "). Under the former head are 

 placed the quartz-porphyries, granophyres, etc., while the 

 latter covers such rocks as minettes and aplites. 



An important feature of Brogger's system is the con- 

 ception of a rock series. In a tabular scheme the various 

 types are ranged in rows and columns, the rows correspond- 

 ing- to families, and the columns to series. The members 

 of one series thus belong to different families. The ideal 

 rock series consists of a number of such types, connected 

 by intermediate gradations, and falling under the same 

 broad structural division (plutonic, hypabyssal, or volcanic). 

 They have certain mineralogical and chemical characters in 



