CHAPTER Till. 



GROUP B. INTERMEDIATE DIVISION. 



NOMENCLATURE. 



rocks ot this division are distinguished from those of the preced- 

 ing by their specific gravity and composition. The specific 

 gravity of the holocrystalline members lies generally between 27 and 2'8 

 and the silica-percentage between 55 and 65 The ferro-magnesian 

 constituents play a less important part than in the rocks of the basic 

 division. It is, however, quite impossible to give any precise definition 

 of the group. The basic rocks shade into the intermediate rocks, and 

 these again into the acid rocks, in the most gradual manner. 



Various names such as diorite, tonalite, banatite, propylite, quartz- 

 norite, trachy-dolerite, andesite, porphyrite, have been applied to different 

 members of the group. Some of the rocks to which the term granite has 

 been applied must be regarded as belonging to this rather than to the 

 acid division ; especially some of the hornblende- and augite-granites. 



Diorite. The history of this term has been briefly referred to on 

 p. 133. It cannot be said to have at present any very definite signification. 

 Almost all writers agree that a rock must be holocrystalline in texture 

 and must contain a plagioclase felspar in order to justify the application 

 to it of the term diorite, but beyond this there is no general agreement. 

 The ferro-magnesian constituent may be biotite, hornblende, augite 

 (diallage), _enstatite (hypersthene) or mixtures of these in varying 

 proportions. Many of the rocks to which the term augite-diorite has 

 been applied, including some of the typical augite-diorites described by 

 STRENG from Minnesota, are basic in composition. Precisely similar rocks 

 occurring near Baltimore have recently been described by G. H. WILLIAMS 

 as gabbros. It appears therefore that, as at present used, the term, diorite 

 has only the vaguest possible signification. Metamorphosed gabbros 

 and dolerites, unmetamorphosed gabbros (augite-diorites) and plagioclase 

 rocks containing original hornblende and belonging to the basic and 

 intermediate groups have all been termed diorites. Some order will be 

 introduced into this chaos if we separate altogether the basic rocks with 

 secondary hornblende from the diorites, under GUMBEL'S name epidiorite, 



