THE ORIGIN AND DESCENT OF ROCKS. 453 



(b) The porphyritic aphanites or porphyries, when Hght-colored, 

 are leucophyres; when dark-colored, are melaphyres (f.n.). They may 

 be classified further, according to the kind of phenocryst imbedded in 

 the aphanitic ground-mass, as 



Quartz-porphyries, or quartzophyres ; 



Feldspar-porphyries, or feldspaphyres (not f elsophyres) ; 



Hornblende-porphyries, or hornblendophyres ; and so on. 



These may be subclassed by color, as 



Quartz-leucophyres, light-colored quartz-porphyries; 



Quartz-melaphyres, dark-colored quartz-porphyries; 



Feldspar-leucophyres; 



Feldspar-melaphyres; and so on. 



III. The glasses are classified, according to color and luster, into 

 obsidians or pitchstones when dark and lustrous; perlites, when a sphe- 

 roidal fracture gives them a pearly appearance; and pumice when 

 greatly inflated by included gases. 



In general discussions, it is regarded as serviceable to use the term 

 granitoids in a broad generic sense, to include all crystalline rocks of 

 the general granitoid type, including the granites, syenites, gneisses, 

 etc. In a similar broad way, the term gabbroids may be used to include 

 the dark crystaUine rocks in which the ferromagnesian minerals pre- 

 dominate, as the diorites, gabbros, dolerites, peridotites, etc. In this 

 convenient and comprehensive way, two contrasted groups of igneous 

 rocks may be designated. As the granitoids are usually acidic and 

 the gabbroids usually basic, the grouping represents a broad fact of 

 importance. 



