MATERIALS OF THE EARTH 61 



Feldspar-porphyries, or felspaphyres (not felsophyres) ; 



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-le ucophyres ; 



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 spheroidal fracture 

 gives them a pearly appearance; and pumice when greatly inflated by in- 

 cluded gases. 



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

 itoids in a broad generic sense, to include all crystalline rocks of the general 

 granitoid type, including the granites, gneisses, etc. In a similar broad way, 

 the term gabbroids may b3 used to include the dark crystalline rocks in which 

 the ferromagnesian minerals predominate, as the diorites, gabbros, dole rites, 

 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. 



STRUCTURAL FEATURES COMMON TO ALL ROCKS 



Certain structural features peculiar to one group of rocks or 

 another have been mentioned. Thus stratification is character- 

 istic of sedimentary rocks, columnar structure and massiveness 

 of certain igneous rocks, foliation of many metamorphic rocks, etc. 

 Joints also have been mentioned, and they affect all rocks, extend- 

 ing down to unknown, but often to great depths. 



Faults. Slipping or displacement sometimes takes place along 

 bedding, joint, or fracture planes, and such slippings are faults. 

 The planes of slipping, the fault-planes, may be vertical, or inclined 

 at any angle whatsoever. The angle of departure from the vertical 

 is the hade. When the fault has a hade, the slipping may be 

 such that the overhanging side goes down relatively (gravity or 

 normal fault, Fig. 33), or such that it goes up relatively (thrust 

 fault, Fig. 34) . The hade may approach or even reach 90. Thrust 

 faults are developed under lateral pressure. Thus faults developed 

 during the folding of strata by lateral pressure are chiefly thrust 

 faults, as in the Appalachian Mountains. Gravity faults imply 



