THE COMPOSITION OF THE EARTH 29 



Syenite (B, Fig. 10) is a rock composed chiefly of feldspar, 

 with smaller amounts of the ferromagnesian minerals, partic- 

 ularly hornblende, and little or no quartz. It is usually gray 

 or reddish, and often closely resembles granite both in color 

 and texture. Syenite is a neutral rock. Like granite, it 

 may have a porphyritic texture. Granite and syenite are 

 called feldspathic rocks (Fig. 10), because feldspar predominates 

 in both. Syenite is a much less common rock than granite. 



Diorite is made up chiefly of hornblende and subordinately 

 of feldspar (C, Fig. 10) . Gabbro consists mainly of augite, with a 

 subordinate amount of feldspar. Diorite and gabbro are gen- 

 erally dark-colored. They are sometimes not distinguishable, 

 because it is not apparent to the naked eye whether the dom- 

 inant mineral is hornblende or augite. The rock may then be 

 called dolerite (meaning deceptive). Some diorites are neutral 

 rocks, while others are basic. Gabbro is basic. Diorite and 

 gabbro are widely distributed and common rocks. 



Peridotite (D, Fig. 10) is a basic rock composed entirely of 

 the dark-colored minerals olivine, hornblende, or augite. 

 These may occur alone or in mixtures. Both feldspar and 

 quartz are absent. The rock is black or dark green, and is 

 much less common than the preceding ones. 



Dense rocks. Most or all of the grains in the rocks of 

 this class are too minute to be distinguished by the naked eye. 

 When nonporphyritic, many of these rocks have a rather uni- 

 form, stony appearance. Such a rock, when dark-colored, 

 may be called basalt; when light-colored, felsite. (The stu- 

 dent must guard against confusing felsite with certain fine- 

 grained sandstones.) Similar names are used when the tex- 

 ture is porphyritic ; if the ground mass is light-colored, the 

 rock is felsite-porphyry, if dark-colored, basalt-porphyry. Fur- 

 ther subdivisions of the porphyries may be made in terms 

 of the minerals which form the visible crystals. Thus there 

 is quartz-felsite-porphyry, feldspar-basalt-porphyry, etc. The 

 light rocks of this class are chiefly feldspathic, while the dark 

 ones are mainly ferromagnesian. 



