MATERIALS OF THE EARTH 29 



by the feldspar, the red and pink varieties of the mineral giving rise to red 

 granite, and the whitish varieties to gray granite. 



Granites vary widely from their type form by the addition and substitu- 

 tion of other minerals, and these sometimes become as prominent as the type 

 minerals. Thus other feldspars sometimes take the place of the orthoclase 

 (p. 79), or accompany it; hornblende and other minerals take the place of 

 the biotite, or occur with it, and so on. Whenever one of these replacing 

 or accessory minerals is abundant, its name is often prefixed, as hornblende- 

 granite. Granite grades almost insensibly into other types of igneous rock, 

 as syenite, diorite, etc. Variations also arise from the absence of one of the 

 three leading minerals. 



The granites were formed from a lava rich in silica (68-70%), alumina, 

 potash, and soda, but generally poor in lime, iron, and magnesia. Incidentally 

 other substances are present. Granite is generally an intrusive massive rock 

 which solidified slowly under conditions which permitted complete crystal- 

 lization, with the development of large crystals. 



When rock of the composition of granite is banded, it is gneiss (p. 84). 

 The texture of graphic granite (see pegmatite, p. 86) is notably peculiar, due 

 to the simultaneous crystallization of the quartz and feldspar (Fig. 12). 



The syenites. The term syenite (from Syene on the Nile, where this sort 

 of rock occurs) is now applied to a rock consisting essentially of feldspar and 

 hornblende (with or without mica) ; but there is a complete gradation from 

 the granites to the syenites. The syenites are richer in iron and magnesium 

 than the granites, and poorer in silica (about 58-60%). The syenites also 

 grade into other classes of rock as do the granites, and are named by similar 

 prefixes, as augite-syenite, etc. The syenites are red or gray, according to 

 the color of the feldspar, and are usually darker than the granites, which they 

 resemble. The texture of syenite is like that of granite. 



The diorites. The diorites are rocks which crystallized from lavas having 

 about the same amount of silica as the lavas of the syenites, but poorer in 

 the alkalies, and richer in the earthy bases. In current usage, diorite is 

 defined as a rock composed of an intimate mixture of crystals of hornblende 

 and a plagioclase feldspar. It differs from syenite in having plagioclase 

 feldspar (p. 77) instead of orthoclase. By substitutions and the addition 

 of accessory minerals, the diorites grade toward the granites and syenites 

 on the one hand, and toward the gabbros on the other. 



The gabbros. The name gabbro was formerly applied to a coarse-grained 

 basic rock consisting of labradorite (p. 78) and diallage (p. 76); but the 

 name has been extended gradually until it embraces a large group of rocks 

 whose principal minerals are plagioclase (normally labradorite) and pyroxene 

 (normally diallage), with magnetite or ilmenite (titanium iron oxide). The 

 gabbros are usually dark colored and rather heavy. The pearly luster of 

 the cleavage faces of the diallage gives a peculiar sheen to a fresh surface of 

 the rock, in many cases. 



