260 MATERIALS AND THEIR ARRANGEMENT 



minerals is abundant, its name is often prefixed, as hornblende-granite. Granite 

 grades insensibly into other types of igneous rock, as syenite, diorite, etc. Varia- 

 tions also arise from the absence of one of the leading minerals. 



Granites were formed from lavas rich in silica (normally 68-70%), alumina, 

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

 generally an intrusive massive rock. When rock of the composition of granite is 

 banded, it is gneiss. 



Graphic granite, composed chiefly of inter-grown crystals of quartz and feld- 

 spar, has a peculiar texture (Fig. 240). Pegmatite is a variety of coarsely cry- 

 stalline granite composed chiefly of quartz, feldspar, and muscovite (p. 255). It 

 occurs principally in dikes and veins associated with granitic and other similar 

 rocks. Rock of similar texture may have the composition of syenite (syenite 

 pegmatite), diorite (diorite pegmatite) etc. 



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

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

 blende, with or without mica; but there is a complete gradation from granites to 

 syenites. Syenites are richer in iron and magnesium than granites, and poorer in 

 silica (about 58-60%). Syenites also grade into other classes of rock as do granites, 

 and special varieties are named by similar prefixes, as augite-syenite, etc. Syenites 

 are red or gray according to the color of the feldspar, and most of them are rather 

 darker than granite, which they resemble. The texture of syenite is like that of 

 granite, and its mode of occurrence the same. 



The diorites. 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 feld- 

 spar. It differs from syenite in having plagioclase feldspar (p. 254) 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. In color most diorites are rather darker than the gray granites. 



The gabbros. The gabbros embrace a large group of rocks whose principal 

 minerals are plagioclase (normally labradorite) and pyroxene (normally diallage), 

 with magnetite or ilmenite (titanium iron oxide). Most gabbros are 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. Silica constitutes 

 about 46 to 55 per cent of gabbros. 



The peridotiles. Peridotites were formed from a magma in which silica was 

 low (39-45%), as were also alumina, lime, and the alkalies, but in which magnesia 

 was high (35-48%). The rock consists largely of the mineral olivine (p. 255.) asso- 

 ciated with pyroxene, magnetite, and other basic minerals. Little or no feldspar 

 is present. Peridotites are much less abundant than the preceding rocks. 



Closely allied with the peridotites are rocks made up largely of a single basic 

 mineral, as augitite, pyroxenite, hornblendite, rocks essentially formed of the min- 

 erals augite, pyroxene, and hornblende, respectively. 



The basalts. The term basalt is used in a somewhat comprehensive way for 

 dark, compact, igneous rocks the crystals of which are in most cases so minute as 

 not to be distinguished readily by the eye. The leading minerals are a plagioclase 

 feldspar and pyroxene (usually augite), with olivine, and magnetite or ilmenite 

 usually present. There is a considerable range in chemical composition, but the 



