ROCKS 4] 
place of felspar, while in the other neither felspars nor 
felspathoids are present. 
1. ROCKS WITH DOMINANT ALKALI FELSPAR 
This group includes the granites, quartz-porphyries, and 
rhyolites—all acid rocks with a percentage of silica ranging 
up to 80, and the syenites, trachytes, and phonolites— 
intermediate rocks with a silica percentage ranging up to 
70 or thereabout. They exhibit all kinds and degrees of 
texture and structure, some being crystalline, others hemi- 
crystalline, and yet others essentially vitreous. As a rule, 
the coarse-grained holocrystalline types are of deep-seated 
origin, while the finer grained microgranitic and porphyritic 
types are usually hypabyssal. Many of these finer grained 
holocrystalline rocks occur as dykes, veins, and sills. The 
hemicrystalline and vitreous types have as a rule flowed out 
as lavas, or have consolidated as intrusive rocks not far from 
the surface. 
Granite is a holocrystalline aggregate of quartz, alkali 
felspar (orthoclase and microcline, usually accompanied by 
some plagioclase), and a ferromagnesian mineral (mica or 
hornblende), the constituents occurring as crystalline granules 
of approximately similar size (= gvanztocd structure). The 
rock varies in texture from microcrystalline to very coarsely 
crystalline. The colour, which largely depends on that of the 
felspar, is usually light or dark grey or reddish ; occasionally 
it is greenish. In coarse or medium grained granite the 
essential minerals are readily distinguished. The felspar 
appears opaque, and is sure to show some of its crystal faces 
and cleavage-planes with their vitreous or pearly lustre. The 
quartz, on the other hand, is quite irregularly outlined. It 
is usually dark grey but transparent, and shows a kind of 
glassy lustre on its uneven broken surfaces; there is no 
trace of cleavage. The mica occurs in lustrous plates and 
scales which are readily separated into the thinnest lamelle. 
The hornblende is recognised by its dark green colour and 
its common prismatic or columnar appearance. Accessory 
minerals may or may not be numerous, some of the com- 
monest being apatite, sphene, zircon, magnetite, etc, 

