ROCKS 45 
coarse or fine-grained streaks and veins, poor in ferromagnesian con- 
stituents. These veins often look as if they occupied fissures or clefts. 
Such fissures may be supposed to have originated while the plutonic mass 
was only in part solidified—the still mobile, residual, acid magma having 
been squeezed into clefts in the solidified portions during movements of 
the gradually cooling rock. The granites are among the most widely 
distributed of eruptive rocks. Although most are of deep-seated origin, 
yet a few appear to have been intruded at a less depth from the surface. 
Quartz-porphyry.—This is a hemicrystalline rock. The 
groundmass is sometimes composed of a microgranitic aggre- 
gate of quartz and felspar (chiefly orthoclase), at other times 
it is cryptocrystalline. Thus, to the naked eye the ground- 
mass in some cases appears very finely crystalline, and in 
other cases dense and smoothly compact. Scattered through 
the groundmass are numerous phenocrysts of quartz and 
felspar (either or both), together with a ferromagnesian 
mineral (biotite or hornblende). The felspar is usually 
orthoclase, but plagioclase (chiefly oligoclase) is often 
associated with it (see Plate XII. 1). The quartz not infre- 
quently occurs in the form of corroded _bi-pyramidal 
crystals, and often contains inclusions of the groundmass. 
The colour of the rock depends largely upon that of the 
orthoclase, which is often exceedingly abundant, and may 
be red or white. Accessory minerals are also numerous, 
Ssucae aS apatite, sphene, zircon, magnetite: etc. As the 
quartz-porphyries are mostly very old rocks, secondary 
minerals are generally present, especially kaolinite, chlorite, 
epidote, and muscovite. Gvanophyre is a quartz-porphyry, 
the: groundmass of which consists of a micropegmatitic 
Mtecrowth of quartz and felspar (see Plate IX. 1). 
The quartz-porphyries, like the granites, are very widely 
distributed. 
Rhyolite (Lzparite ; Quartz-trachyte).—This rock is usually 
somewhat light-coloured — grey, yellowish, reddish, or 
greenish. It is hemicrystalline—the groundmass varying 
considerably in character in one and the same rock. Some- 
times it is glassy for the most part, or it may be crypto- 
crystalline, or microcrystalline. Scattered throughout are 
phenocrysts of sanidine, plagioclase, quartz, biotite, and 
occasionally hornblende or augite. 



