ROCKS 47 
associated with alkali-granites, alkali-syenites, and elzolite-syenites. 
Orthoclase-porphyry is not so common a rock as quartz-porphyry. It 
occurs in S. Scotland, where it is associated with volcanic rocks of Old 
Red Sandstone age. 
Trachyte is a hemicrystalline rock, usually light or dark 
grey or yellowish, but sometimes brownish or even reddish. 
The texture of the groundmass is commonly close-grained, 
apparently sometimes compact; frequently, however, it has a 
rough, porous structure. Disseminated through it, pheno- 
crysts are usually conspicuous, especially sanidine, in addition 
to which plagioclase, hornblende, biotite, and pyroxene 
frequently occur. 
Under the microscope the groundmass would appear to consist 
essentially of lath-like microlites of sanidine, frequently showing fluxion 
structure, and often entangling a few granules of a ferromagnesian 
mineral, which is usually augite (see Plate IX. 4). Some interstitial 
glass or microfelsitic matter may be present. Accessory minerals are 
numerous, amongst them being apatite, magnetite, zircon, sphene ; while 
in certain trachytes, sodalite and olivine occasionally make their 
appearance. 
The glassy varieties of trachyte are known as 7vachyte-obsidian and 
Trachyte-pitchstone, and so closely resemble the rhyolitic glasses that 
they can hardly be distinguished from these except by chemical analyses. 
They contain a lower percentage of silica (about 62). 
Trachyte is one of the commonest effusive rocks of Tertiary and later age 
—occurring in most volcanic districts in the old and the new worlds. 
Trachytes, however, are not exclusively young rocks ; rocks of this type 
occur among the Old Red Sandstone and the Carboniferous volcanic 
series of Scotland. 
Phonolite is a greenish or greyish to white or yellow, and sometimes 
brown rock composed essentially of sanidine and nepheline or leucite 
(either or both). The texture is usually compact, with a somewhat greasy 
lustre, or it may be fine-grained with dull lustre. The most conspicuous 
phenocrysts are sanidine and nepheline (or leucite), besides which the 
unassisted eye may often distinguish pyroxene or amphibole. The rock 
is characterised by the absence of quartz, by its somewhat flaggy 
structure, by its conspicuous crystals of sanidine, and by the bell-like 
clink it gives out when struck with the hammer. 
The microscope proves the groundmass to consist of microlites and 
small crystals of sanidine and nepheline (or leucite), which not infre- 
quently show parallel arrangement—a structure to which probably the 
flaggy structure of the rock is in some measure due. Interstitial glass 
is rarely present. The microscope reveals many other minerals besides 
those of macroscopic size, such as the common accessories, sphene, 
zircon, etc. ; while one or more of the following may be present : biotite, 
