There is a form of granite called pegmatite, which is found in veins or dikes. It is very coarsely granular. The crystals of feldspar sometimes measure a foot or more in diameter, while the quartz and muscovite crystals corre spond in size. Pegmatite is the source of nearly all of the white mica of commerce. It is also remarkable as being the home of a variety of extremely rare minerals — com pounds of thorium, lanthanum, yttrium, etc— among which may be mentioned uraninite or pitchblende, the present source of radium. Pegmatite is characterized by the interesting parallel intergrowth of feldspar and quartz called graphic granite. Pegmatite is particu larly noteworthy, however, as having been de posited from an intensely heated aqueous solu tion, perhaps from vapor of water (mingled with other gases, such as carbonic acid, hydro fluoric acid, boracic acid, etc.), which was un der such great pressure that it bordered on the fluid condition. The minerals of pegmatite. more particularly the quartz crystals, are fre quently filled with fluid inclusions of water or carbonic acid. A great variety of minerals and a few rocks are considered to have been formed in this manner, and are said to be of pneumatolytic origin (Greek, trveivnirew, to turn into air), and such processes are termed pneumatolytic processes. Furthermore, since these processes usually succeed some great vol canic outbreak, they are termed post-volcanic processes.
The granites as a family have a range in silica which varies between 65 and 80 per cent.
Where a magma of the chemical composition of granite erupts it may form dikes, sheets or flows. The rocks may have a variety of colors, but they are prevailingly light ashy gray. Where crystalline, they possess the same min eral constituents asgranite, except that the feldspar is apt to be the glassy variety of or thoclase called sanidine. The porphyritic text ure prevails among the intrusive and extrusive forms alike.
The geologically older intrusives of this family are called quartz or orthoclase por phyry according to whether the phenocrysts im bedded in the felsitic ground-mass are quartz or orthoclase. The strictly extrusive forms of granitic composition are known as rhyolites (from the Greek word which means to flow). so called because of the flow-structure which is commonly developed in them. The rhyolites are rarely holocrystalline, containing nearly al ways more or less glass and occasionally con sisting wholly of it. These entirely glassy forms of rhyolite are called obsidian. Perlite and pitchstone are varieties of obsidian. Occa sionally the rhyolites are frothy in character, due to the rapid escape of steam resulting from the relief from great pressure. This form is known as pumice. RhYrrlite is also called liparite because extruded abundantly from the volcanoes of the Lipari Islands.
%make,— The hornblende granites, as al ready explained, are those in which the mica has been replaced by hornblende. An occur rence of such a rock at Syene (now Assuan) on the Nile, was called syenite from its local ity. Of late the term has been restricted to quartzless rocks consisting primarily of ortho clase and hornblende. Rocks of the syenite family usually carry varying amounts of plagioclase and may contain subordinate quartz. Apatite, zircon. titanite and magnetite are uni formly present as accessory constituents. In texture they are granitoid and, by an increase of quartz, grade over completely into the gran ites, or by an increase of the plagioclase pass over into the diorites (described below). Where augite replaces the hornblende the rock is called augite-syenite; where mica replaces it, minnette. Nepheline and orthoclase constitute nepheline-syenite.
The range in silica percentage for the typical syenites is between 55 and 65 per cent.
When a magma having the chemical com position of •a typical syenite erupts it forms dikes, sheets and flows, closely resembling the eruptive forms of granite, but without quartz. The loosely applied mining term porphyry is used indiscriminately for the eruptive members of both the granite and syenite families. True eruptive syenite, however, is called trachyte, from the Greek word which means rough. The most typical occurrences are light-colored, ashy gray rocks, either aphanitic or porphyritic in texture, with usually some glass in the ground mass. When porphyritic the phenocrysts are of the glassy variety of orthoclase called sanidine. In fact, the entire felspathic constituent is chiefly of this variety.
The eruptive rock having the composition of nepheline-syenite is called phonolite because of the ringing sound which thin plates make when struck with a hammer.
Rock-Families: II. Basic Division; Dior ites.— These diorites are plutonic rocks having a silica percentage running from 50 to 65 per cent. They are of medium acidity and the magmas from which they are formed may be considered as approachine closely the hypothet ical parent magma from which, by the process of cleavage, the other rock-families, acid, basic and very basic, may be thought of as having originated. Mineralogically they consist of hornblende and plagioclase feldspar. Those in which mica replaces a part of the hornblende are called mica-diorites, while the same con stituents plus quartz go by the name of quartz diorites or granite-diorites. These rocks mark the transition to the granites. Augite-diorites are rocks of the diorite type, in which horn blende is replaced in part by augite. They grade over into the gabbros.