Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/59

Rh potash mica, lepidomelane (Proc. Roy. Soc., xvii. p. 209). Professor Heddle finds that the black mica of most Scottish granites is a distinct species, which he calls Haughtonite (Mineralog. Mag., No. 13, 1879, p. 72). A large number of accessory minerals occur in granite, no fewer than forty-four being cited by Zirkel (Lehrl. d. Petrog., . p. 481). Upon the presence of these supplementary minerals numerous varie ties of granite have been founded. Thus, if tourmaline be present, the rock is a schorlaceous or tourmaline granite ; when cassiterite or tin-stone occurs, it forms a stanniferous granite ; the presence of epidote gives rise to an epidote granite ; and so on with other minerals. The most common accessory constituent of granite is hornblende, a mineral which appears to replace to some extent the mica, and thus produces a hornblendic or syenitic granite. This rock was formerly, and by some petrographers is still, termed syenite ; it is the syenites of Pliny, so named from Syene in Upper Egypt, where a smiliar rock was quarried by the ancient Egyptians. By modern petrographers, however, the term syenite is usually restricted to a rock which is au aggregate of orthoclase and hornblende, in other words, a granite in which the quartz has disappeared while the mica has been superseded by hornblende. A beautiful schorla ceous rock, which is apparently a variety of granite, has been described by Pisani under the name of luxullianite (Com- ptes Rendus, lix., 1864, p. 913). It occurs in the parish of Luxullian, near Lostwithiel, in Cornwall, where it is found in the form of boulders, but has not been detected in situ. This rock is composed of toiirmaline, or schorl, with quartz and orthoslase ; the last named mineral occurring in large flesh-coloured crystals, which by contrast with the dark basa produce a very beautiful effect. Two varieties of tourmaline, one brown and the other bluish, have been detected by Professor Bonney (Mineralog. Mag., No. 7, 1877, p. 215). The sarcophagus of the duke of Wellington, in St Paul s Cathedral, is wrought out of a splendid block of luxullianite. Many varieties of granite are founded upon structural characteristics. Occasionally the constitu ents are developed in such large individuals as to form a giant granite. Crystals of orthoclase, associated with quartz in a peculiar parallel arrangement, produce the variety known as graphic granite or Lapis Judaicus names which refer t-&amp;gt; the resemblance which the rock presents, when cut in certain directions, to lines of Hebrew charac ters. Graph! 3 granite was termed by Hauy pegmatite, but this namo is now generally applied to a coarse admixture of orthoclase, quartz, and silvery mica. When any of the component minerals occur in large crystals, embedded in a fine-grained base, a porphyritic granite is produced. Gene rally the crystals are those of orthoclase, as in many of the West of England granites, and in the characteristic rock of Shap Fell in Westmoreland. Granitite is a name applied to a variety of granite made up of orthoclase and quartz, with more or less plagioclase and a small proportion of mica. A granite composed of only felspar and quartz is called haplite or semi-granite. Some of the micaless varieties are known as granulite. When, instead of the mica disappear ing, the felspar is absent, the resulting aggregate of quartz and mica is termed greisen ; it is frequently a tin-bearing rock. Occasionally the granite, when fine in grain, loses its mica, and an intimate mixture of orthoclase and quartz is thus obtained ; such a rock is known as a felstone. Crystals of orthoclase disseminated through a felsitic matrix, either compact or microcrystalline, give rise to a felspar porphyry ; while crystals or rounded grains of quartz in a similar felsitic base produce a quartz-porphyry or guartz-f el- site. By Cornish miners these quartz-porphyries are termed elvans (elvanite of Jukes) ; but tliis name is also applied to fine grained granites and to almost any rock which occurs as a dyke running through the killas or clay slate. Few questions have been more warmly discussed than the origin of granite. W r hen this rock is found forcing its way through older rocks, and appearing at the surface in large bosses from which veins are sent forth in all directions, there can be little doubt of its eruptive character. The small width of some of these granitic veins, or apophyses, suggests that the rock must have existed in a condition of perfect fusion or complete liquidity, and not simply as a viscous paste, before it could have been injected into such narrow fissures as those which are now occupied by granite. In many cases, the rocks which are penetrated by the granitic veins are altered in such a manner as to indicate a considerable elevation of temperature : a limestone in the neighbourhood of the veins may become saccharoidal, and shales may become indurated or even converted into horn- stone, while new minerals are often developed in the vicinity of the intruded veins. In these veins the granite is apt to change its mineralogical constitution, becoming either fine-grained or felsitic, or even reduced at the extre mities of the vein to quartz. From the days of Hutton it has been generally admitted that most granite is of igneous origin. Since it appears to have been solidified at great depths beneath the surface, it has been distinguished as a plutonic rock, while those eruptive rocks which have risen to the surface, and have there consolidated, are termed volcanic rocks. The older geologists regarded granite as the primitive rock of the earth s crust, forming the floor of all stratified deposits and the nucleus of mountain chains. Such a view, however, has been long exploded. It is known indeed that granite, so far from being in all cases an original rock, may be of almost any geological age. Some is undoubtedly as old as the Silurian period, while other granites are certainly as young as the Tertiary rocks, and perhaps of even more recent date. By many field-geologists granite has of late years been regarded as a metamorphic rather than as a truly igneous rock. Meta- morphism, however, is a term which has been so vaguely used that most of our eruptive rocks may, in a certain sense, be said to be metamorphic. Still, in the case of granite, it has often been pointed out that a passage may be traced from this rock into gneiss, and that gneiss itself may be regarded as an altered sedimentary rock. Thus so experi enced an observer as Professor Ramsay expresses his opinion that &quot; granite is sometimes merely gneiss still further meta morphosed by heat in the presence of moisture&quot; (Phys. Geol. of Gt. Brit., 5 ed., 1878, p. 42). For a number of instances in which granite is said to pass into gneissose rocks, and these in turn, by numerous gradations, into un doubtedly stratified deposits, see Green s Geology, part i. p. 307, and also GEOLOGY, vol. x, p. 309. Chemists have also brought forward arguments against the igneous origin of granite. Thus it has been argued that the specific gravity of the quartz of granite is about 2 6, while that of silica after fusion is only 2 2. It must be remembered, however, that the quartz of granite hns solidified under great pressure, as proved by Mr Sorby s observations, and it is probable that such pressure would increase the density of the silica. Moreover, it has been pointed out by the late D. Forbes (GeoL Mag., iv., No. 10, 1867, p. 443) that the siliceous tests of certain infusoria, which assuredly have not been fused, are as low as 2 - 2. Another argument which has been advanced against the they contain minerals of a basic character which could not have existed in a state of fusion in the presence of free silica, without forming a combination with the latter. Again, some of the accessory minerals in granite would suffer change by an elevation of temperature, while many of them contain water which, it is assumed, would be expelled on fusion. Probably, however, these minerals are in most 
 * igneous origin of granitic rocks is based on the fact that