Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/248

Rh 234 -'ales, associated with the marine Lower Silurian rocks, or (b) l subaerial, as probably in the uartz-porphyry of _rran, and perhaps ' in the series of the “ green-s ates and pnrphyries ” of the Silurian system in Cumberland, which Professor It-amsay has conjectured to ' be the products of a subaerial volcano. Pitchslo-no (kclinilc) is a glassy rock, having a pitch-like or resinous lustrc, and a black or dark-green colour ranginvr through shades of green, brown, and yellow to nearly white. It is essentially an orthoclase rock, and may be regarded as the natural glass of many of the more granular or crystalline orthoclase rocks, such as the quartz- porphyries or felsites. Examined microscopically, it is found to consist of glass in which are ditfused, in greater or less abundance, hair-like microlites, angular or irregular grains, or more deﬁnitely formed crystals. The pitchstone of Corriegills, in the island of Arran, presents abundant green, feathery, and dcndritie microlites of a pyroxenic eharaeter (see ﬁg. 3). According to Durocher the mean composition of pitchstone is—— silica, 70'6; alumina, 15'0; potash, 1'6; soda, 2'4; lime, 1'2; magnesia, 0'6; oxides of iron and manganese, 2'6 ; loss by ignition, 6'0. The mean specific gravity is given as 2'34. Pitchstone is found either as intrusive dykcs, veins, or bosses, probably in close connexion with former volcanic activity, or in sheets which, like the porphyritic pitelistone of the Isle of Eigg, ﬂowed out at the surface as lava-st1'eams. Lz'parz'tc (Rhyolitc, Quart:-trachytc) is an orthoclase rock con- taining an excess of silica which usually appears in distinct grains or in doubly terminated pyramids. The orthoclase, which is of the variety termed sanidine, is sparingly accompanied with trielinic felspar. Other frequent ingredients are magnesia-mica, hornblende, augite, apatite, and magnetite. Considerable diversity exists in the texture of this rock. Some varieties are coarse and granitoid in character, and are regarded by some petrographers as the cquiva- lents in Tertiary times of the granite of older geological periods. From this crystalline aspect intermediate varieties may be obtained like the quartz-porphyries, passing by degrees into more or less distinctly vitreous rocks. Throughout all these gradations, how- ever, a characteristic ground mass can be seen under the microscope having a glassy, enamel-like, or poreellanous cha1'aeter. An analysis by Vom Hath of a i'hyolite from the Euganean Hills gave—silica, 76'03; alumina, 13'3'2; soda, 5'29; potash, 3'83; protoxide of iron, 1 '74; magnesia, 0'30; lime, 0'85; loss, 0'32 ; total, 101'68,— speciﬁc gravity, 2'553. Liparite is a volcanic rock of late geologi- cal date occurring in the form of erupted lavas. Obsidian is a volcanic glass representing the vitreous condition of a highly silicated sanidine-rock, such as liparite. It resembles bottle glass, having a perfect _conchoidal fracture, and breaking into sharp splinters, semi-trans arent or translucent at the ed es. The colours of the rock are blac, brown, or greyish-green, rare y yellow, blue, or red, but not infrequently streaked or banded with paler and darker lines. “Then a thin slice of obsidian is prepared for the inicroscrope it is found to be very pale yellow, grey, or nearly colourless. On being magniﬁed it shows that the usual dark colours are almost always produced by the presence of minute crys- tals, needles, and black hair-like bodies. In rare examples the obsidian appears as a perfect glass without any foreign admixture. The minute crystals and hair-like bodies sometimes so increase in abundance as to make the rock lose the aspect of a glass and assume that of a dull ﬂint-like or enamel-like stone. This devitriﬁcation can only be properly studied with the microscope. Again little granules (sphcrulitcs) of a dull grey enamel (pearlstone) appear, and in some parts of the rock so abundantly as to alter its character and convert it from obsidian into pcarlstone. The average chemical composition of the rock is—silica, 71 '0; alumina, 13'8; potash, 1 4'0; soda, 5'2; lime, 1'1; magnesia, 0'6; oxides of iron and man- ganese, 3-7; loss, 0'6; total, 100'0,—mean speciﬁc gravity, 2'40. Obsidian occurs as a product of the volcanoes of late geological periods. Pearlstmw (Pcrlite) is another vitreous condition of sanidine lava. As its name denotes, it consists of vitreous or enamel-like globules, occasionally assuming polygonal forms by mutual pressure. These globules sometimes constitute the entire rock, their outer portions shading elf into each other so as to foi'm a compact mass ; in other cases they are separated by and cemented in a compact glass or enamel. They consist of successive very thin shells, which, in a transverse section, are seen as concentric rings, usually full of the same kind of hair-like crystallites and crystals as in obsidian. Occasionally there are found among them true spherulites where the internal structure is radiating ﬁbrous. When such sphcrulitcs occupy the main mass they give rise to syvhrrulitc-rock. Pumice is a general term for the cellular and ﬁlamentous or fi'oth-like parts of lavas. In the great majority of cases it is a form of the obsidians, showing under the microscope the usually vitreous characters, and possessing a speciﬁc gravity of 2'0 to 2'53, though, owing to its porous nature, it possesses great buoyancy and readily ﬂoats on water. At Hawaii, however, some of the pyroxenic or olivine lavas give rise to a pumiceous froth which has the usual outward characters of ordinary pumice. GEOLOGY ' usually nosean. ‘ of iron and manganese, 3'5; loss by ignition, 3'2 per cent. [ii. GEOGNOSY. The rocks enumerated up to this point are all orthoclase-rock~', and markedly siliceous, frequently showing their excess of silica in the form of quartz grains or crystals. In the succeeding group free quartz is not found as a marked constituent, although occasionally it occurs in some quantity. In this series syenite may be regunlml as the C1 uivalent of granite in the quartzose series, orthoclase por- phyry ot quartz-porphyry and fclstonc, and trachyte of liparite. Syem'tc.—Ae-coiiliiig to the niodei'ii nomenclature, this name, which was formerly given in England to a granite with horn- blcndc replacing mica, is now restricted to a rock consisting essen- tially of a mixture of orthoclase and l10l‘nl)l¢-Iltlc, to which plagio- elase, quartz, and mica are occasionally added. The name syenite, first used by Pliny in reference to the rock of Syene, was introduced by 'erner as a scientiﬁc designation, and applied to the rock of the Plauensehcr-Grund, Dresden. Werner aftcrwai-ds, ll0'evel‘, niadc that rock a greenstone. The base of all sycuites like that of granites is crystalline, without a trace of airy amorphous s11b.~;tai1:-c between the crystals, such as most igneous rocks contain. llenee the texture is of that crystalline kind commonly known as granitic. The typical syenite of the Plauenscher-Grund, formerly described as a eoarsc-grained mixture of ﬂesh-coloured orthoclase and black hornblende, containing no quartz, and with no indication of plagioclase, was regarded as a normal ortlioelasc-hornblcndc rock. Microscopical research has, however, shown that wcll—striated tri- clinic felspar, as well as quartz, occurs in it. Its composition is shown by the following analysis:—silica, 59283; alumina, 16'85; protoxide of iron, 7'01; lime, 4'43; magnesia, 2'61; potash, 6'57; soda, 2'-14; water, &c., 1'29; total, 101 '03. The average speciﬁc gravity of sycnitc is from 2'75 to 2'90. Syenitc occurs 1indci' conditions similar to those in which granite is found; it has been erupted in large irregular masses, especially among inetainorphic rocks, as well as in sniallci' bosses and veins. Ortlioclasc Porpliyry (Qmzrtzlcss Pozpliyry) is an ortlioclasc rock containing no quartz, or a very sparing admixture of that lnincral, but with a little plagioclase, and not unfrequcntly with some horn- blende and dark biotite. It contains from 55 to (55 per cent. of silica. It differs thus from quartz-porphyry and fclstonc in its smaller pro- portion of silica, but the distinction is one which, except by chemical or microscopical analyses, must often be diflicult to estab- lish between the ﬁne compact fclstones and the orthoclase por- pliyries, especially when the latter contain fi'ec quartz. This rock is sometimes termed syenite-porphyry, since it is associated with syenite much in the same way that elvanite is with granite. It is like syenite a plutonie rock, and occurs in veins, dykcs, and in- trusive sheets. Probably, however, many of the so-called “fel- stoncs ” which occur as lavas, contemporaneously ejected with the older Palacozoie formations, are really orthoclase-poi-phyrics. Trachytc, a tei'n1 originally applied to a la1'gc series of modern volcanic rocks possessing a cha.ractcristic roughness (-rpaxiﬁs) under the ﬁnger, is now restricted to rocks consisting essentially of sanidine, with more or less trielinic felspar, usually with liorn- blende, biotite, or augite, and sometimes with magnetite and apatite. In microscopic structure the rock is distinguishable froui the quartz-trachytcs or lipa1'ites by the absence or feeble development of any niicrofelsitic ground-mass, and in general by the presence of a porphyi'itic base, consisting either of a pure glass or of one with dc- vitriﬁcation products. The average composition of trachytc may be stated thus :—silica, 60'0; alumina, 17'0; protoxide of iron, 8'0; magnesia, 1 '0; lime, 3'5; soda, 4'0; potash, 5'0; loss by ignition, 1'5. Average speciﬁc gravity, 2'65. Trachytc is :1 vol- canic rock of Tertiary and post-Terti-.iry datc. Phenol-its (O7£7zk.st07zc), a term suggested by the metallic ringing sound emitted by the compact varieties when struck, is applied to a. mixture of sanidine felspar and ncpheline with hornblende and An average specimen contained silica, 57'7; alu- mina, 20 '6; potash, 6'0 ; soda, 7'0; liiue, 1'5; magnesia, 0'5; oxvildles ,,. speciﬁc gravity may be taken as about 2'58. Phonolite is sometimes found splitting into thin slabs which can be used for rooﬁng pur- poses. Occasionally it assumes a porphyritic texture from the presence of large crystals of sanidine or of hornblende. When the rack is partly decomposed and takes a somewhat porous texture, it resembles trachyte in appearance. Like trachyte, phonolite is a thoroughly volcanic rock and of late geological date. It occurs sometimes ﬁlling the pipes of volcanic oriﬁces, sometimesas sheets which have been poured out in the form ' of lava-streams, and sometimes as dykes and veins. In the rocks enumerated up to this point the essential felspar constituent is orthoclase; in the felspar rocks now to be described the corresponding ingredient is nearly always some trielinic form. In the volcanic rocks of this series there is usually some mineral of the hornblende or augite family present in such quantity as to give a green or even black colour to the mass. I’o7'ph_1/rile may be used as the designation of rocks which consist essentially of some ti'iclinie felspar, and show a glassy or partially devitriﬁed ground-mass containing abundant crystals of plagioclase