Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/249

Rh CRYSTALLINE ROCl{S.] with magnetite or titaniferoiis iron, and sometimes hornblende, augite, or mica. These rocks include many varieties which have not yet been thoroughly examined. The texture varies from coarse ierystalline-granular to exceedingly close-grained, and passes oe- easionally even into vitreous. Porphyrite _is a volcanic rock very e.liaracteristic of the later palzeozoic formations, occurring there as intcrstratiﬁed lava-beds, and in eruptive sheets, dykes, veins, and irregular bosses. '1 _ _ _ Dioritri (Grernstone in part) is a crystalline mixture of oligoelase or some allied felspar and hornblende with magnetic iron and apatite. Wh--re free quartz occurs the rock is called q-uartz-(her-ztc. The more compact dark varieties have_ been termed aphamte. lhe -.u'er.ige chemical composition of this rock may be taken to be— silita, 53:? ; alumina, 16'O; potash, 1'3; soda, 2'2; lii_ne,_ 6'3; in-ignesia, 6 '0; oxides of iron and manganese, 14'O ; loss by ignition, 1'0. The mean speciﬁc gravity is about 2'95. Diorite occurs as an eruptive rock under conditions similar to those of _quartz-porphyry .-ind syenite. It is found in palzeozoie volcanic regions, as in North “Wales, in “neck"’-like masses which may mark the position of some of the volcanic oriﬁces of eruption. _ I’rop_a/lite is a name given to certain Tertiary volcanic rocks consist- ing of a plagioclase felspar and lioi'iibleiide in a tine-grained ground- iiiass. They are subject to eonside1'able alteration, the hornblende being converted into epidote. Some quartziferous propylites have been described by Zirkel from Clarence King's Szu'L'ey of the 40th I’vIrrtllL'l, wherein the quartz abounds in liquid cavities containing briskly moving bubbles, and sometimes double enclosures with an interior of liquid carbonic acid. Ilumblcmlc-Amlcsita is a rock of late geological date consisting of a plagioelase felspar and hornblende often with a little sanidine. The ground-inass is frequently quite crystalline, or shows a small proportion of a felsitic nature, with microlites and granules. When the rock contains free quartz it is called Dacite. In the next series of rocks aiigite plays a similar part to that taken by hornblende in the foregoing species. Dz'vtbasc.—Tliis name has been given to certain dark green or black eruptive rocks found in the older geological formations and consist- ing essentially of a ti'ielinic or plagioclase felspar, augite, magnetic oi-titanifcrous iron, sometimes olivine, and usually with more oi' less nf a diffused greenisli substance which has resulted from the altera- tion of the augite or olivine. The texture is sometimes quite crys- talline; iii other cases it shows a felsitie ground-mass. Except that the so-called diabases are con lined to Palacozoie rocks and the basalts 1o Tertiary and post-Tei'tiary formations, thei'e seems no essential dis- tinetion between these two groups, though, of course, as the diabases are much older and have been fai' longer exposed to metainorpliie processes, they are in general less fresh than the basalts. .l[claplz_2/rc is a term which has been so variously used that the sense in which it is taken must always be explained. Thus Seuft described it as a rock having an indistinetly mixed character, with colours ranging from dirty greenish-brown, or i'cddish—grey, or greriiisli black-brown, to a complete black; hard and tough when fresh, and then showing crystals of reddish-grey labrado1'ite, with inrignctic titaiiifcrous iron, and usually with carbonates of lime and iron, and fcrriigiiious ehlorite (dclessite), in crystalline grains ; eoiii- pavt or earthy, or sometimes porpliyritic or aniygdaloidal. Naumann ll -ﬁnes inelapliyre as a close-grained rock, very often amygda- l'lltli1l, composed essentially of labradorite, with an undetermined silicate, some titaniferous iron, carbonates of lime and of iron, and sometiincs crystals of augite, rubcllan, and mica. Zirkel called it generally crypto-crystalline, sometimes porphyritie, very often
 * lll]_Vg(l:l.lOl(l{tl rock, consisting of a mixture of oligoclase and aiigitc

with magnetic iron. Lastly, Rosenbuscli proposes to restrict it to those plagioelase-aiigitc rocks which contain olivine, and possess a li~tin.-t poi-phyry gound-mass. There can be littlc_doubt that, like the so-called diabases, the mclaphyres are merely older forms of the great basalt-fainily. AzLgit«_:-Amlcsite is the name given to certain dark eruptive rocks of 'l‘ertiary and post-Tertiary date which consist of a ti'icliiiic felspar (oligoclase, or some species ratlicr richer in silica than labradorite) -and augite, with sometimes saiiidiiie, hornblende, biotite, magnetite, or apatite, and in some varieties quartz. The composition of an example from Santorin, erupted in the year 1866, was found to be as follows :—silica, 67'35 ; alumina, 157:2 ; magnesia, 1'16; lime, 3'60; soda, 5'04; potash, 1'86; oxides of iron, 1'94; loss by ignition, 0'36 ; total, 101 '06. Mean speciﬁc gravity, 2'75. BrLsalt.—Under this term are included those widespread and ini- portant'volcanic rocks which consist of a triclinic felspar, probably always lab1'adorite, augite, olivine, and magnetic or titanifcroiis iron, with apatite, and sometimes saiiidinc or neplieline. The more eo-.irsely crystalline varieties are known as dolcritc, while those of in- l4'l'lnC(llﬂte texture have been termed amzmcsitc, the more elose-grained black heavy kinds being distinctively basalts. The chemical coin- position of an average variety may be set down as silica, 4.3'0; aluinina, 15'0; magnesia, 6'5; lime, 10'5; soda, 3'5; potash, 1'5; oxides of iron and manganese, 15'0; loss by ignition, 3'0. Mean specific gnivity, 2'95. GEOLOGY 235 Zirkel has described under the name of basalt certain rocks in which the part of the felspar is played by another mineral, in some cases neplicliiie, in others lcucite. Basalt occurs as dykes and veins, intrusive bosses and sheets and successive contemporaneous ﬂows. It often presents a coluinnar structure, as at Statfa and the Giant’s Causeway, whence the structure has been popularly termed basaltic. Taehylitc is a black volcanic glass often found in association with basalt, of which indeed it is merely the vitreous condition. Tliusa basalt-dyke sometimes shows a thin crust of this pitch-like substance on its walls, where the molten rock was ﬁrst suddenly cooled. Palagonite is a volcanic glass related to basalt, and found in fragments in some tulls. Gabbro (Diallaf/c-rock) is a compound of a triclinic felspar and diallage often with olivine, and also very generally with magnetic or titaniferous iron and apatite, more rarely with hornblende, biotite, or quartz. An average chemical composition is silica, 50 '0; alumina 15'0; magnesia, 7'0; lime, 10'0; soda, 2'5; potash, 0'5; oxides of iron and manganese, 12'5 ; loss by ignition, 2'5. Mean specific gravity, 2'95. A very few crystalline massive rocks occur without felspar as an essential constituent; but they are of comparatively little impoi'tanee as rock-masses, though interesting in themselves and sometimes of considerable beauty. Pilrritc is a rock rich in olivine, usually more or less serpentinized, with augite, magnetite, or ilmenite, and a little brown biotite, horn- blende, or apatite. Eulysita is a mixture of olivine, augte, and red garnet. Garnet-oli'vi2w-rock is composed of olivine, diallage, and garnet. ' Enstatite-oZi'mIne—rocIc consists of olivine and enstatite (bronzite or hypersthene) with magnetite or chromite. Lherzolite is a mixture of olivine, pyroxene, pieotite, and usually some magnetite. Eclogite is a compound of garnet and oniphacite, or smaragdite (hornblende). Dzmitc is a mixture of olivine and chromite, found with serpentine. Limburgzta is composed of crystals of olivine, augitc. and mag- nctite, in a base more or less vitreous. (1).) Sclzistose or Foliuted I2oclcs.—These form an exceed- ingly well-deﬁned characteristic series. They are distin- guished from the massive rocks by the possession of an internal arrangement into more or less closely parallel layers or folia, consisting of materials which have assumed a crystalline character along these layers. The layers may be composed of only one mineral, but are usually of two or more, which occur either in distinct, often alternate, laininaz or intermingled in the same layer. More- over, the layers are not usually continuous for more than a short space. As a rule they are strikingly leiiticular, thickening out and then dying away, and reappearing after an interval on the same or a different plane. They are likewise characteristically welded, as it were, into each other, the crystalline particles of one lamina being so iiitei'— mingled with those of the layers above and below it that the whole coheres as a tough not easily ﬁssile mass. Though arranged in distinct layers, a schistose rock is usually dis- tinguished from an ordinary sedimentary one by the irregu- larity of its lamination, and by a remarkable and eminently distinctive puckering or crumpling of the folia. The vast plications which can be seen from a distance running up the face of a mountain are repeated on a smaller scale in hand specimens, and even down to such proportions as can only be seen with a microscope. As already stated, the origin of these rocks has been the subject of much discussion. That they are metamorphosed sediments, and not original chemical precipitates, is the general opinion of geologists. See part iv. A foliated rock showing this characteristic irregular ﬁssility in a marked degree is termed a “schist.” This word, placed after the distinguishing mineral of the rock, is used as the name of the rock, as mica-schist, chlorite-schist, hornblende-schist. If the mass loses its ﬁssile tendency owing to the felting together of the component mineral into a tough coherent whole, the word rock is usually sub- stituted for schist, as in hornble1ide—rock, actinolite—rock, and so on. There are thus three kinds of ﬁssility among rocks: ——(l) that of original deposit, as in sliale,—this is termed