Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/243

Rh nocKs.] all ages. Hzcmatite (peroxide of iron) oecurs crystallized in veins through crystalline rocks, also massive and earthy in beds, and sometimes in minute scales (rubin-glimmer) disseminated through the minerals of many crystalline rocks. Magnetite (Fe34) has an extensive diffusion in the form of minute octohedra or grains through crystalline rocks. In some of these rocks indeed, as in basalt, it plays the part of a chief constituent. It also occurs in many metamorphic rocks both scattered in detached crystals and segregated into veins or beds. Titanoferritc or titaniferous iron IS likewise found as a plentiful ingredient in many crystalline rocks, pm'ti('11l:11'ly among the older basalts and dolerites. Hydrous iron oxide or limonite is diffused through almost all rocks. It is the usual brown or yellow colouring substance of minerals, and may be l0ol((‘d for wherever rocks containing iron have been exposed to the weather. It occurs also mixed with clay and other impurities in beds, as in the bog-iron-ore of lakes and marshes. 2. Rocks. I. General Chtiracters. A rock may be deﬁned as a mass of mineral matter, composed of one, more usually of several, kinds of mine- rals,—having, as a rule, no deﬁnite external form, and liable to vary considerably in chemical composition. The crust of the earth is built up of rocks, including under this term, not only hard solid masses like granite and limestone, but even all loose incoherent deposits such as mud, soil, peat, and blown sand. Rocks may be distinguished by external a11d internal characters. i. Eaclernal C'ha-racters. 1. Structure, or the manner in which the component particles have been built 111) into the mineral masses called rocks, is the fundamental character. Viewed broadly, there are two leading types of structure among rocks-—crystalline or massive, and fragmental. (41,) Crystalline-—consisting of a network of interlaced crystals and crystalline particles. Sometimes those crystals are large (half an inch or more in length), as i11 many granites, when the texture is called coarse or macro- crystalline; in other cases they are so minute as not to be discernible with the naked eye, when the texture is micro- crystalline or compact. While the crystalline structure is ])l.I‘tlCl1ld.1‘ly characteristic of rocks which have crystallized from igneous fusion, it is not altogether peculiar to them. It may be produced by chemical deposit from aqueous solutions, or it may be developed in rocks previously granular by chemical inﬁltration and metamorphism. Under the head of crystalline it is usual to include the glass}; or -vitreous structure. Rocks possessing this character are natural glasses produced by igneous fusion, such as obsidian a11d pitehstone. In most of these rocks, however, the process of devitriﬁcation may be observed ; the glass has evidently become more and more stony as it cooled, by the appearance in it of small spherules, or hairs, or crystals, until in some cases it has become entirely lithoid. These stages are best studied with the microscope, and belong to the internal rather than the external characters. When larger crystals than those of the compact base are scattered through a rock, the texture is said to bepovyi/L_g/ritic. Many rocks, when in a melted condition, have had a cellular texture given to them by their imprisoned steam, like the open, cavernous texture of ill-baked bread. Several varie- ties of this texture are distinguished,-—as vesic2cl(u', when there are comparatively few and small holes; scoriaceous, when the cavities occupy about as much space as the solid part, and are of very unequal sizes and forms; pumiceous, when the cells are much more numerous than the solid portion, and when, consequently, a piece of the rock may even ﬂoat in water; aing/_r/clrzloidal, when by subsequent inﬁltration the cells have been ﬁlled up with concretions of calcite, calcedony, zeolite, &c., which, from the elongated flattened form of the cells, are frequently almond-shaped. GEOLOGY Foliatcd rocks have their crystalline ingredients arranged in more or less deﬁned layers, which usually inosculatc. Sclwstose rocks are those where the foliated arrangement has been so produced that the rock splits into rude rough laminae or plates. Most of the crystalline rocks have resulted from igneous fusion. Some, like limestone, have been formed as deposits in water. The foliated rocks are generally believed to have acquired their peculiar character from the re-crystallization of their ingredients along original divisional planes, such as the lines of deposit. ' (6.) Fragmental or Clastic.—These are all derivative from previously formed masses. They vary in texture from coarse masses consisting of accumulated blocks, several feet or even yards in length, to such ﬁne aggregates as only show their secondary origin by microscopic investigation. They are said to be con_(/lomeratic when they consist of beds of rounded water-worn pebbles like compacted gravel; ag_qlome7'atic, when the blocks are large, rounded, or sub- angular, and tumultuously thrown together; brecciated, when the fragments are angular and not water-worn. _Most elastic rocks are bedded, that is, arranged in beds or layers. Each bed may consist of many thin layers or laminae, which, when they enable the rock to split up into thin leaves, give what is called a slaaly or ﬁssile structure. Many fragmental rocks show a c0ncretz'onary structure. When the concretions are like the roe of a ﬁsh, and of a calcareous nature, they form the oolitic structure ; when of larger size, like peas, they give the pisolitic structure. There is often also a crystalline structure developed in rocks originally quite fragmental ; many limestones, for example, made up originally of water-worn fragments of shells, corals, &c., slowly acquire a crystalline character from the action of percolating and slightly acidulous water. The action of rai11 on the exposed parts of a recent coral reef produces this change in the dead coral. 2. Colom'.—-—This character varies so much even in the- same rock, according to the freshness of the surface ex- amined, that it possesses but a subordinate value as a means of discriminating rocks. Nevertheless, when cautiously used, it may be made to afford valuable indications as to- the probable nature and composition of rocks. It is in this respect always desirable to comparea freshly-broken with a weathered piece of the rock. White indicates usually the absence or comparatively small amount of the metallic oxides, especially iron. It may either be the original colour of the rock, as in chalk and calc-sinter, or may be developed‘ by weathering, as the white crust on ﬂints and 011 many porphyries. rock. Its existence may be due either to the presence of carbon, when weathering will not change it much, or to some iron-oxide (magnetite chieﬂy), or some silicate rich in iron (as hornblende and augite). Many rocks (basalts and dolerites particularly) which look quite black 011 a fresh surface, become red, brown, or yellow on exposure. Yellow, as a dull earthy colouring matter, almost always indicates the presence of hydrated peroxide of iron. Bright, metallic, gold-like yellow is usually that of iron-sulphide. Brown occurs as the original colour in some carbonaceous rocks (lig- nite), and ferruginous beds (bog-iron-ore, clay-ironstone, It very generally, on weathered surfaces, points to the oxida- tion and hydration of minerals containing iron. Red, in the vast majority of cases, is due to the presence of granular peroxide of iron. This mineral gives dark blood-red to pale ﬂesh—red tints. As it is liable, however, to hydration, these hues are often mixed with brown and yellow. Green, as the prevailing tint of rocks, occurs among metamorphic schists, when its presence is usually due to some of the hydrous magnesian silicates (chlorite, talc, serpentine). It occurs also among the igneous rocks. especially those of older Blac/r seldom occurs on a weathered surface of '