Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/245

Rh -to consist of crystalline grains .nocKs.] crystallized or amorphous (niclzt indim'dualisz'rte7z) substance, sometimes glassy and sometimes microfelsitic. Rocks really formed of compacted sediment become sometimes so close-grained that their origin may not be apparent to the naked eye. - Their truly derivative char- acter is well brought out by the microscope. In ﬁg. 4, for example, the structure of a piece of ﬁne greywacke is shown. It will be observed that the component particles are not crystals, but broken and n1ore or less rounded fragments of different mine- rals. The larger white pieces are quartz, the darker por- tions consist of granules of slate, felrspar, and other substances, with a little siliceous ferruginous cement. Many exceedingly compact and even ﬂinty argillaceous rocks are in this way shown by microscopic examination to be formed of water-worn particles. Rocks which have been clunges as to acquire a new crystalline character, and to receive the name Meta- morphic, exhibit many char- acteristie features of struc- ture under the microscope. Limestones, for example, which have been altered into -saccharoid marble are found FIG. 3.—.licroscopic .'truct1n‘eof Pitch- slonr-. A glassy base, with numerous f L-athered and needle-shaped micro- lites, and a sanidine crystal. so affected by subsequent of calcite, showing the charac- teristic cleavage of that mine- ral 5). The foliated ro ks (schists) show a curious blending of the characters of igneous and sedimentary rocks. has they have often a distinctly granular structure, resembling that of sedimentary deposits, with, at the same time, an arrange- ment of the micaceous folia reminding us of the ﬂuid structure of igneous rocks. In fig. 6, for instance, the quartz-grains are to be ob- served in layers separated by folia of mica which curve and twist likethe microlites in an obsidian (compare fig. 2. -‘Inch light 113-3 been C355 I-‘1G.5.——.Iicroscopic StrnctureofSaccha— on the origin and history of 1'°'“1“'hite Murb1e(can-am). igneous rocks by microscopic example, to see in what order the several mineral con1pon- ents have crystallized out of the original glass. Thus in basalt the magnetite l1as ap- peared before the augitc, in which it has been abundantly enclosed. Again, the move- ment of the still liquid or viscid rock, when many of its crystals had already been pro- duced, is beautifully shown by the “ﬂuid structure ” (fig. 2), where minor crystals and particles are drawn into curving lines which bend ren:-d Fm. 4.—1Iici-oscopie Structure of a fragmental rock. Grcywacke. investigation. It is easy, for Fxc. 6.—_IEcrost-epic Structure of Mica- schist. - GEOLOGY 231 the large crystals, and also by the frequent fracture of the larger crystals and the insertion of portions of the general ground-mass of the rock between the separated pieces. That intensely saline water was present during the forma- tion of many crystalline rocks is proved by the presence in their crystals of minute cavities ﬁlled with ﬂuid and containing cubes of halite (common salt). Liquid carbonic acid has been observed in such cavities. Most interesting and important information is likewise afforded by the microscope regarding the subsequent changes which rocks have undergone through the inﬂuence chieﬂy of percolating water. Every gradation of alteration from the fresh mineral to its complete pseudomorph may be observed. In this way many serpentines have been shown to have been originally olivine rocks. It can be seen, too, how certain minerals decay, and to what products their alteration gives rise, even when the general mass of the rock looks to the eye still tolerably fresh. There can be little doubt that the application of microscopic analysis is destined to throw much light upon both the formation and the subsequent history of the sedimentary rocks. These have not yet been so sedulously explored as the igneous groups. The obscure subject of metamorphism is especially open to elucidation by the microscopic method} 2. Clzemical A7zalg/sz's.———This method of investigation must supplement the work of the microscope. A mere chemical analysis gives the ultimate chemical constitution of a rock, but may afford little clue to its mineral structure, which can only be thoroughly examined by means of the microscopic method. On the other hand, many rocks do not allow of satisfactory determination of their constitution by means of the microscope. For these chemical analysis is, of course, indispensable. But our knowledge of no rock can be considered complete until the rock has been sub- jected to both processes of investigation. II. Classiﬁcation and Description. A precise yet convenient classiﬁcation of rocks is still required. We may adopt chemical characters as the basis of arrangement, and group rocks according as they may be sulphates, carbonates, silicates, &c.; but in so doing we place together rocks which, from a geological point of view, have no real affinity. Again, we may select mineralogical composition as the groundwork of the classiﬁcation; but in this case also great violence may be done in the geological relationships of the rocks. In many respects the long estab- lished geological arrangement according to manner of pro- duction is a useful one—igneous, aqueous, and metamorphic rocks. There is, of course, the obvious objection to it that it starts upon a preconceived theory of the origin of the rocks, and this objection must be admitted to be serious. Every year, however, is diminishing its force by making us more certain of the mode of formation of different rocks; and, probably, some modiﬁcation of it will in the end be very generally adopted. I11 the meantime the most eligible course seems to be to choose a scheme of arrangement which, confessedly imperfect and temporary, shall recognize at once the mineralogical, chemical, and geological relations of the rocks. With this object the following classiﬁcation will be adopted here. I. C'r_2/stallizze and Glassy .Von-Fragmental Ifoclcs. 1. Simple Rocks (composed of one mineral substance), —chieﬂy of aqueous rocks formed from chemical precipitates. 1 The reader will ﬁnd this subject fully treated in Zirkel's J[z'/cr0- skopische Bcscha_[]'enhez't der Jlineralicn and Gesteizte (1873), Rosen- bnsch's J1 r'l:rosl:opz'sche P/L3/s1'o_r/rap/Le (lcr J[1'neralL'en uml G’cstc1'ne (1873—7), Vogelsang's If r_1/stalliten (1874), and Dela.Vallée and Renard, Snr les rochcs plutozdcmzcs de la B-_’I_r/iqzte (Acad. Royale de Belglque, 1876).