Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/244

Rh 230 geological formations, where some of the hornblende, olivine, or other similar silicates have been altered. Among the sedimentary rocks it is principally due to the proto-silicate of iron in glaiiconite. Carbonate of copper colours some rocks a bright emerald or verdigris green. The mottled character so common among many stratified rocks is fre- quently traceable to unequal weathering, some portions being more oxidized than others ; while some, on the other hand, become deoxidized from the reducing action of decay- ing organic matter. To the latter cause may be attributed the circular green spots so often found among red strata. 3. Lustre, as an external character of rocks, does not pos- sess the value which it has among minerals. In most rocks the granular texture prevents the appearance of any distinct lustre. Where a rockshows a completely vitreous lustre it will usually be found to consist of a volcanic glass. A splendent semi-metallic lustre may often be observed upon the foliatioii planes of schistose rocks and upon the laminae of micaceous sandstones. As this silvery lustre is almost invariably due to the presence of mica, it is commonly called distinctively micaceous. A metallic lustre is met with sometimes in beds of anthracite ; more usually its occurrence among rocks indicates the presence of metallic oxides or sulphides. 4. Hardness and 1"ran_qz'b'ilit_z/.———A rock which can easily be scratched with thenail is almost always much decomposed, though some cliloritic and talcose schists are soft enough to be thus affected. Compact rocks which can easily be scratched with the knife, and are apparently not decomposed, may be limestones, or other fragmeiital masses. Crystalline rocks, as a rule, cannot be scratched with the knife unless considerable force be used. The ease with which a rock may be broken is the measure of its frangibility. Most rocks break most easily in one direction ; attention to this point will sometimes throw light upon their internal structure. 5. Fracture is the surface produced when a rock is split or broken, and depends for its character upon the texture of the mass. Finely granular compact rocks are apt to break with a splz'.nter_z/ fracture where wedge—shaped plates adhere by their thicker ends to, and lie parallel with, the yneral surface. When the rock breaks off into concave and convex rounded shell—like surfaces, the fracture is said to be conchoidal, as may be seen in obsidian and other vitreous rocks, and in exceedingly compact limestones. The fracture may also be foliatecl, slat;/, or slzalg/, according to the structure of the rock. Many black, opaque, compact rocks are translucent on the thin edges of fracture, and afford there, with the aid of a lens, a glimpse of their internal composition. 6. Feel.—Practice enables a geologist to discriminate some rocks by the feel of their weathered or fresh surfaces. The hydrous magnesian silicates, as already mentioned, have a marked soapy or greasy feeling under the ﬁngers. Some micaceous schists, with ‘margarodite or an allied mica, likewise exhibit the same character. 7. Smell.—.Iany rocks when freshly broken einit dis- tinctive odours. Those containing volatile hydrocarbons give sometimes an appreciable bitzmzizzous odour, as is the case with some of the dolerites, which in central Scotland have been intruded through coal-seams and carbonaceous shales. Limestones have often a fetid odour; rocks full of decomposing sulphides are apt to give a sulplmrous odour; those which are highly siliceous yield, on being struck, an em- rocks to emit a strong earthy smell when breathed upon. 8. Specz:/Zc_r}ra.'vity is an important character among rocks as among minerals. It varies from 0'6 among the hydro- carbon compounds to 3'1 among the basalts. As already stated, the average specific gravity of the rocks of the ear§h(’)s crust may be taken to be about 2'5, or from that to - '. GEOLOGY [ii. Gsooxosv. 9. .l[«zgnefz'.sm is a distinguishing feature of many igneous and some metamorphic rocks. In some cases it exists in such development as powerfully to affect the magnetic needle, so that observations with that instrument among rocks of this character are deceptive. But even when much more sparingly present, the existence of magnetic iron in a rock may be shown by reducing the rock to powder in an agate mortar, washing carefully the triturated powder, and drying the heavy residue, from which grains of magnetite may be extracted with a magnet. This may be done with any basalt ii. Internal C’/iaracters. These are revealed chiefly by the inicroscope and clieniical analysis. By the former we learn what are the Ctillllmllclll minerals of arock, how they are built up into its nins.-, and what changes they have undergone. By the latter we ari- taught the chemical constitution of rocks, and are enabled to bring into close relations rocks which have externally no resemblance to each other, or, on the other hand, ti (siliotw tthat rocks externally similar are chemically very is me . 1. J[z'croscopz'c ]f.mminat2'o7z..~'l'liis method of inquir_v has made great advances during the last :20 years, especially from the labours of Gernian pctrographers. Slices are cut from the rocks to be examined, and after being polished on one side with great care, are cemented by that side with Canada balsam to glass, and are then ground down until they attain the requisite transparency. In this way the miiintest features in the structure of a rock can be leisurely studied. By the application of polarized light to these thin slices a niarvellously delicate niethod of petrographical analysis is afforded. Among the igneous rocks three leading types of micro- scopic structure have been established, chieﬂy through the researches of Professor Zirkel of Leipsic :——(1.) Purely-crystalline.———Granite (ﬁg. 1) is a good example, consisting, as it does, en- tirely of crystals interlaced with each other. (2.) Half- crystalline.—In this divi- sion, which embraces most of the eruptive masses, the rocks consist of a non-crys- talline amorphous matrix with crystals scattered through it. This matrix may be either (a) entirely glassy (figs. 2 and 3) ; (b) partly devitriﬁed through separation of peculiar little granules and needles which are not “niicrolites” of the com- ponent parts of the rock; (c) an aggregation of such little granules, needles, and hairs, between which no glass, or almost none, ap- pears (microcrystallitic) ; or (d) microfelsitic, nearly re- lated to the two previous groups, and consisting of an amorphous mass marked usually with indeﬁnite or I-‘mi. ‘.?.—.Iici-oscopic Structure of 0b.-i- dizin. A volcanic glass, “Ill! niimerous half-eﬁ‘3CCl1 gra-1111195 and {H9-* microlites, which have been dl'lI“Tl out lfvents-R (L 3-) f t1;7n?°'f’J' SW1‘ 1'} ‘ln‘é°'}.‘§Zi‘i'o‘3ii"$§§i'§"tiiilflfﬁﬁll-‘iJl‘I-37 inc.-— oc 's o s c ass are (x 18 diameter.) much less common than those of the other two. In their most typical condition they consist entirely of a noii- FIG. 1.——-Microscopic Structure of ll('l.‘T— Purely crystalline—Section of (':raiiitt- (x 18 diameter). The white iiiiiu-nil is quartz; that with sliading, ortho- cluse. Some ﬂakes of mica are shown as striated fornzs.
 * nyreum.atic odour. It is very characteristic of argillaceous