through it in the most opposite directions. The chiastoliteschist (which also forms a variety of clay-slate) is found on the contact margins of plutonic igneous rocks, e. g. next to granite. Near Gefrees in the Fichtelgebirge. Also abundant about Skiddaw, Cumberland. (9) NODULAR or SPOTTED SCHIST.) This schist contains small concretions of different structure, KNOTENSCHIEFER, FLECK- oder FRUCHTSCHIEFER. (Germ.) SCHISTE NODULEUX ou RUBANÉ. hardness, and colour to that of the general mass. They are, for (Fr.) In the most part, harder and darker, and they either form small knots or only spots with indistinct margin; sometimes they resemble the currants in a fruit-pudding, hence their different names. Their composition has not yet been determined with accuracy by the various mineralogical chemical analyses which they have undergone, according to which they have been successively taken for a kind of fahlunite, for hornblende, serpentine, chiastolite, or andalusite. It is very possible that at different places they are somewhat differently composed. reference to their origin, it is of special interest that according to the careful investigations of Carius, the schist with nodules does not differ in the quality or proportionate quantity of its ingredients from the same schist without nodules farther removed from the contact, so that no new substance appears to have been added to form those concretions, but they appear rather to have arisen from a new arrangement of the previously existing ingredients. At the margin of the granite in the Western Erzgebirge and Voigtland, these nodular schists are very frequent, and are observed there just as much in the claymica-schist as in the ordinary clay-slate. A similar appearance occurs at Wechselburg in Saxony, in a rock which is decidedly mica-schist. (r) ALUM-SCHIST. This schist contains much carbon, and ALAUNSCHIEFER. (Germ.) is thereby rendered black. Pyrites is SCHISTE ALUMINEUX. (Fr.) always mixed with it in fine particles, through whose decomposition alum and iron-vitriol are formed. In the case of this variety, we can only decide from the bedding whether it belongs to argillaceous mica-schist or to clay-slate, for the carbon which it contains thoroughly obliterates the slender landmarks by which the difference might otherwise be established. It is characteristic of most of the alum-schists, that they are of very much contorted or displaced texture, and are frequently pervaded by irregular swollenshaped fragments of quartz and lustrous but bent lamina (mica), and sometimes also lenticular concretions of bituminous limestone or anthraconite. Reichenbach in Voigtland. (8) CARBONACEOUS SCHIST, BLACK CHALK. In the case of this variety, very rich in carbon, we can only ZEICHNENSCHIEFER, SCHWARZE KREIDE. (Germ.) determine by its bedding whether it belongs to argillaceous S drawing or writing. Ludwigstadt in the Thuringian Forest, where it belongs to clay-slate. All the above-mentioned varieties in composition are equally applicable to the ordinary clay-slate as to the argillaceous mica-schist, and we shall therefore have to enumerate them again when we come to consider that rock, but our previous descriptions will suffice for both. Argillaceous mica-schist is usually also distinctly stratified in addition to its foliated texture, which, as already said, is not parallel to the stratification; otherwise, as to its bedding and extent, it exactly resembles mica-schist, with this only difference, that it more usually than that rock is interstratified with the oldest sedimentary and distinctly fossiliferous rocks. By the name of argillaceous mica-schist we do but seek to establish a stage of transmutation between clayslate proper and mica-schist. References. Frick, Pleischl, Sauvage, and Kjerulf have contributed various analyses to Poggend. Ann. 1835, vol. xxxv. p. 188; in the Journ. f. prakt. Chemie, 1844, vol. xxxi. p. 45; and 1855, vol. lxv. p. 192. List, on Sericitschiefer, in the Jahrb. d. Vereins f. Naturk. in the Duchy of Nassau, 1850, No. 6, p. 128. Lipold, on Sericitschiefer in the Alps, Jahrb. d. geol. Reichsanst. 1854, pp. 201 and 359. Gümbel, on Ottrelitschiefer, in Corresp-Bl. d. zool. mineral. Ver. z. Regensburg, 1853, p. 53, and on Phyllit, in the same, 1854, p. 12. Naumann, on Knotenschiefer, Erläuter. z. geogn. Karte v. Sachsen, 1838, No. II. p. 264, and 1845, No. V. p. 50. Kersten, on Knotenschiefer, in Journ. f. prakt. Chemie, vol. xxxi. p. 108. Carius, on Knotenschiefer, in the Annalen d. Chemie. u. Pharm. 1855, vol. xciv. p. 45; and in v. L. u. Br. Jahrb. 1856, p. 595. Müller, on Knotenschiefer, in the Berg- u. Hüttenm. Zeitung, Durocher, on Chiastolith and Knotenschiefer, in Bullet. de la CHAPTER III. SEDIMENTARY AND FRAGMENTAL ROCKS. ALL sedimentary rocks are stratified; or at least, their beds lie one above the other in parallel planes. The greater part consists of the débris of older rocks mechanically washed together and deposited from a state of suspension in water. A few only are the result of chemical precipitate of mineral substances. Many contain organic remains (fossils) more or less distinct; some consist entirely of such. As a consequence of their origin, the sedimentary rocks are rarely of genuine crystalline conformation. Some, however, which appear to be actual chemical precipitates from aqueous solutions, such as gypsum and rock-salt, usually possess a crystalline structure. Following the different origin of these rocks, we may divide them into (a) Mechanical deposits. (b) Chemical precipitates. (c) Rocks resulting from organic processes. (a) Phytogenic, caused by the accumulation of vege table matter. (B) Zoogenic, caused by the accumulation of animal remains. The minerals which chiefly predominate in sedimentary rocks are not the same as those which are most abundant in the igneous and the metamorphic rocks. We find in the sedimentary rocks little or no felspar, hornblende, or pyroxene. The following are those which occur with greatest frequency:-Quartz, which in general terms we may call the most abundant mineral of the earth; clay (itself, however, a compound rather than a distinct mineral); carbonates of lime and magnesia, as calcspar (limestone) and dolomite; sulphate of lime, as gypsum and anhydrite; chloride of sodium, as rock-salt; finally, coal and iron-ores. Gypsum (or anhydrite), salt, coal, and iron, usually form distinct and separate beds of comparatively small extent the principal and most important sedimentary rocks are composed chiefly of the first-named of the above minerals, quartz, clay, and carbonate of lime (or magnesia). They may be accordingly divided into argillaceous rocks, calcareous rocks, and quartzose rocks. The marl rocks occupy an intermediate place between the calcareous and argillaceous. The quartzose rocks may be divided into the arenaceous or sandstones, and the conglomerates, to which we may add certain other fragmental rocks containing less quartz, usually termed tufa or tuff. The material for all these several rocks was mostly derived from the disintegration of more ancient previously existing rocks. The igneous rocks, by the decay of their felspar, hornblende, augite, and mica, have supplied the following substances towards the formation of the sedimentary rocks :-argillaceous mud, and weak solutions of lime, magnesia, silica, potash, soda, oxide of iron; their quartz has furnished grains of sand; in some cases their mica has remained undecomposed, and is found as mica in minute lamina in the sedimentary rocks. The older sedimentary rocks have also in process of time become disintegrated, and have furnished similar materials to form the more recent, and every solid rock has at times furnished pebbles, and other fragments for the formation of conglomerates. The several sedimentary deposits have been divided into so-called formations, according to the order of their superposition, and consequently of their age, and these again have been gathered into groups, which answer to longer periods of deposit. Post-Tertiary. It may therefore be useful here to present the following TABLE OF GEOLOGICAL PERIODS.* (Recent Formations of every kind. Mud, sand, gravel, calcareous and volcanic tuff, coral reefs, bog iron-ore, turf, peat, &c., guano, infusorial beds. Pleistocene, or Post-Pliocene Formation. Diluvial or glacial deposits, loam and breccias of bonecaverns, brick-earth and fluviatile loam or loess, valley gravels, bog iron-ore, calcareous tuff, coral-reefs, &c. * In different countries these are somewhat differently divided and named. |