ΤΗ CHAPTER IV. DERIVATIVE OR STRATIFIED ROCKS. HEIR Origin and Composition.-In Chapter III. we have examined the igneous rocks which come from the interior of the earth, and we have seen that they are essentially siliceous, some of them, especially the granites, containing crystals of pure silica, and all of them containing silicate of alumina. Now pure sand consists of grains of quartz, and pure clay consists of silicate of alumina combined with water. It is obvious, therefore, that clay and sand may be formed by the detrition and destruction of an igneous rock, such as granite, and this process may be actually watched wherever granite rocks are exposed to the erosive action of water. All the sandy (arenaceous) and all the clayey (argillaceous) deposits, therefore, which are met with in the earth's crust, have been derived from some previously existing rocks, and these older rocks must either have been igneous rocks, or must originally have been derived from some igneous rock. Similarly the siliceous and calcareous rocks of organic origin have been formed out of the silica and carbonate of lime held in solution by the sea-water; these materials having been primarily derived from the decomposition of the soluble silicates contained in igneous rocks. The varieties of rock produced by the intermixture of the aluminous, quartzose, and calcareous elements are very numerous. The proportions of these ingredients in any deposit will depend partly on the character of the rocks exposed to erosion in the area whence the chief supply is derived, and partly on the amount of matter contributed by organisms to the sediment which is accumulating below. Thus the chemical composition of any stratified rock is generally very variable: sands and sandstones frequently contain fragments of other minerals besides quartz; clay almost always contains a certain amount of lime and iron; and many limestones are rendered impure by an admixture of clay or siliceous matter. Classification and Description.-As already stated, the whole group is naturally divided into two seriesA, the unaltered rocks; B, the altered or metamorphic rocks. In the present chapter we shall deal only with the unaltered rocks, and it will be convenient to describe them under the following heads : : 1. Arenaceous rocks. 1. Arenaceous Rocks. Breccia is a mass of angular fragments, the interstices between them being filled either by small débris, sand and dust, or by an infiltrated cement of carbonate of lime or other mineral matter deposited by percolating water. The agencies by which ordinary breccias are formed have been described on p. 191; but Professor Bonney' has called attention to another class of breccias, which might be mistaken for the former, but have really been formed in a very different manner. These he terms "crush breccias," and believes them to have resulted from pressure or strain acting on a rock generally uniform, but containing weaker beds or portions, which have been crushed in situ into fragments of all shapes and sizes. They occur principally in the older rocks, and when the fragments are re-cemented by infiltrated matter they closely resemble an ordinary breccia. Conglomerate is a mass of consolidated gravel or shingle, the pebbles being rounded and water-worn, and set 1 "Geol. Mag.," Dec. 2, vol. x. p. 435. in some kind of matrix which binds them together into a firm rock. The pebbles may consist of any kind of hard rock or mineral which is capable of resisting continued attrition; and siliceous substances, from their great durability, are naturally the commonest constituents. When the rock consists chiefly of quartz pebbles it is called a Quartz-conglomerate; when the pebbles are limestone, it is a Limestone-conglomerate, and so on. The matrix enclosing the pebbles also varies in composition; sometimes it is only sand, and the rock appears to have been consolidated simply by pressure, so that the pebbles may be removed by a slight blow; in other cases the matrix consists of an infiltrated cement, either calcareous, ferruginous, or siliceous, and is sometimes so hard that a blow produces a clean fracture through both matrix and pebbles. Sandstone is consolidated sand, and the particles are usually quartz. Particles of flint or chert are rare, but occur in some Tertiary sands. Sandstones vary greatly as regards their degree of consolidation, from a mere sandrock which is just compact enough to stand with a vertical face, to a hard and compact gritstone, which is capable of being used as a millstone. The size of the grains also varies from the minutest particles of quartz to grains as large as peas, the rock then commencing to pass into a conglomerate. In the case of sand carried by water the particles are buoyed up in the liquid, their relative weight being less and their friction on the bottom and against one another being proportionately diminished; under such circumstances a great amount of drifting and mechanical action must be required to wear down any quartz fragments into round grains. As a matter of fact, Mr. Sorby finds that the sand brought down by rivers is often very little worn, and that in sea-shore sands the proportion of well-worn grains does not often exceed one-half. But when drifted and worn by the wind in the open air the amount of friction is necessarily very much greater, and in sands so accumulated all the grains show signs of attrition, most of them being completely rounded, and their surface more or less roughened. Some of the sandstones associated with shallow-water deposits are doubtless ancient blown sands formed on terrestrial surfaces, and subsequently covered by strata of aqueous origin. But Mr. A. R. Hunt bas pointed out that the sand of submarine shoals and sand-banks is kept almost constantly in motion by the action of the waves, and his examination of fine sand from the Skerries shoal in the English Channel showed that a large proportion of the grains were rounded. There are many varieties of sandstone produced by the admixture of other mineral substances with the quartz grains. Micaceous sandstone contains flakes of mica which are sometimes so abundant as to cause the rock to split into plates and slabs with glittering surfaces. Felspathic sandstone contains grains of felspar, distinguishable by their dull white, yellow, or reddish colour, and peculiar fracture, imparting an earthy appearance to the rock. When coarse and resulting from the direct detrition of granitic or gneissic rocks, so that the grains of quartz felspar and mica are angular and plainly visible, it is called Arkose. When very fine, so that the constituents cannot be distinguished without a microscope, it may be called a sandy mudstone. When compacted by pressure or by the infiltration of a siliceous cement, it is known as Greywacke. Calcareous sandstones. Of these, three kinds may be distinguished-1, Cornstone, a fine-grained rock, probably a calcareous silt cemented by crystalline calcite; 2, Calcareous grit, a coarse-grained sandstone, the separate grains of which are cemented together by crystalline calcite; 3, Sandstone, the component grains of which are embedded in large crystals of calcite. These crystals form a kind of mosaic, and each includes a portion of the original substance of the rock. Argillaceous sandstones, or consolidated silts, are always fine-grained, and generally more or less laminated, so as to split easily along the planes of bedding; they are then called Flagstones, and generally contain a proportion of felspathic or micaceous material. Glauconitic sandstone is one in which grains of glauconite are conspicuous, giving the whole a greenish tint. The softer kinds are usually called Greensand. Some glauconitic sandstones become calcareous grits by the infiltration of a calcitic cement. Gaize is a fine-grained micaceous and glauconitic sandstone, occurring among the cretaceous rocks of France and England. Malmstone, as understood in England, is a siliceous rock, but the silica is chiefly in a colloid state, and derived from an organic source. It is a fine-grained whitish rock, which looks like chalk at a little distance, and it is sometimes calcareous from an admixture of fine chalky matter; but a pure malmstone is light and porous, microscopical examination showing that it consists of very minute globules of colloid silica with numerous broken sponge spicules. Its porosity is due to minute cavities left by the solution of many of the spicules. This rock is often called Firestone. Siliceous sandstones.-Under this head may be placed those sandstones which are compacted into hard rock by a siliceous cement. They are distinguished from quartzite by the fact that the cement is opal or chalcedony, while in quartzite it consists of crystalline quartz formed in optical continuity with the quartz of the original sand. Chalcedony is the cement most frequently found, but opaline cements are known, and an opaline sandstone containing precious opal has recently been found in the interior of New South Wales near Wincannia. Peldon and Calliard are miners' terms applied to beds of hard fine-grained siliceous stone, which have a smooth and even fracture. Chert is a name applied to any siliceous nodule or layer consisting of earthy or organic particles embedded in a matrix which shows the radiating fibrous structure of chalcedony. Flint differs in having a matrix which is partly chalcedonic and partly microcrystalline. Both have a glassy or flinty fracture. 2. Argillaceous Rocks. Clay may be defined as a consolidated mud which contains sufficient hydrated silicate of alumina to be plastic, or capable of being moulded. Kaolin, or porcelain clay, is |