Page:EB1911 - Volume 21.djvu/350

Rh the processes involved in the formation of igneous rocks cannot be successfully handled. But every day sees an increase in the amount of data available, and encourages us to believe that sooner or later some of the simpler igneous rocks at any rate will be completely explicable on physico-chemical principles.

Rock masses of igneous origin have no sooner consolidated than they begin to change. The gases with which the magma is charged are slowly dissipated, lava-flows often remain hot and steaming for many years. These gases attack the components of the rock and deposit new minerals in cavities and fissures. The beautiful zeolites, so well known to collectors of minerals, are largely of this origin. Even before these “post-volcanic” processes have ceased atmospheric decomposition begins. Rain, frost, carbonic acid, oxygen and other agents operate continuously, and do not cease till the whole mass has crumbled down and most of its ingredients have been resolved into new products. In the classification of rocks these secondary changes are (generally considered unessential; rocks are classified and describe as if they were ideally fresh, though this is rarely the case in nature.

Epigenitic change (secondary processes) may be arranged under a number of headings, each of which is typical of a group of rocks or rock-forming minerals, though usually more than one of these alterations will be found in progress in the same rock. Silicification, the replacement of the minerals by crystalline or crypto-crystalline silica, is most common in acid rocks, such as rhyolite, but is also found in serpentine, &c. Kaolinization is the decomposition of the felspars, which are the commonest minerals of igneous rocks, into kaolin (along with quartz, muscovite, &c.); it is best shown by granites and syenites. Serpentinization is the alteration of olivine to serpentine (with magnetite); it is typical of peridotites, but occurs in most of the basic rocks. In uralitization secondary hornblende replaces augite; this occurs very generally in diabases; chloritization is the alteration of augite (biotite or hornblende) to chlorite, and is seen in many diabases, diorites and greenstones. Epidotization occurs also in rocks of this group, and consists in the development of epidote from biotite, hornblende, augite or plagioclase felspar.

The sedimentary rocks, which constitute the second great group, have many points in common that distinguish them from the igneous and the metamorphic. They have all originated on the surface of the earth, and at the period of their formation were exposed only to the temperature of the air and to atmospheric pressure (or the pressures which exist at the bottoms of seas and lakes). Their minerals are in most cases not susceptible to change when exposed to moist air or sea, and many of them are hydrated (chlorite, micas, &c.), or oxidized (iron ores), or contain carbonic acid (calcite, dolomite). The extent, however, to which this is the case depends largely on the rapidity with which they have accumulated; coarse rocks quickly piled up often consist of materials only partly weathered. When crystalline, the sedimentary rocks are usually soluble at low temperatures. The members of this group occur in beds or strata, hence they are often known as the stratified rocks; the upper beds are always of later formation than those which underlie them, except (as may happen when great disturbance has taken place) the whole series is inverted or overturned. Many of the stratified rocks have been formed by the agency of moving water (rivers, currents, &c.) and are grouped together as “aqueous” rocks, others have been deposited by the wind in deserts, on sandy beaches, &c. (these are “aeolian”). Others are the remains of animals or of plants, modified by the action of time, pressure and percolating water. Lastly, we find beds of crystalline nature, such as rock-salt and gypsum, which have been formed by the desiccation of saline waters; other crystalline stratified rocks, such as dolomite and many bedded iron-stones, are replacement products due to the introduction of mineral matter in solution, which replaced the original rock mass partially or wholly.

When the rocks exposed at the earth's surface give way before the attack of the agencies of denudation, they crumble down and are resolved into two parts. One of these consists of solid material (sand, clay and angular débris) insoluble in carbonated waters; the other part is dissolved an washed away. The undissolved residues, when they finally come to rest, form clastic sedimentary rocks (sandstone, conglomerate, shale, &c.). The dissolved portions are partly transferred to the sea, where they help to increase its store of salts, and may again be precipitated as crystalline sedimentary rocks; but they are also made use of by plants and by animals to form their skeletal and vital tissues From this latter portion the rocks of organic origin are built up. These may also contain certain ingredients derived from the atmosphere (nitrogen, carbon in coals, &c.)

We have thus three types of sediments of distinct origin, which may be named the clastic (or fragmental), the crystalline and the organic.

The clastic materials may accumulate in situ, and then differ chiefly in their disintegrated and weathered state from the parent rock masses on which they rest The best example of these are the soils, but in elevated regions angular broken rock often covers large areas. More usually they are transported by wind or water, and become sorted out according to their size and density. The coarsest débris comes first to rest and is least worn and Weathered; it includes screes, gravels. coarse sands, &c., and consolidates as conglomerates, breccias and pebbly grits. The bedding of these rocks is rudimentary and imperfect, and as each bed is traced along its outcrop it frequently changes its character with the strata on which it rests. The most finely divided sediment travels farthest, and is laid down in thin uniform sheets of wide extent. It is known as mud and clay; around the shores of our continents, at distances of a hundred miles and more from land, great sheets of mud are spread over the ocean floors. This mud contains minute particles of quartz and of felspar, but Consists essentially of finely divided scaly minerals, which by their small size and flat shape tend to remain suspended in water for a very long time Chlorite, white micas and kaolin are the best examples of this class of substances. Wind action is even more effective than water in separating and removing these fine particles. They to a very large extent escape mechanical attrition, because they are transported in suspension and are not swept along the ground or the bottom of the sea; hence they are mostly angular. Fragments of intermediate magnitudes (from of an inch to  of an inch are classed as sands. They consist largely of quartz, because it does not weather into scaly minerals like felspar, and having but a poor cleavage does not split up into flakes like mica or chlorite. These quartz grains have been rolled along and are usually rounded and worn (Pl. IV, fig. 1). More or less of garnet felspar, tourmaline, zircon, rutile, &c., are mixed with the quartz, because these are hard minerals not readily decomposed.

The mechanical sorting by the transport in agencies is usually somewhat incomplete, and mixed types of sediment result, such as gravels containing sand, or clays with coarser arenaceous particles Moreover, successive layers of deposit may not always be entirely similar, and alternations of varying composition may follow one another in thin laminae: e.g. laminae of arenaceous material in beds of clay and shale. Organic matter is frequently mingled with the finer-grained sediments.

These three types have been named the psephitic (or pebbly; Gr., pebble); psammitic (or sandy, Gr. , sand), and pelitic (or muddy: Gr., mud).

Two groups of clastic sediments deserve special treatment. The pyroclastic (Gr., fire, and , broken) rocks of volcanic origin, consist mostly of broken pieces of lava (bombs, ash, &c.) (Pl. IV. fig 2), and only accidentally contain other rocks or fossils. They are stratified, and may be coarse or fine, but are usually much less perfectly sorted out, according to their fineness, than ordinary aqueous or aeolian deposits. The glacial clays (boulder clays), representing the ground moraines of ancient glaciers and ice sheets, are characterized by the very variable size of their ingredients and the striated, blunted sub-angular form of the larger rock fragments. In them stratification is exceptional and fossils are very rare.

The crystalline sedimentary rocks have been deposited from solution in water. The commonest types, such as rock-salt, gypsum, anhydrite, carnallite, are known to have arisen by the evaporation of enclosed saline lakes exposed to a dry atmosphere. They occur usually in beds with layers of red clay and marl; some limestones have been formed by calcareous waters containing carbonate of lime dissolved in an excess of carbonic acid; with the escape of the volatile gas the mineral matter is precipitated (sinters, Sprudelstein, &c.). Heated waters on cooling may yield up part of their dissolved mineral substances; thus siliceous sinters are produced around geysers and hot springs in many parts of the world. There seems no reason to separate from these the vein stones which fill the fissures by which these waters rise to the surface They differ from those above enumerated in being more perfectly crystallized and in having no definite stratification, but only a banding parallel to the more or less vertical walls of the fissure. Another subdivision of this class of rocks is due to recrystallization or crystalline replacement of pre-existing sediments. Thus limestones are dolomitized or converted into ironstones, Hints and cherts, by percolating waters which remove the lime salts and substitute for them compounds of iron, magnesia, silicon, and so on. This may be considered a kind of metamorphism; it is generally known as (q.v.).

The rocks of organic origin may be due to animals or plants. They are of great importance, as limestones and coals belong to this group. They are the most fossiliferous of all rocks; but elastic sediments are often rich in fossils though crystalline sediments rarely are. They may be subdivided, according to their dominant components, into calcareous,