Lithology.—All rocks are either igneous or sedimentary; that is, have either been produced by the action of heat, or been arranged by mechanical or other means in layers or beds. • I. The igneous rocks differ amongst themselves in their composition, structure, and -age; they are made up of different materials; they have various textures, as granular, •compact, or glassy; and they have been ejected at different periods of the earth's history. From these characteristics, they have been grouped thus: 1. The volcanic rocks (q.v.), -comprising all that have been formed during the present and tertiary periods, and which are popularly known as lavas and volcanic ash. They have been ejected from volcanoes either in a fluid state, spreading over the land, and cooling as compact lavas; or spread ing below shallow water, and becoming vesicular pumice, or as ash scattered in layers i over the country; or they have risen into cracks and crevices of rocks as dykes and veins. Their principal constituents are feldspar and augite, and the different varieties depend on the predominance of the one or other of these ingredients. The felds pathic lavas are generally light-colored, and have a rough prickly feel to the finger. The •chief varieties are trachyte, pearlstone, phonolite, obsidian, and pumice. The augitie lavas are of a dark-green or black color, weathering brown externally, and are generally heavier than the feldspathic lavas. The most common forms are dolorite, basalt, and leucite. 2. The trappean rocks (q.v.), which generally belong to the primary and sec ondary strata, and are composed of the same materials as the volcanic rocks, except that the silicates of magnesia and lime crystallize in the latter as augite, while they assume the more obtuse form of hornblende in the trappean rocks. Trap-rocks are always associated with a pipe or dyke connecting them with the underlying mass from which the materials were obtained. They have either overflown the surface, and formed :a bed conformable to, and contemporaneous with the subjacent strata, or inserted them selves between already formed strata, forming injected sheets that are not contempo raneous. The predominance of the one constituent material over the other gives the basis for grouping the trappean rocks into the feldspathic traps, which are light-colored and generally compact rocks, the chief varieties being feldstone and pitchstone, and hornblendie traps or .greenstones, containing the most abundant and best known rocks -of this division. They are of a greenish color, varying from very light, when the feld spar is white and abounding, to almost black, when the constituent minerals are finely -divided and colored with iron. In texture, also, there is considerable difference, some being fine-grained and compact, while in others the crystalline structure is very evident. Time principal varieties are greenstone, basalt, and melaphyre. Porphyry occurs in both the volcanic and trappean rocks when the feldspar is aggregated in large and evident •crystals, scattered through the body of the rock. 3. The granitic rocks (q.v.). The striking characteristic of these rocks is the abundance of silex in a separate and uncom bined state as pure quartz. Granites are associated with the primary strata; they form also the support of the sedimentary deposit, wherever their base has been exposed to view. They occur in beds overspreading the sedimentary deposits or intercalated with them, in dykes, or as the apparent fundamental and unstratified rock. The chief vari eties are true granite, syenite, and protogene.
II. The sedimentary rocks occur in layers or strata. They are either aqueous, aVrial, -chemical, or organic in their origin. 1. The aqueous rocks (q.v.) are argillaceous (q.v.), -composed more or less of clay, as kaolin shale and clay-slate; or arenaceous (q.v.), in which the constituent portions are so large as to be evident to the eye, as in sandstone.
The aqueous rocks were deposited in thin layers, which, however, frequently cohere, so as to form solid masses or beds of considerable thickness. Originally deposited hori zontally, they have in many cases been subjected to disturbances that have elevated or depressed them; hence have arisen faults (q.v.) and dislocations (q.v.), as well as the exposing of the edges of the strata on the surface of the earth (strike, q.v.) at variou. angles (dip, q.v.). 2. The aerial rocks, which cannot be easily separated from aqueous rocks, except by their anomalous stratification (see DRIFT). They play so important a part on sandy coasts and arid interiors at the present day, that it cannot be doubted that they helped in former periods to bring the earth into its present condition. 3. The chemical rocks have been formed by the evaporation of liquids containing substances in solution. The materials thus deposited are salt, gypsum, lime, and silex. Salt is gen erally associated with gypsum, and occurs' in a great range of formations front the Devouian or carboniferous, up to the most recent. The'salt mines at North Melt; in
Cheshire, belong to the triassic period. Rock-salt occurs' in a coarsely crystalline mass, get3rally colored with iron, and more or less mixed with clay and other impurities. The deposits are often of great thickness, but apparently of limited extent, and were prob ably precipitated in isolated brine-lakes. Gypsum seems to have been formed under similar circumstances. It is abundant in the magnesian limestone, in the London clay, and in the Paris basin. Lime has not been deposited in masses, like gypsum, but only from the exposure to the atmosphere of small quantities of liquid saturated with it, which, by evaporation, have left stalagmitic or tufaceous deposits. Silicious sinter has been deposited in a similar manner as it is at the present day around the hot springs of Iceland. 4. The organic rocks are those which have been entirely, or to a large extent, formed from the remains of animals—as chalk and other more compact limestones—or vegetables, as coal, lignite, and diatomaceous deposits.
Changes are continually taking place in the'sedimentary rocks, altering their structure and texture. Among the chief agents including these metamorphic changes are chemical attraction, the infiltration of water, the pressure of the superincumbent strata, and above all, heat and magnetism. Some of the older strata have been so much altered that they are generally spoken of as metamorphiC rocks (q.v.).
apply this title to that division of geology which considers the stratified rocks in their chronological order, as exhibiting different phases of the history and development of the globe Itself, and in their fossil contents forth the progress of life upon its surface. Referring to the article a notice of the animal and vegetable organisms that have been preserved in the rocks, we shall here give a rapid sketch of the various periods in the earth's geological history.
The original, and, as it is supposed, molten condition of the globe is hid in mystery and uncertainty. The geologist takes up the history at the point where air and water make their appearance, and where the inorganic substances were subject to the same influences as those now in operation. It is very doubtful whether the fundamental crust is in any place exposed or has ever been uncovered by man. The earliest rocks observed, though probably not the oldest, are those described by Logan as the Lauren tian system (q.v.). The typical beds occur in Canada; strata of the same age were sub sequently detected in Scotland by Murchison and Geikie. The strata'have been very much metamorphosed by the action of heat, and by the many chemical and physical forces which heat has set in motion, so that their original condition is entirely altered, the whole series being converted into gneissose strata. A structure supposed to repre sent a great foraminifer (Eamon Canadense) has been detected in these rocks, as well as indistinct traces of other fossils. Even in the succeeding Cambrian. series (q.v.), fossils are very rare, consisting of a few zoophytes, crustaceans, and annelids. The rocks of this period consist of thick masses of sandstones and slates or shales. The Silurian period (q.v.) is represented by immense marine depoSits, which in some districts are rich in the remains of invertebrate animals, while other extensive tracts have not yielded a single fossil. No certain evidence of plants has yet been observed, except the round spore-cases iu the upper transition beds, yet the economy,of life would require then, as now, oxygen producers and carbonic acid consumers. Perhaps the anthracite of the graptolitic shales, and the oil from the bituminous silurian shales of North America, may be in part or in whole of vegetable origin. The first traces of the exis tence of dry land occur in the old red sandstone (q.v.). The great mass of the strata of this period consist of immense thicknesses of limestone, composed of corals and shell fish, of beds of shide and of sandstone, crowded in some places with fish-remains. A few land-plants and air-breathing animals, the tenants of the dry land, are preserved in the strata of the middle and upper divisions. The carboniferous measures (see CARBONIF EROUS SYSTEM) are ushered in by a great thickness of deep-sea limestone. The coal bearing strata are alternately sea, estuary, or lake deposits of sandstone, Jude, and limestone, and dry land surfaces with the vegetation converted into coal. THE waters teemed with fishes of great size and strange form; and the dry.land was covered with a rank and luxuriant vegetation of ferns and coniferous trees, and strange forms like gigantic mares' tails and club-mosses. A few air-breathing reptiles and shells have been found in these strata. The permian period (q.v.) exhibits a group of organisms differ ing little from those of the preceding epoch, with the. exception of a few added reptiles.