ith mud and animal remains. 3. Sandstone pillar in Saxony. 4. Volcanic island (Ferdinandea). both of Nothosaurus. 9. Plate-formed granite rock. 10. Turbinites dubius. 11. Cross-section of julted stratification.

Geology.

guished between marine and fluviatile formations. He was not able, however, to free Limself from the absurd hypotheses of his day.

In England, the diluvialists were busy framing idle theories, to give a plausibility to their creed that the Noachian deluge was the cause of all the past-changes on the earth's surface. Differing somewhat in detail, they all agreed in the notion of an interior abyss, whence the waters rushed, breaking up and bursting through the crust of the earth, to cover its surface, and whither, after the deluge, they returned again. Such absurd dreams, obviously opposed to the observed order of nature, greatly hindered the progress of true science.

Leibnitz (1680) proposed the bold theory that the earth was originally in a molten state from heat, and that the primary rocks were formed by the cooling of the surface, which also produced the primeval ocean, by condensing the surrounding vapors. The sedimentary strata resulted from the subsiding of the waters that had been put in motion from the collapse of the crust on the contracting nucleus. This process was several times repeated, until at last an equilibrium was established.

Hooke (1688) and Ray (1690,) differing as much from Burnet as from Leibnitz, advocated views similar to those of Pythagoras. They considered the essential condition of the globe to be one of change, and that the forces now in action would, if allowed sufficient time, produce changes as great as those of geological date. They were followed in the same direction by Vallisneri (1720), Moro (1740). Buffon (1749), Lehman (1756), and Fuchsel (1773), each contributing something additional. Werner (1780) greatly advanced the science by establishing the superposition of certain groups, by giving a system and names, and by showing the practical applications of geology to mining, agriculture, and medicine. He had very crude notions regarding the origin of the strata, supposing that the various formations were precipitated over the earth in succession from a chaotic fluid; even the igneous rocks he held to be chemical precipitates from the waters. Hutton (1788), rejecting all theories as to the beginning of the world, returned to the opinions of Pythagoras and Ray. He held that the strata which now compose the continents were once beneath the sea, and were formed out of the waste of pre-existing continents by the action of the same forces which are now destroying even the hardest rocks. He introduced the notion of a periodical elevation of the sedimentary deposits from the internal heat raising the bed of the sea. Lyell, in our own day, adopted and improved these views, eliminating the baseless theories which were mixed up with them, and demonstrating that existing forces might produce all the phenomena of geology.

The determination of the order of the strata, and the grouping of them in chronological order, were begun by Lehman (1756), and carried on by Fuchsel (1773), Pallas (1785), and Werner. Smith made the most important contribution to this subject when, in 1790, he published his Tabular View of the British strate. He showed their superposition, and characterized the different groups by their peculiar fossils. The publication of his geological map of England (1815) may be said to form an epoch in the history of geology. Since then, the science has advanced by rapid strides; and it is not too much to expect that ere long all the chief geological features of the accessible parts of the world will be known and published.

Geology, in its restricted and usual sense, takes cognizance of the solid substance of of the earth, or rather of as much of it as is accessible to man's observation. He has not, by his own efforts, penetrated at any point more than a few hundred yards from the surface; but natural sections, and the peculiar arrangement of the stratified rocks (the key to which he has to some extent obtained), have given him an aquaintance with a greater thickness than could have resulted from his own labors. He has thus by actual observations, coupled with reasonings upon them, been able to construct an ideal section representing a depth of perhaps 10 m., or about a 400th part of the distance from the surface to the centre. He does not, and cannot with certainty, know anything of the structure or condition of what is deeper. This does not, however, prevent the attempt to know something of what is beyond; and in making the attempt, there are many facts which serve as bases for inductions, or at least theorizings, as to the condition of the interior of the globe. As the conclusions depend upon the balancing of evidence, upon the value given to one set of facts as set against another, they will differ according to the importance given by each individual to the one or other set of facts. The long entertained opinion of the existence of a central heat seems to be on the whole fairly established, and upon such facts as these: 1. There is a regular and gradual increase in the temperature of all deep mines, equal to 1° F. for every 55 ft. of descent after the first 100. 2 Deep wells have always a high temperature. This has been carefully determined in artesian wells, not only by applying the thermometer to the water at the surface which has risen from a known depth, but also by sinking the instrument to various depths. The results have shown an increment similar to that exhibited in mines. Hot or boiling natural springs rise through great and deep fissures. 3. Igneous rocks-that is to say rocks which have cooled from a state of fusion by heat -invariably come from below, upwards, and thus testify to an amount of internal heat able either to retain these rocks in a state of fusion, or to convert them into a fluid condition before their ejection. 4. Physics also contributes important evidence. The specific gravity of granite or basalt is scarcely 3, while that of the earth, according to VI.-18a.

Geology.

the recent experiments of Airy, is about 6. If unopposed, the influence of gravitation would so increase the density of the composing rock as to give a greater specific gravity for the earth than 6. There must, then, be some expansive force acting to reduce the gravity, and the only force with which we are acquainted that could so act, is heat. On the other hand, physics raises difficulties which militate against the fluid condition of any considerable portion of the earth's interior, and in these difficulties it is supported by astronomy. But although we may admit that the rate of increase of heat from the surface downwards goes on at the rate indicated by observation in mines and wells, we need not draw the conclusion that the interior is fluid below 25 m., or even 2,000 m.; for we are ignorant of the effects of enormous pressure in altering the point of fusion. The strict province of geology is the observed or observable portion of the earth's crust. The early geologists were no more than geognosts-they observed and described the rock-mineralogy of districts, and thus laid the foundations for those generalizations which have raised geology to its present position. The materials of the earth's crust were at first grouped together according to their composition, structure, and origin; but gradually it became evident that the rocks themselves occurred in groups, and that they had a particular order in nature; until at last, all the sedimentary strata were arranged in a single continuous and chronological series, from characters drawn less from their lithological structure than from their organic contents. Both systems of classification are important that of the geognost as well as that of the modern geologist. The one is the result, to a large extent, of work in the laboratory and the study, and may be accomplished by the examination of hand specimens; the other must be determined in the field, and only from the examination of rocks in the mass, and in their natural position. The term lithology has been applied to the one aspect, while stromatology (strūma, a layer) may with equal fitness be given to the other.

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 spreading below shallow water, and becoming vesicular pumice, or as ash scattered in layers 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 augitic 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 secondary 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 themselves 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 hornblendic 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 feldspar 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. The 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 varieties are true granite, syenite, and protogene.

II. The sedimentary rocks occur in layers or strata. They are either aqueous, aerial, 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 horizontally, 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 various 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 generally associated with gypsum, and occurs in a great range of formations from the Devonian or carboniferous, up to the most recent. The salt mines at Northwich, in Cheshire, belong to the triassic period. Rock-salt occurs in a coarsely crystalline mass, generally 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 probably 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.).

Stromatology-We 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 setting forth the progress of life upon its surface. Referring to the article PALEONTOLOGY for 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 Laurentian system (q.v.). The typical beds occur in Canada; strata of the same age were subsequently 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 represent a great foraminifer (Eozoon 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 in 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 existence 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 shellfish, of beds of shale 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 (sce CARBONIF EROUS SYSTEM) are ushered in by a great thickness of deep-sea limestone. The coalbearing strata are alternately sea, estuary, or lake deposits of sandstone, shale, 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.