appear in some strata in the form of little heaps of various shapes, and are conjectured to be due to the intermittent escape of gas from decomposing organic matter in the original sand or mud, as we now sometimes see in the mud flats of rivers and estuaries. The order of superposition is the foundation of geological chronology. As strata are laid down upon one another in a more or less nearly hori zantal position, the underlying beds must be older than those which cover them. This obvious truth is termed the "law of superposition," and furnishes the means of deter mining the chronology of rocks, and, though other methods are employed, they must all be based upon the observed order of superposition. The only cases where the apparent superposition may be deceptive is where the strata have been inverted. In the Alps, the rocks composing great mountain masses have been so completely overturned that the highest beds appear to be covered by others which ought properly to be underneath them. Such conditions, however, are exceptional. Alternations of strata show that certain repetitions occur with tolerable regularity. Sandstones are interleaved with shale above, and then pass into shale; the latter may become sandy at top and be finally covered with sandstone, or may become calcareous and pass into limestone. A sand stone group indicates water of comparatively little depth, moved by changing currents, and bringing the sand alternately from one side to the other. Limestone above the shale shows that the water cleared, owing to a deflection of the sediment-carrying cur rents, or to a continued and perhaps more rapid subsidence, and that corals, crinoids, mollusks, and other lime-secretinn• organisms established themselves upon the spot. Shale overlying limestone tells of fresh inroads of mud by which animal life was destroyed. In studying associations of strata we find that certain kinds commonly occur together, because the conditions under which they were formed were apt to arise in succession. A seam of coal is almost invariably found to lie in a bed of fire-clay, or on some argillaceous stratum. This is because the fire-clay formed the soil on which the plants grew that went to form the coal. When the clay was laid down under suitable circumstances vegetation sprang up upon it. Turning to the relative persistence of strata we observe that some kinds of sediment are much more widely spread than others, and therefore some kinds of sedimentary rocks possess far greater persistence than others. Usually the coarser the grain the more local the extension of the rock. Conglomerates are thus by far the most variable of all sedimentary formations. Sandstones are less liable to extremes, but are apt to thin out and then swell again. Shales are far more persistent, the same zone often being traceable for many miles. Limestones display remarkable continuity, and coal-seams often present remarkable persistence. Overlap is where strata have been laid down in a subsiding region wherein the area of deposit grad ually increased, and the sediment spread over a progressively augmenting surface. whereby the later portions of a sedimentary series are extended over and repose upon the earlier portions. As to the relative lapse of time represented by strata and intervals between them, we can form no satisfactory estimates.
In all speculations in the case we must hear in mind that the length of time repre sented by a given depth of strata is not to be estimated merely from their thickness or lithological character. The intervals between the deposit of two successive laminm of shale may have been as long as, or longer than, that required for the formation of one of the laminte. In like manner, the interval needed for the transition from one stratum or kind of stratum to another may Often have been more than equal to the time required for the formation of the strata on either side. But the relative chronological impor tance of the bars or lines in the geological record can seldom be satisfactorily discussed merely on lithological grounds. This must mainly be decided on the evidence of organic remains. By this kind of evidence it can be made nearly certain that the inter vals represented by strata were in many cases much shorter than those not so repre sented; in other words, that the time during which no deposit of sediment went on was longer than that wherein deposit did take place. Passing from individual strata to groups of strata, the geologist makes two bases of classification: 1. Lithological charac ters; 2. Organic remains. The first is uncertain, and everywhere variable. It is by the remains of plants and animals iinbedded among the stratified rocks that the most satisfactory subdivisions of the geological record can be made. A chronological suc cession of organic forms can be made out among the rocks of the earth's crust. A cer tain common type is found to characterize particular groups of rock, and to hold true even though the Ethological constitution of the strata should greatly vary. Moreover, though comparatively few species are universally diffused, they possess remarkable per sistence over wide areas, and even when they are replaced by others, the same general fades of fossils remains.• Hence the stratified formations of two countries geographi cally distant, and having little or no lithological resemblance to each other, may he compared and paralleled, zone by zone, simply by means of their inclosed organic remains.
Joints traverse all rocks, more or less distinctly, by vertical or highly inclined divisional planes. Soft rocks, such as loose sand and uncompacted clay, do not show these joint lines; but wherever a mass of clay has been subjected to some pressure and consation, it will usually be found to have acquired them. It is by means of the in tion of joints that rocks can be removed in blocks, and the quarryman takes ad age of these natural planes of division.
iitip The inclination of rocks would satisfy the most casual observer that the rocks now visible at the earth's surface are seldom in their original position. The inclination Of rocks is termed their dip, and the amount is expressed in degrees measured from the plane of the horizon; thus rocks standing vertical have a dip of 90°. The edges of strata where they come to the surface are the outcrop or basset. A line drawn at right angles to the dip is called the strike of the rocks. Miners call this strike the level course or level-bearing. The movements which the crust of the earth has Undergone have not only folded up and corrugated the rocks, but have fractured them in all directions; the result being. called dislocations of rocks, These dislocations may be simple fissures, that is, rents without any vertical displacement of the mass on either side; or may be faults, that is, rents where one side has been pushed up or has sunk down. Another system of divisional planes is known as cleavage, by which rocks are sornetimel traversed. When this cleavage is well developed it divides the rock into parallel laminae, which run at a high angle quite independently of stratification or any other divisional planes. Cleavage is most perfect in fine-grained material, hence it is admirably shown in argil laceous rocks. With regard to igneous rocks as a part of the structure of the earth's
crust, the general law which has governed their intrusion within the crust may be thus stated: Every fluid mass impelled upwards by pressure from below, or by the expansion of its own imprisoned vapor, has sought egress along the line of least resistance. What that line was to be, has depended iu each case upon the structure of the terrestrial crust and the energy of the eruption. In ninny instances it has been determined by an already dislocation; in others by the planes of stratification, or by the surface of junc tion of two inconformable formations, or by irregular rents and cracks, or by the more complex lines of weakness. Sometimes the intruded mass has actually fused and obliterated some of the rock which it has invaded, incorporating such portion into its own substance. The shape of the channel of escape has necessarily determined the form of the intrusive rock, as the mold regulates the form of a casting of molten iron. Igneous rocks may be arranged under four heads, according to the shape in which they have solidified: 1. Amorphous masses; 2. Sheets; 3. Veins and dykes; 4. Necks. The first are chiefly crystalline coarse-textured rocks, of which granite and syenite are the most conspicuous; sheets are masses of crystalline rock intruded between other rocks. Many of the older volcanic rocks occur in this form, such as feldstone, quartz, porphyry, dolerite, basalt, and others. Coal seams, when invaded by intruded sheets of igneous matter, assume different aspects, according to the thickness and nature of the invading sheet, the depth of the coal seam, and other conditions. In some cases the coal has been fused, and has acquired a blistered or vesicular texture, the gas 'cavities being either empty or filled with mineral matter, such as calcite. In other cases the coal has nearly disappeared, the remaining portion being black soot or ashes. In others, still. it has become hard and brittle, and has been converted into a kind of anthracite, owing to the loss of its more volatile portions. Veins of igneous rock may occur indifferently in igneous, aqueous, or metamorphoric rocks, and range in diameter from thread-like fila ments to masses many feet or yards across. There are veins of segregation and veins of intrusion. Those of segregation are peculiar to crystalline rocks; they abound in many granites, and are found in gneiss and schist. They run as straight, curved, or branching ribands, seldom exceeding a foot in thickness. Most frequently they are finer in texture than the reek which they traverse.:but occasionally the reverse is the case; especially in granite. These veins arc so welded to the rock that they cannot easily be separated along the plane of union. Veins of intrusion are portions of once melted matter injected into rents in previously solidified rocks. Dykes are wall-like masses of igneous rock, filling vertical or highly inclined fissures. They present as great a variety of thickness as is shown by the veins. Sometimes they occur as plates of rock only an inch or two in thickness;,at other times they attain a breadth of ten or twelve fathoms. The name is given because the formation presents the appearance of a wall, and their sides are often as parallel and perpendicular as those Of a piece of masonry. Necks are the filled up pipes or funnels of former volcanic vents. When a volcano ceases action its vent is left full of igneous matter which soon solidifies. The wash of surface by rains and other agencies in old volcanic regions has laid these necks bare. They are usually circular or elliptical, but occasionally of a more branching or • irregular form. The materials are sometimes crystalline, sometimes fragmental, and there may be some form of lava. The fissures which so abundantly traverse the crust of the earth have, in many instances, served as places for the _deposit of mineral matter forming mineral veins. These veins or lodes vary in thickness from less than an inch up to fathoms, and the same vein may, in not far separated parts, have both extremes of thickness. Such veins usually send out shoots, and in some mining districts this has been done to such an extent that it becomes hardly possible to identify the main vein among its numerous offshoots. In some rocks, more especially in limestones, large sub terranean cavities have been filled with veinstones and ore. Various theories have been proposed to account for the infilling of mineral veins. Of these, the most noteworthy are: 1. The theory of lateral segregation, which teaches that the substances in the veins have been derived from the adjacent rocks by a process of solution and redeposit; and, 2. The theory of filling from below, according to which the minerals and ores were introduced from below, dissolved in water or steam, or by sublimation, or by igneous fusion and injection. The fact that the nature and amount of the minerals, and especi ally of the ores in a vein, vary.with the nature of the surrounding rocks, seems to show that such rocks have had a certain influence on the precipitation of mineral matter in the fissures passing through them; but that this mineral matter came chiefly from below appears almost certain. The phenomena of the ascent of hot water in volcanic districts afford a close analogy to what has occurred in mineral veins. It is known that at the present time various minerals, including silica, both crystalline and calcedonic, and various metallic sulphides, are being deposited in fissures up which hot water rises. At the same time it is conceivable that to some extent there may be a decomposition of the rocks on either side of a fissure, and that a portion of the mineral matter abstracted may be laid down in another form along the walls of the fissure, or, on the other hand, that the rocks on either side of the fissure may be permeated for some distance by the ascending waters, and that some of the mineral substances carried up in solution may be deposited in the pores and cavities of these rocks as well as in the fissure itself. The last division to notice under structural geology is unconformability. Where one series of rocks has been laid down continuously and without disturbance upon another series they are said to be "conformable." Though such rocks may usually be presumed to have followed each other continuously without any great disturbance of geographical condition, we cannot always be safe in such an inference. But an unconformability leaves no room to doubt that it marks a decided break in the continuity of deposit. Hence, no kind of geological structure is of higher importance in the interpretation of the history of the stratified formations of a country. Paramount, ;hough, the effect of unconformability may be in the geological structure of a country, it must nevertheless be in almost all cases local. The disturbance by which it was produced can have affected but a comparatively circumscribed region, beyond the limits of which the con tinuity of sedimentation may have been undisturbed.