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parts, and will repose directly upon the shelving bottom, with none of those older strata underneath them. This relation is called Overlap (see fig. 59). The higher or newer members are said to overlap the older. This structure may often be detected among formations of all geological ages. It brings before us the shore line of ancient land-surfaces, and shows how, as these sank under water, the gravels, sands, and silts gradually advanced and covered them. Relative Lapse of Time represented by Strata and by the Intervals between them.-Of the absolute length of time represented by any strata or groups of strata we can form no satisfactory estimates. Certain general conclusions may indeed be drawn, and comparisons may be made between different series of rocks. Sandstones full of false bedding were probably accumulated more rapidly than finely-laminated shales or clays. It is not uncommon in certain Carboniferous formations to find huge coniferous trunks imbedded in an inclined position in sandstone. These trees seem to have been carried along and to have sunk, their heavier or root-end touching the bottom, and their upper end pointing upward in the direction of the current, exactly as in the case of the snags of the Mississippi. The continuous deposit of sand at last rose above the level of the trunks and buried them. It is clear then that the rate of deposit must have been sufficiently rapid to have allowed a mass of 20 or 30 feet of sand to accumulate before the decay of the wood; though modern instances are known where, under certain circumstances, submerged trees may last for some centuries. Continuous layers of the same kind of deposit suggest a persistence of geological conditions; numerous alternations of different kinds of sedimentary matter point to vicissitudes or alternations of conditions. As a rule, we should infer that the time represented by a given thickness of similar strata was less than that shown by the same thickness of dissimilar strata, because the changes needed to bring new varieties of sediment into the area of deposit would usually require the lapse of some time for their completion. But this conclusion might often be erroneous. It would be best supported when, from the very nature of the rocks, wide variations in the character of the waterbottom could be established. Thus a group of shales followed by a fossiliferous limestone would almost always mark the lapse of a much longer period than an equal depth of sandy strata. Limestones made up of organic remains which lived and died upon the spot, and whose remains are crowded together generation above generation, must have demanded many years for their formation.

But in all speculations of this kind we must bear in mind that the length of time represented by a given depth of strata is not to be estimated merely from their thickness or lithological characters. It has already been pointed out that the interval between the deposit of two successive lamine of shale may have been as long as, or even longer than, that required for the formation of one of the lamina. In like manner, the interval needed for the transition from one stratum or kiud of strata 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 importance 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, as will be shown in part v. By this kind of evidence it can be made nearly certain that the intervals represented by strata were in many cases much shorter than those not so represented,-in other words, that the time during which no deposit of sediment went on was longer than that wherein deposit did take place.

Groups of Strata.-Passing from individual strata to large masses of stratified rock, the geologist finds it necdful

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for convenience of reference to subdivide these into groups, He avails himself of two bases of classification-(1) litho logical characters, and (2) organic remains.

1. The subdivision of stratified rocks into groups according to their mineral aspect is an obvious and easily applied classification. Moreover, it often serves to connect together rocks formed continuously in certain circumstances which differed from those under which the strata above and below were laid down, so that it expresses natural and original subdivisions of strata. In the middle of the English Car- ' boniferous system of rocks, for example, a zone of sandy and pebbly beds occurs, known as the Millstone Grit. No abrupt and sharp line can be drawn between these strata and those above and below them. They shade upward and downward into the beds between which they lie. Yet they form a conspicuous belt, traceable for many miles by the scenery to which it gives rise. The red rocks of central England, with their red sandstones, marls, rock-salt, and gypsum, form likewise a well-marked group or rather series of groups. It is obvious, however, that characters of this kind, though sometimes wonderfully persistent over wide tracts of country, must be at best but local. The physical conditions of deposit must always have been limited in extent. A group of strata showing great thickness in one region will be found to die away as it is traced into another. Or its place is gradually taken by another group which, even if geologically contemporaneous, possesses totally different lithological characters. Just as at the present time a group of sandy deposits gradually gives place along the sea-floor to others of mud, and these to others of shells or of gravel, so in former geological periods contemporaneous deposits were not always lithologically similar. Hence mere resemblance in mineral aspect usually cannot be regarded as satisfactory evidence of contemporaneity except within comparatively contracted areas. The Carboniferous Limestone of Ireland is a thick calcareous group of rocks, full of corals, crinoids, and other organisms, which bear witness to the formation of these rocks in the open sea. But if these limestones, with their characteristic marine fossils, are traced into the north of England and Scotland, they are found to pass into sandstones and shales, with numerous coal-seams, and only a few thin beds of limestone. The soft clay beneath the city of London is represented in the Alps by hard schists and contorted limestones. We conclude therefore that lithological agreement when pushed too far is apt to mislead us, partly because contemporaneous strata often vary greatly in their lithological character, and partly because the same lithological characters may appear again and again in different ages. By trusting too implicitly to this kind of evidence, we may be led to class together rocks belonging to very different geological periods, and on the other hand to separate groups which really, in spite of their seeming distinction, were formed contemporaneously.

2. It is by the remains of plants and animals imbedded among the stratified rocks that the most satisfactory subdivisions of the geological record can be made, as will be more fully stated in parts v. and vi. A chronological suc cession of organic forms can be made out among the rocks of the earth's crust. A certain common facies or type of fossils is found to characterize particular groups of rock, and to hold true even though the lithological constitution of the strata should greatly vary. Moreover, though comparatively few species are universally diffused, they possess remarkable persistence over wide areas, and even when they are replaced by others, the same general facies of fossils remains. Hence the stratified formations of two countries geographically distant, and having little or no lithological resemblance to each other, may be compared and paralleled zone by zone, simply by means of their enclosed organic remains.

II. JOINTS.

All rocks are traversed more or less distinctly by vertical or highly inclined divisional planes termed Joints. Soft rocks indeed, such as loose sand and uncompacted clay, do not show these lines; but wherever a mass of clay has been subjected to some pressure and consolidation, it will usually be found to have acquired them. It is by means of the intersection of joints that rocks can be removed in blocks; the art of quarrying consists in taking advantage of these natural planes of division. Joints differ in character according to the nature of the material which they traverse; those in sedimentary rocks are usually distinct from those in crystalline masses.

1. In Sedimentary Rocks.-Joints vary in sharpness of definition, in the regularity of their perpendicular and horizontal course, in their lateral persistence, in number, and in the directions of intersection. As a rule, they are most

sharply defined in proportion to the fineness of grain of the rock. In limestones and close-grained shales, for example, they often occur so clean-cut as to be invisible until revealed by fracture or by the slow disintegrating effects of the weather. The rock splits up along these concealed lines of division whether the agent of demolition be the hammer or frost. In coarse-textured rocks, on the other hand, joints are apt to show themselves as irregular rents along which the rock has been shattered, so that they present an uneven sinuous course, branching off in different directions. In many rocks they descend vertically in straight lines at not very unequal distances, so that the spaces between them are thus marked off into so many walllike masses. But this symmetry often gives place to a more or less tortuous course with lateral joints in various random directions, more especially where the different strata vary considerably in lithological characters. A single joint may be traced sometimes for many yards, or even for several miles, more particularly when the rock is finegrained, as in limestone. But where the texture is coarse and unequal, the joints, though abundant, run into each other in such a way that no one in particular can be identified for so great a distance. The number of joints in a mass of stratified rock varies within wide limits. Among strata which have undergone little disturbance the joints may be separated from each other by intervals of several yards. But in other cases where the terrestrial movement appears to have been considerable, the rocks are so jointed as to have acquired therefrom a fissile character that has nearly or wholly obliterated their tendency to split along the lines of bedding. An important feature in the joints of stratified rocks is the direction in which they intersect each other. result of observation we learn that they possess two dominant trends, one coincident in a general way with the direction in which the strata are inclined to the horizon, and the other running transversely at a right angle or nearly so. The former set is known as dip-joints, because they run with the dip or inclination of the rocks, the latter is termed strike-joints, inasmuch as they conform to the general strike or mean outcrop. It is owing to the existence of this double series of joints that ordinary quarrying operations can be carried on. Large quadrangular blocks can be wedged off, which would be shattered if exposed to the risk of blasting. A quarry is usually worked to the dip of a rock, hence the strike-joints form clean-cut faces in front of the workmen as they advance. These are known as "backs," and the dip joints which traverse them as "cutters." The way in which this double set of joints occurs in a quarry may be seen in fig. 15, where the parallel lines which traverse the shaded and unshaded faces mark the successive strata. The broad white spaces running along the length

As the

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FIG. 15.-Joints In limestone quarry near Mallow, co. Cork (G. V Du Noyer.) "cutters" from

ends looking towards the spectator are which the rock has been quarried away on one side. the enclosed pebbles as well as the surrounding matrix. In some conglomerates the joints may be seen traversing Large blocks of hard quartz are cut through by them as sharply as if they had been sliced in a lapidary's machine, and the same joints can be traced continuously through many yards of the rock. Such facts show that the agency to which the jointing of rocks was due must have operated with considerable force. Further indication of movement is often supplied by the rubbed and striated surfaces of joints. These surfaces, termed slickensides, have evidently been ground against each other. They are often coated with hæmatite, calcite, chlorite, or other mineral, which has taken a cast of the striæ and then seems itself to be striated.

Joints form natural lines for the passage downward and upward of subterranean water. They likewise furnish an effective lodgment for surface water which, frozen by a lowering of temperature, expands into ice, and wedges off blocks of rock in the manner already described. As they serve, in conjunction with bedding, to divide stratified rocks into large quadrangular blocks, their effect on cliffs and other dislocated aspect so familiar in mountain scenery. exposed masses of rock is seen in the apparently splintered,

Occasionally a prismatic or columnar form of joints may be observed among stratified rocks. When this occurs among unaltered strata it is usually among those which observed by Mr Jukes in the Paris Basin, some beds are have been chemically formed, as in gypsum, where, as divided from top to bottom by vertical hexagonal prisms. A columnar structure has often been superinduced upon stratified rocks by contact with intrusive igneous masses. Sandstones, shale, and coal may be observed in this condi tion. The columns diverge perpendicularly to the surface of is vertical the columns are horizontal, or when it undulates the injected and altering substance, so that when the later, Beautiful examples of

the columns follow its curvatures.

this character occur among the coal-seams of Ayrshire. 2. In Crystalline (Igneous) Rocks.-While in stratified rocks the divisional planes consist of lines of bedding and angle, in massive igneous rocks they include joints only, of joint, cutting each other usually at a high if not a right and as these do not as a rule present the same parallelism as lines of bedding, unstratified rocks, even though as full the stratified formations. Granite, for example, is traversed of joints, have not the same regularity of arrangement as in by two sets of chief or "master-joints," cutting each other long quadrangular, rhomboidal, or even polygonal columns. somewhat obliquely. Their effect is to divide the rock into

Rendus, lxxxvi., 1878) on the production of faults and joints. 1 See an interesting series of experiments by M. Daubrée (Comptes X. 38

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convincing evidence of movement. Judging from what takes place at the present time on the bottoms of lakes and of the sea, we confidently infer that when the strata now constituting so much of the solid framework of the land were formed, they were laid down either horizontally or at least at low angles. When, therefore, we find them inclined at all angles, and even standing on end, we conclude that they have been disturbed. Over wide spaces they have been upraised bodily with little alteration of their original horizontality; but in most places some departure from that original position has been effected.

But a third set may usually be noticed cutting across the columns, though less continuous and dominant than the others. When these transverse joints are few in number or occasionally absent, columns many feet in length can be quarried out entire. Such monoliths have been from early times employed in the construction of obelisks and pillars. In rocks of finer grain than granite, such as many diorites and dolerites, the numerous perpendicular joints give the rock a prismatic character. The prisms however are unequal in dimensions, as well as in the number and proportions of their sides, a frequent diameter being 2 or 3 feet, though they may sometimes be observed three times thicker, and extending up the face of a cliff for 300 or 400 feet. It is by means of joints that precipitous faces of rock are produced and retained, for, as in the case of those in stratified masses, they serve as openings into which

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The inclination thus given to rocks is, termed their dip. Its amount is expressed in degrees measured from the plane of the horizon. Thus a set of rocks half-way between the horizontal and vertical position would be said to dip at an angle of 45°, while if vertical they would be marked with the angle of 90°. The edges of strata, where they come up to the surface, are termed their outcrop or basset. When they crop out, that is, rise to the surface, along a perfectly level piece of ground, the outcrop runs at a right angle to the dip. But any inequalities of the surface, such as valleys, ravines, hills, and ridges will cause the outcrop to describe a circuitous course, even though the dip should remain perfectly steady all the while. If a line of precipitous gorge should run directly with the dip, the outcrop will there be coincident with the dip. The occurrence of a

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FIG. 16.-Joints in granite, Killiney Hill, Dublin. (G. V. Du Noyer.) frost drives every year its wedges of ice, whereby huge slices are stripped off. They likewise give rise to the formation of those fantastic pinnacles and fretted buttresses so generally to be observed among igneous rocks in which they occur.

But undoubtedly the most striking series of joints to be found among igneous rocks is in the regularly columnar, or as it is often called, basaltic structure. This structure has been already (ante, p. 249) described in connexion with modern volcanic rocks. It may be met with in rocks of all ages. It is as well displayed among the felsites of the Lower Old Red Sandstone, and the basalts of the Carboni. ferous Limestone in central Scotland, as among the Tertiary lavas of Auvergne or the Vivarais.

3. In Foliated Rocks.-The schists likewise possess their joints, which approximate in character to those among the massive igneous rocks, but they are on the whole less distinct and continuous, while their effect in dividing the rocks into oblong masses is considerably modified by the transverse lines of foliation. These lines play somewhat the same part as those of stratification do among the stratified rocks, though with less definiteness and precision.

III. INCLINATION OF ROCKS.

The most casual observation is sufficient to satisfy us that the rocks now visible at the earth's surface are seldom in their original position. We meet with sandstones and conglomerates composed of water-worn particles, yet forming the angular scarps of lofty mountains; shales and clays full of the remains of fresh-water shells and land-plants, yet covered by limestones made up of marine organisms, and these limestones rising into great ranges of hills, or undulating into fertile valleys, and passing under the streets of busy towns. Such facts, now familiar to every reader, and even to many observers who know little or nothing of systematic geology, point unmistakably to the conclusion that the rocks have in many cases been formed under water, sometimes in lakes, more frequently in the sea, and that they have been elevated into land.

But further examination discloses other and not less

FIG. 17.-Vertical strata, originally deposited horizontally or at low angles.

gently shelving valley in that position will cause the outcrop to descend on one side and to mount in a corresponding way on the other, so as to form a V-shaped indentation in its course. A ridge, on the other hand, will produce a deflexion in the opposite direction. Hence a series of parallel ridges and valleys running in the same direction as the dip of the strata underneath would cause the outcrop to describe a widely serpentinous course. Again, should the rocks be vertical, the outcrop will necessarily correspond with the dip, and continue to do so irrespective altogether of any irregularities of the ground. The lower therefore the angle of inclination the greater is the effect of surface inequalities upon the line of outcrop; the higher the angle the less is that influence, till when the beds stand on end it ceases.

A line drawn at a right angle to the dip is called the strike of the rocks. From what has just been said this line must coincide with outcrop when the surface of the ground is quite level, and also when the beds are vertical. At all other times they are not strictly coincident, but the outcrop wanders to and fro across the strike according to the changes in the angle of inclination and in the form of the ground. The strike may be a straight line or may of the dip. If, for instance, a set of beds dips for half a curve rapidly in every direction, according to the behaviour mile continuously to the north, the strike will run for that distance as a straight east and west line. If the dip gradually changes to north-west and west, and then by southwest to south, it is obvious that the strike must curve round by north-east, north, and north-west till it once more

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FIG. 18.-Geological map.of a portion of a rocky coast-line, and the country inland. (J. B. Jukes.)

sections Fig. 18 represents a geological map in which
series of strata dips in a south-south-easterly direction (S.
northern to 50° at the southern end of the beach.
28- E). The angle of inclination increases from 35° at the
the inhor along
the inner margin, where the ground ascends in a

On the

line

or

cliff (BB) to the inland country (CC), the outcrop is seen
to be deflected a little so as to cross the plateau along a
slightly more northerly line than on the beach.
would show the structure represented in Bg. 19. Such a
section, expressing graphically the result of careful measure-

A section drawn at a right angle to the strike along the line DD

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Fia. 19.-Section along the line DD on fig. 18.

D Level

ment in the field, would give not only the order of succes

sion of beds at the surface, but their actual depth at any dip in fig. 19 is somewhat more than 850 feet.

The total thickness of rock measured at right angles to the

point boneath it.

These Thus a bore or shaft sunk at the point various strata, if restored to their original position, would

marked d on the map would have to pass through rather lie one over the other to that depth. If they were on end more than 425 feet of rock before reaching the stratum b. they would occupy exactly that breadth of ground. But

the inclined position of strata makes them cover more horizontal space; in the present instance it increases that space to 1200 feet.

A convenient rule was given many years ago by the late Mr Charles Maclaren of Edinburgh for estimating the thickness of strata inclined at angles of less than 45°. The real thickness of a mass of inclined strata is th of its apparent thickness for every 5° of dip. Thus if a set of beds dip steadily in one direction at 5° for a horizontal space of 1200 feet measured across the strike, their actual thickness will beth or 100 feet. If the dip be 15° the true thickness will be ths or 400 feet. and so on.

IV. CURVATURES OF ROCKS.

A little reflexion will show that though, so far as regards the trifling portions of the rocks visible at the surface, we might regard the inclined surfaces of the strata as parts of straight lines, they must nevertheless be parts of large curves. Take, for example, the section given in fig. 19. At the north end of that section we observe the beds to plunge one after another into the earth at an angle of 35°. By degrees the inclination increases until it reaches 50°. As there is, no dislocation or abrupt change of angle, but a gradual transition, it is evident that the beds at the north end cannot proceed indefinitely downward at the same angle which they have at the surface, but must bend round to accommodate themselves to the higher inclination which sets in southwards. By prolonging the lines of the bed for some way beneath the sea-level, we can show graphicall, the nature of the curve. In every instance therefore where, in walking over the surface, we traverse a series of strata which gradually, and without dislocations, increase or diminish in inclination, we cross part of a great curvature in the strata of the earth's crust.

Such foldings, however, can often be distinctly seen, either on some cliff or coast-line, or in the traverse of a piece of hilly or mountainous ground. The observer cannot long continue his researches in the field without discovering that the rocks of the earth's crust have been almost everywhere thrown into curves, usually so broad and gentle as to escape observation except when specially looked for. The outcrop of beds at the surface is commonly the truncation of these curves. The strata must once have risen above the present surface, and in many cases may be found descending to the surface again with a contrary dip, the intervening portion of the undulation having been worn

away.

If then the inclination of rocks is so closely connected with their curvature, a corresponding relation must hold between their strike and curvature. In fact, the prevalent strike of a region is determined by the direction of the axes of the great folds into which the rocks have been thrown. If the curves are gentle and inconstant there will be a corresponding variation in the strike. But should the rocks be strongly plicated, there will necessarily be the most thorough coincidence between the strike and the direction of the plication.

The curvature occasionally shows itself among horizontal or gently inclined strata in the form of an abrupt inclination, and then an immediate resumption of the previous flat or sloping character. The strata are thus bent up and continue on the other side of the tilt at a higher level. Such bends are called monoclines or monoclinal folds, because they present only one fold, or one half of a fold, instead of the two which we see in an arch or trough. The most notable instance of this structure in Britain is that of the Isle of Wight, of which a section is given in fig. The Cretaceous rocks on the south side of the island rapidly rise in inclination till they become nearly vertical.

20.

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FIG. 20.-Section of the Isle of Wight-a monoclinal curve. a, Chalk; 8, Woor wich and Reading beds; c, London clay; d, Bagshot series; e, Ileaden series, f, g, Osborne and Bembridge series.

remarkable cases of the same structure have been brought to light by Mr J. W. Powell in his survey of the Colorado region.

It much more frequently happens that the strata have been bent into arches and troughs, so that they can be seen dipping under the surface on one side of the axis of a fold, and rising up again on the other side. Where they dip away from the axis of movement the structure is termed an anticline or anticlinal fold; where they dip towards the

A

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Fig. 21.-Plan of anticlinai and synclinal folds.

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FIG. 22.-Section of antcilinal and synclinal folds on the line CD (fig. 21).

the structure given in fig. 22. Here we see that, at the part of the anticlinal axis (A) where the section crosses, bed No. 4 forms the crown of the arch, Nos. 1, 2, and 3 being concealed beneath it. On the east side of the axis the strata follow each other in regular succession as far as No. 13, which, instead of passing here under the next in order, turns up with a contrary dip and forms the centre of a trough or syncline (B). From underneath No. 13 on the east side, the same beds rise to the surface which passed beneath it on the west side. The particular bed marked EF has been entirely removed by denudation from the top of the anticline, and is buried deep beneath the centre of the syncline.

Such foldings of strata must always die out unless they are abruptly terminated by dislocations. In the cases given in fig. 21, both the arch and trough are represented as diminishing, the former towards the north, the latter towards the south. The observer in passing northwards

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