The Student's Elements of Geology Part 6

In both instances the force may be irresistible, but though adequate, it need not be visible by us, provided the time required for its development be very great. The lateral pressure arising from the unequal expansion of rocks by heat may cause one ma.s.s lying in the same horizontal plane gradually to occupy a larger s.p.a.ce, so as to press upon another rock, which, if flexible, may be squeezed into a bent and folded form. It will also appear, when the volcanic and granitic rocks are described, that some of them have, when melted in the interior of the earth's crust, been injected forcibly into fissures, and after the solidification of such intruded matter, other sets of rents, crossing the first, have been formed and in their turn filled by melted rock. Such repeated injections imply a stretching, and often upheaval, of the whole ma.s.s.

We also know, especially by the study of regions liable to earthquakes, that there are causes at work in the interior of the earth capable of producing a sinking in of the ground, sometimes very local, but often extending over a wide area. The continuance of such a downward movement, especially if partial and confined to linear areas, may produce regular folds in the strata.

CREEPS IN COAL-MINES.

The "creeps," as they are called in coal-mines, afford an excellent ill.u.s.tration of this fact.-- First, it may be stated generally, that the excavation of coal at a considerable depth causes the ma.s.s of overlying strata to sink down bodily, even when props are left to support the roof of the mine. "In Yorks.h.i.+re," says Mr. Buddle, "three distinct subsidences were perceptible at the surface, after the clearing out of three seams of coal below, and innumerable vertical cracks were caused in the inc.u.mbent ma.s.s of sandstone and shale which thus settled down." (Proceedings of Geological Society volume 3 page 148.) The exact amount of depression in these cases can only be accurately measured where water acc.u.mulates on the surface, or a railway traverses a coal-field.

(FIGURE 59. Section of carboniferous strata at Wallsend, Newcastle, showing "creeps." (J. Buddle, Esq.) Horizontal length of section 174 feet. The upper seam, or main coal, here worked out, was 630 feet below the surface.

Section through, from top to bottom: Siliceous sandstone.

Shale.

1. Main coal, 6 feet 6 inches, with creeps a, b, c, d.

Shale eighteen yards thick.

2. Metal coal, 3 feet, with fractures e, f, g, h.)

When a bed of coal is worked out, pillars or rectangular ma.s.ses of coal are left at intervals as props to support the roof, and protect the colliers. Thus in Figure 59, representing a section at Wallsend, Newcastle, the galleries which have been excavated are represented by the white s.p.a.ces a, b, while the adjoining dark portions are parts of the original coal seam left as props, beds of sandy clay or shale const.i.tuting the floor of the mine. When the props have been reduced in size, they are pressed down by the weight of overlying rocks (no less than 630 feet thick) upon the shale below, which is thereby squeezed and forced up into the open s.p.a.ces.

Now it might have been expected that, instead of the floor rising up, the ceiling would sink down, and this effect, called a "thrust," does, in fact, take place where the pavement is more solid than the roof. But it usually happens, in coalmines, that the roof is composed of hard shale, or occasionally of sandstone, more unyielding than the foundation, which often consists of clay.

Even where the argillaceous substrata are hard at first, they soon become softened and reduced to a plastic state when exposed to the contact of air and water in the floor of a mine.

The first symptom of a "creep," says Mr. Buddle, is a slight curvature at the bottom of each gallery, as at a, Figure 59: then the pavement, continuing to rise, begins to open with a longitudinal crack, as at b; then the points of the fractured ridge reach the roof, as at c; and, lastly, the upraised beds close up the whole gallery, and the broken portions of the ridge are reunited and flattened at the top, exhibiting the flexure seen at d. Meanwhile the coal in the props has become crushed and cracked by pressure. It is also found that below the creeps a, b, c, d, an inferior stratum, called the "metal coal," which is 3 feet thick, has been fractured at the points e, f, g, h, and has risen, so as to prove that the upward movement, caused by the working out of the "main coal," has been propagated through a thickness of 54 feet of argillaceous beds, which intervene between the two coal-seams. This same displacement has also been traced downward more than 150 feet below the metal coal, but it grows continually less and less until it becomes imperceptible.

No part of the process above described is more deserving of our notice than the slowness with which the change in the arrangement of the beds is brought about.

Days, months, or even years, will sometimes elapse between the first bending of the pavement and the time of its reaching the roof. Where the movement has been most rapid, the curvature of the beds is most regular, and the reunion of the fractured ends most complete; whereas the signs of displacement or violence are greatest in those creeps which have required months or years for their entire accomplishment. Hence we may conclude that similar changes may have been wrought on a larger scale in the earth's crust by partial and gradual subsidences, especially where the ground has been undermined throughout long periods of time; and we must be on our guard against inferring sudden violence, simply because the distortion of the beds is excessive.

Engineers are familiar with the fact that when they raise the level of a railway by heaping stone or gravel on a foundation of marsh, quicksand, or other yielding formation, the new mound often sinks for a time as fast as they attempt to elevate it; when they have persevered so as to overcome this difficulty, they frequently find that some of the adjoining flexible ground has risen up in one or more parallel arches or folds, showing that the vertical pressure of the sinking materials has given rise to a lateral folding movement.

In like manner, in the interior of the earth, the solid parts of the earth's crust may sometimes, as before mentioned, be made to expand by heat, or may be pressed by the force of steam against flexible strata loaded with a great weight of inc.u.mbent rocks. In this case the yielding ma.s.s, squeezed, but unable to overcome the resistance which it meets with in a vertical direction, may be gradually relieved by lateral folding.

DIP AND STRIKE.

(FIGURE 60. Series of inclined strata dipping to the north at an angle of 45 degrees.)

In describing the manner in which strata depart from their original horizontality, some technical terms, such as "dip" and "strike," "anticlinal"

and "synclinal" line or axis, are used by geologists. I shall now proceed to explain some of these to the student. If a stratum or bed of rock, instead of being quite level, be inclined to one side, it is said to DIP; the point of the compa.s.s to which it is inclined is called the POINT OF DIP, and the degree of deviation from a level or horizontal line is called THE AMOUNT OF DIP, or THE ANGLE OF DIP. Thus, in the diagram (Figure 60), a series of strata are inclined, and they dip to the north at an angle of forty-five degrees. The STRIKE, or LINE OF BEARING, is the prolongation or extension of the strata in a direction AT RIGHT ANGLES to the dip; and hence it is sometimes called the DIRECTION of the strata. Thus, in the above instance of strata dipping to the north, their strike must necessarily be east and west. We have borrowed the word from the German geologists, streichen signifying to extend, to have a certain direction. Dip and strike may be aptly ill.u.s.trated by a row of houses running east and west, the long ridge of the roof representing the strike of the stratum of slates, which dip on one side to the north, and on the other to the south.

A stratum which is horizontal, or quite level in all directions, has neither dip nor strike.

It is always important for the geologist, who is endeavouring to comprehend the structure of a country, to learn how the beds dip in every part of the district; but it requires some practice to avoid being occasionally deceived, both as to the point of dip and the amount of it.

(FIGURE 61. Apparent horizontality of inclined strata.)

If the upper surface of a hard stony stratum be uncovered, whether artificially in a quarry, or by waves at the foot of a cliff, it is easy to determine towards what point of the compa.s.s the slope is steepest, or in what direction water would flow if poured upon it. This is the true dip. But the edges of highly inclined strata may give rise to perfectly horizontal lines in the face of a vertical cliff, if the observer see the strata in the line of the strike, the dip being inward from the face of the cliff. If, however, we come to a break in the cliff, which exhibits a section exactly at right angles to the line of the strike, we are then able to ascertain the true dip. In the drawing (Figure 61), we may suppose a headland, one side of which faces to the north, where the beds would appear perfectly horizontal to a person in the boat; while in the other side facing the west, the true dip would be seen by the person on sh.o.r.e to be at an angle of 40 degrees. If, therefore, our observations are confined to a vertical precipice facing in one direction, we must endeavour to find a ledge or portion of the plane of one of the beds projecting beyond the others, in order to ascertain the true dip.

(FIGURE 62. Two hands used to determine the inclination of strata.)

If not provided with a clinometer, a most useful instrument, when it is of consequence to determine with precision the inclination of the strata, the observer may measure the angle within a few degrees by standing exactly opposite to a cliff where the true dip is exhibited, holding the hands immediately before the eyes, and placing the fingers of one in a perpendicular, and of the other in a horizontal position, as in Figure 62. It is thus easy to discover whether the lines of the inclined beds bisect the angle of 90 degrees, formed by the meeting of the hands, so as to give an angle of 45 degrees, or whether it would divide the s.p.a.ce into two equal or unequal portions. You have only to change hands to get the line of dip on the upper side of the horizontal hand.

(FIGURE 63. Section ill.u.s.trating the structure of the Swiss Jura.)

It has been already seen, in describing the curved strata on the east coast of Scotland, in Forfars.h.i.+re and Berwicks.h.i.+re, that a series of concave and convex bendings are occasionally repeated several times. These usually form part of a series of parallel waves of strata, which are prolonged in the same direction, throughout a considerable extent of country. Thus, for example, in the Swiss Jura, that lofty chain of mountains has been proved to consist of many parallel ridges, with intervening longitudinal valleys, as in Figure 63, the ridges being formed by curved fossiliferous strata, of which the nature and dip are occasionally displayed in deep transverse gorges, called "cluses," caused by fractures at right angles to the direction of the chain. (Thurmann "Essai sur les Soulevemens Jura.s.siques de Porrentruy" Paris 1832.) Now let us suppose these ridges and parallel valleys to run north and south, we should then say that the STRIKE of the beds is north and south, and the DIP east and west. Lines drawn along the summits of the ridges, A, B, would be anticlinal lines, and one following the bottom of the adjoining valleys a synclinal line.

OUTCROP OF STRATA.

(FIGURE 64. Ground-plan of the denuded ridge C, Figure 63.)

(FIGURE 65. Transverse section of the denuded ridge C, Figure 63..)

It will be observed that some of these ridges, A, B, are unbroken on the summit, whereas one of them, C, has been fractured along the line of strike, and a portion of it carried away by denudation, so that the ridges of the beds in the formations a, b, c come out to the day, or, as the miners say, CROP OUT, on the sides of a valley. The ground-plan of such a denuded ridge as C, as given in a geological map, may be expressed by the diagram, Figure 64, and the cross- section of the same by Figure 65. The line D E, Figure 64, is the anticlinal line, on each side of which the dip is in opposite directions, as expressed by the arrows. The emergence of strata at the surface is called by miners their OUTCROP, or Ba.s.sET.

If, instead of being folded into parallel ridges, the beds form a boss or dome- shaped protuberance, and if we suppose the summit of the dome carried off, the ground-plan would exhibit the edges of the strata forming a succession of circles, or ellipses, round a common centre. These circles are the lines of strike, and the dip being always at right angles is inclined in the course of the circuit to every point of the compa.s.s, const.i.tuting what is termed a qua- quaversal dip-- that is, turning every way.

There are endless variations in the figures described by the ba.s.set-edges of the strata, according to the different inclination of the beds, and the mode in which they happen to have been denuded. One of the simplest rules, with which every geologist should be acquainted, relates to the V-like form of the beds as they crop out in an ordinary valley. First, if the strata be horizontal, the V- like form will be also on a level, and the newest strata will appear at the greatest heights.

(FIGURE 66. Slope of valley 40 degrees, dip of strata 20 degrees.)

Secondly, if the beds be inclined and intersected by a valley sloping in the same direction, and the dip of the beds be less steep than the slope of the valley, then the V's, as they are often termed by miners, will point upward (see Figure 66), those formed by the newer beds appearing in a superior position, and extending highest up the valley, as A is seen above B.

(FIGURE 67. Slope of valley 20 degrees, dip of strata 50 degrees.)

Thirdly, if the dip of the beds be steeper than the slope of the valley, then the V's will point downward (see Figure 67), and those formed of the older beds will now appear uppermost, as B appears above A.

(FIGURE 68. Slope of valley 20 degrees, dip of strata 20 degrees, in opposite directions.)

Fourthly, in every case where the strata dip in a contrary direction to the slope of the valley, whatever be the angle of inclination, the newer beds will appear the highest, as in the first and second cases. This is shown by the drawing (Figure 68), which exhibits strata rising at an angle of 20 degrees, and crossed by a valley, which declines in an opposite direction at 20 degrees.

These rules may often be of great practical utility; for the different degrees of dip occurring in the two cases represented in Figures 66 and 67 may occasionally be encountered in following the same line of flexure at points a few miles distant from each other. A miner unacquainted with the rule, who had first explored the valley Figure 66, may have sunk a vertical shaft below the coal-seam A, until he reached the inferior bed, B. He might then pa.s.s to the valley, Figure 67, and discovering there also the outcrop of two coal-seams, might begin his workings in the uppermost in the expectation of coming down to the other bed A, which would be observed cropping out lower down the valley. But a glance at the section will demonstrate the futility of such hopes. (I am indebted to the kindness of T. Sopwith, Esq., for three models which I have copied in the above diagrams; but the beginner may find it by no means easy to understand such copies, although, if he were to examine and handle the originals, turning them about in different ways, he would at once comprehend their meaning, as well as the import of others far more complicated, which the same engineer has constructed to ill.u.s.trate FAULTS.)

SYNCLINAL STRATA FORMING RIDGES.

(FIGURE 69. Section of carboniferous rocks of Lancas.h.i.+re. (E. Hull. (Edward Hull, Quarterly Geological Journal volume 24 page 324. 1868.)) a. Synclinal. Grits and shales.

c. Anticlinal. Mountain limestone.

b. Synclinal. Grits and shales.)

Although in many cases an anticlinal axis forms a ridge, and a synclinal axis a valley, as in A B, Figure 63, yet this can by no means be laid down as a general rule, as the beds very often slope inward from either side of a mountain, as at a, b, Figure 69, while in the intervening valley, c, they slope upward, forming an arch.

It would be natural to expect the fracture of solid rocks to take place chiefly where the bending of the strata has been sharpest, and such rending may produce ravines giving access to running water and exposing the surface to atmospheric waste. The entire absence, however, of such cracks at points where the strain must have been greatest, as at a, Figure 63, is often very remarkable, and not always easy of explanation. We must imagine that many strata of limestone, chert, and other rocks which are now brittle, were pliant when bent into their present position. They may have owed their flexibility in part to the fluid matter which they contained in their minute pores, as before described, and in part to the permeation of sea-water while they were yet submerged.

(FIGURE 70. Strata of chert, grit, and marl, near St. Jean de Luz.)

At the western extremity of the Pyrenees, great curvatures of the strata are seen in the sea-cliffs, where the rocks consist of marl, grit, and chert. At certain points, as at a, Figure 70, some of the bendings of the flinty chert are so sharp that specimens might be broken off well fitted to serve as ridge-tiles on the roof of a house. Although this chert could not have been brittle as now, when first folded into this shape, it presents, nevertheless, here and there, at the points of greatest flexure, small cracks, which show that it was solid, and not wholly incapable of breaking at the period of its displacement. The numerous rents alluded to are not empty, but filled with chalcedony and quartz.

(FIGURE 71. Bent and undulating gypseous marl.

g. Gypsum. m. Marl.)

Between San Caterina and Castrogiovanni, in Sicily, bent and undulating gypseous marls occur, with here and there thin beds of solid gypsum interstratified.

Sometimes these solid layers have been broken into detached fragments, still preserving their sharp edges (g, g, Figure 71), while the continuity of the more pliable and ductile marls, m, m, has not been interrupted.

(FIGURE 72. Folded strata.)