A Manual of Elementary Geology Part 12

To conclude this subject, I may remind the reader, that were we to embrace the doctrine which ascribes the elevated position of marine formations, and the depression of certain freshwater strata, to oscillations in the level of the waters instead of the land, we should be compelled to admit that the ocean has been sometimes every where much shallower than at present, and at others more than three miles deeper.

[Ill.u.s.tration: Fig. 61. Vertical conglomerate and sandstone.]

_Inclined stratification._--The most unequivocal evidence of a change in the original position of strata is afforded by their standing up perpendicularly on their edges, which is by no means a rare phenomenon, especially in mountainous countries. Thus we find in Scotland, on the southern skirts of the Grampians, beds of pudding-stone alternating with thin layers of fine sand, all placed vertically to the horizon. When Saussure first observed certain conglomerates in a similar position in the Swiss Alps, he remarked that the pebbles, being for the most part of an oval shape, had their longer axes parallel to the planes of stratification (See fig. 61.). From this he inferred, that such strata must, at first, have been horizontal, each oval pebble having originally settled at the bottom of the water, with its flatter side parallel to the horizon, for the same reason that an egg will not stand on either end if unsupported. Some few, indeed, of the rounded stones in a conglomerate occasionally afford an exception to the above rule, for the same reason that we see on a s.h.i.+ngle beach some oval or flat-sided pebbles resting on their ends or edges; these having been forced along the bottom and against each other by a wave or current so as to settle in this position.

Vertical strata, when they can be traced continuously upwards or downwards for some depth, are almost invariably seen to be parts of great curves, which may have a diameter of a few yards, or of several miles. I shall first describe two curves of considerable regularity, which occur in Forfars.h.i.+re, extending over a country twenty miles in breadth, from the foot of the Grampians to the sea near Arbroath.

The ma.s.s of strata here shown may be nearly 2000 feet in thickness, consisting of red and white sandstone, and various coloured shales, the beds being distinguishable into four princ.i.p.al groups, namely, No. 1. red marl or shale; No. 2. red sandstone, used for building; No. 3.

conglomerate; and No. 4. grey paving-stone, and tile-stone, with green and reddish shale, containing peculiar organic remains. A glance at the section will show that each of the formations 2, 3, 4, are repeated thrice at the surface, twice with a southerly, and once with a northerly inclination or _dip_, and the beds in No. 1., which are nearly horizontal, are still brought up twice by a slight curvature to the surface, once on each side of A. Beginning at the north-west extremity, the tile-stones and conglomerates No. 4. and No. 3. are vertical, and they generally form a ridge parallel to the southern skirts of the Grampians. The superior strata Nos. 2. and 1.

become less and less inclined on descending to the valley of Strathmore, where the strata, having a concave bend, are said by geologists to lie in a "trough" or "basin." Through the centre of this valley runs an imaginary line A, called technically a "synclinal line," where the beds, which are tilted in opposite directions, may be supposed to meet. It is most important for the observer to mark such lines, for he will perceive by the diagram, that in travelling from the north to the centre of the basin, he is always pa.s.sing from older to newer beds; whereas, after crossing the line A, and pursuing his course in the same southerly direction, he is continually leaving the newer, and advancing upon older strata. All the deposits which he had before examined begin then to recur in reversed order, until he arrives at the central axis of the Sidlaw hills, where the strata are seen to form an arch or _saddle_, having an _anticlinal_ line B, in the centre. On pa.s.sing this line, and continuing towards the S.E., the formations 4, 3, and 2, are again repeated, in the same relative order of superposition, but with a northerly dip. At Whiteness (see diagram) it will be seen that the inclined strata are covered by a newer deposit, _a_, in horizontal beds. These are composed of red conglomerate and sand, and are newer than any of the groups, 1, 2, 3, 4, before described, and rest _unconformably_ upon strata of the sandstone group, No. 2.

[Ill.u.s.tration: Fig. 62. Section of Forfars.h.i.+re, from N.W. to S.E., from foot of the Grampians to the sea at Arbroath (volcanic or trap rocks omitted). Length of section twenty miles.]

An example of curved strata, in which the bends or convolutions of the rock are sharper and far more numerous within an equal s.p.a.ce, has been well described by Sir James Hall.[48-A] It occurs near St. Abb's Head, on the east coast of Scotland, where the rocks consist princ.i.p.ally of a bluish slate, having frequently a ripple-marked surface. The undulations of the beds reach from the top to the bottom of cliffs from 200 to 300 feet in height, and there are sixteen distinct bendings in the course of about six miles, the curvatures being alternately concave and convex upwards.

[Ill.u.s.tration: Fig. 63. Curved strata of slate near St. Abb's Head, Berwicks.h.i.+re. (Sir J. Hall.)]

[Ill.u.s.tration: Fig. 64. Block section.]

An experiment was made by Sir James Hall, with a view of ill.u.s.trating the manner in which such strata, a.s.suming them to have been originally horizontal, may have been forced into their present position. A set of layers of clay were placed under a weight, and their opposite ends pressed towards each other with such force as to cause them to approach more nearly together. On the removal of the weight, the layers of clay were found to be curved and folded, so as to bear a miniature resemblance to the strata in the cliffs. We must, however, bear in mind, that in the natural section or sea-cliff we only see the foldings imperfectly, one part being invisible beneath the sea, and the other, or upper portion, being supposed to have been carried away by _denudation_, or that action of water which will be explained in the next chapter. The dark lines in the accompanying plan (fig. 64.) represent what is actually seen of the strata in part of the line of cliff alluded to; the fainter lines, that portion which is concealed beneath the sea level, as also that which is supposed to have once existed above the present surface.

[Ill.u.s.tration: Fig. 65. Experimental set-up.]

We may still more easily ill.u.s.trate the effects which a lateral thrust might produce on flexible strata, by placing several pieces of differently coloured cloths upon a table, and when they are spread out horizontally, cover them with a book. Then apply other books to each end, and force them towards each other. The folding of the cloths will exactly imitate those of the bent strata. (See fig. 65.)

Whether the a.n.a.logous flexures in stratified rocks have really been due to similar sideway movements is a question of considerable difficulty.

It will appear when the volcanic and granitic rocks are described, that some of them have, when melted, been injected forcibly into fissures, while others, already in a solid state, have been protruded upwards through the inc.u.mbent crust of the earth, by which a great displacement of flexible strata must have been caused.

But we also know 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 sometimes extending over a wide area. The frequent repet.i.tion, or continuance throughout long periods, of such downward movements seems to imply the formation and renewal of cavities at a certain depth below the surface, whether by the removal of matter by volcanos and hot springs, or by the contraction of argillaceous rocks by heat and pressure, or any other combination of circ.u.mstances. Whatever conjectures we may indulge respecting the causes, it is certain that pliable beds may, in consequence of unequal degrees of subsidence, become folded to any amount, and have all the appearance of having been compressed suddenly by a lateral thrust.

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."[50-A] 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.

[Ill.u.s.tration: Fig. 66. 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.]

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 fig. 66., 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 coal-mines, 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_, fig. 66.: 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 re-united 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 downwards 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.

Between the layers of shale, accompanying coal, we sometimes see the leaves of fossil ferns spread out as regularly as dried plants between sheets of paper in the herbarium of a botanist. These fern-leaves, or fronds, must have rested horizontally on soft mud, when first deposited.

If, therefore, they and the layers of shale are now inclined, or standing on end, it is obviously the effect of subsequent derangement.

The proof becomes, if possible, still more striking when these strata, including vegetable remains, are curved again and again, and even folded into the form of the letter Z, so that the same continuous layer of coal is cut through several times in the same perpendicular shaft. Thus, in the coal-field near Mons, in Belgium, these zigzag bendings are repeated four or five times, in the manner represented in fig. 67., the black lines representing seams of coal.[53-A]

[Ill.u.s.tration: Fig. 67. Zigzag flexures of coal near Mons.]

_Dip and Strike._--In the above remarks, several technical terms have been used, such as _dip_, the _unconformable position_ of strata, and the _anticlinal_ and _synclinal_ lines, which, as well as the _strike_ of the beds, I shall now explain. 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 annexed diagram (fig. 68.), 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.

[Ill.u.s.tration: Fig. 68. Diagram.]

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.

[Ill.u.s.tration: Fig. 69. Apparent horizontality of inclined strata.]

If the upper surface of a hard stony stratum be uncovered, whether artificially in a quarry, or by the 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 their strike, the dip being inwards 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 annexed drawing (fig. 69.), 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. 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.

[Ill.u.s.tration: Fig. 70. Explanatory sketch.]

It is rarely important to determine the angle of inclination with such minuteness as to require the aid of the instrument called a clinometer. We 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 fig. 70. It is thus easy to discover whether the lines of the inclined beds bisect the angle of 90, formed by the meeting of the hands, so as to give an angle of 45, or whether it would divide the s.p.a.ce into two equal or unequal portions. The upper dotted line may express a stratum dipping to the north; but should the beds dip precisely to the opposite point of the compa.s.s as in the lower dotted line, it will be seen that the amount of inclination may still be measured by the hands with equal facility.

[Ill.u.s.tration: Fig. 71. Section ill.u.s.trating the structure of the Swiss Jura.]

[Ill.u.s.tration: Fig. 72. Ground plan of the denuded ridge, fig. 71.]

[Ill.u.s.tration: Fig. 73. Transverse section.]

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 fig. 71., 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.[55-A] 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. 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 fig. 72., and the cross section of the same by fig. 73. The line D E, fig. 72., 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 each 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.

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 upwards (see fig. 74.), those formed by the newer beds appearing in a superior position, and extending highest up the valley, as A is seen above B.

[Ill.u.s.tration: Fig. 74. Slope of valley 40, dip of strata 20.]

Thirdly, if the dip of the beds be steeper than the slope of the valley, then the V's will point downwards (see fig. 75.), and those formed of the older beds will now appear uppermost, as B appears above A.

[Ill.u.s.tration: Fig. 75. Slope of valley 20, dip of strata 50.]

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 (fig. 76.), which exhibits strata rising at an angle of 20, and crossed by a valley, which declines in an opposite direction at 20.[57-A]

[Ill.u.s.tration: Fig. 76. Slope of valley 20, dip of strata 20, in opposite directions.]

These rules may often be of great practical utility; for the different degrees of dip occurring in the two cases represented in figures 74 and 75.

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 (fig. 74.), 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 fig. 75., 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.

In the majority of cases, an anticlinal axis forms a ridge, and a synclinal axis a valley, as in A, B, fig. 62. p. 48.; but there are exceptions to this rule, the beds sometimes sloping inwards from either side of a mountain, as in fig. 77.

[Ill.u.s.tration: Fig. 77. Cross section.]