Part 12 (2/2)

[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.]

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