Page images
PDF
EPUB

A

along the axis of that anticline finds himself getting into progressively higher strata, as the fold sinks down. On the other hand, in advancing southwards along the synclinal axis, he loses stratum after stratum and gets into lower portions of the series. When a fold diminishes in this way it is said to "nose out." In fig. 21 there is obviously a general. inclination of the beds towards the north, besides the outward dip from the anticline and the inward dip from the syncline. Hence the anticline noses out to the north and the syncline to the south..

It occasionally happens that the maximum movement either of upheaval or subsidence has taken place not along line of axis but at some one point. Hence arise, on the one hand, dome-shaped elevations of strata where the dip is outward from a centre (quaquaversal), round which the beds are disposed in successive parallel layers or rings, and, on the other hand, circular basin-shaped depressions, towards the centre of which there is a general inclination of the

rocks.

So great has been the compression to which rocks have been subjected during the process of curvature that the folds may often be found inverted. This has taken place

in

be se

strik

M

23-Section of inclined axes, showing consequent inversion of strata.

abandantly in regions of great plication. The Silurian uplands of the south of Scotland, for instance, have the arches and troughs tilted in one direction for miles together, so that half of each of them the strata lie bottom upwards. It is in large mountain-chains, however, that inversion can en on the grandest scale. The Alps furnish numerous ng illustrations. On the north side of that chain the Tertiary rocks have been so completely turned over any miles that the lowest beds now form the tops of ills, while the highest lie deep below them. Individual mountains, such as the Glärnisch, present stupendous examples of inversion, great groups of strata being folded Over and over above each other as we might fold carpets.

older

for

the

superior grandeur may be observed among the more precipit ous valleys of the Swiss Alps. No more impressive testimony could be given to the potency of the force by which mountains were upheaved.

V. DISLOCATIONS OF ROCKS.

The movements which the crust of the earth has undergone have not only folded and corrugated the rocks, but have fractured them in all directions. These dislocations may be either simple fissures, that is, rents without any vertical displacement of the mass on either side, or faults, that is, rents where one side has been pushed up or has sunk down. It is not always possible in a shattered rock to discriminate between joints and true fissures. The joints indeed have sometimes served as lines along which fissuring has taken place. It is common to meet with traces of friction along the walls of fissures even when no proof of actual vertical displacement can be gleaned. The rock is more or less shattered on either side, and the contiguous faces present numerous slickensided surfaces. Mineral deposits may also commonly be observed encrusting the cheeks of a fissure, or filling up, together with broken fragments of rock, the space between the two walls.

In a large proportion of cases, however, there has been displacement as well as fracture, and the rents have become faults as well as fissures, Faults on a small scale are sometimes sharply-defined lines. as if the rocks had been

[graphic][merged small][merged small]
[graphic]
[graphic]

Nis 192

der the

Fio. .4.-Curved and contorted rocks, near Old Head of Kinsale. (Du Noyer.) Where curvature has been carried so far, we may nearly always discover localities at which it has been so intensified that the strata, have been corrugated and crumpled till it becomes almost impossible to follow out any particular bed through the disturbance. On a small scale instances of such extreme contortion may now and then be found at landslips, where fissile shales have been pressed forward by advancing heavy masses of more solid rock. But it is of course among against the fault, while those on the opposite side are bent the more are pure nives it best illustrations. Few travellers who have considerably broken, jumbled, and crumpled, so that the passed the upper end of the Lake of Lucerne can have failed line of fracture is marked by a belt or wall-like mass of the remarkable cliffs of contorted rocks near fragmentary rock. Where 8 dislocation has occurred But innumerable examples of equal or even through materials of very unequal hardness, such as solid

to notic

Flueles

FIG. 26.-Section of strata, bent at a line of fault.

[blocks in formation]

FIG. 27.-Section of fault, showing the alternate expansions and contractions due to the shifting of one side of a sinuous fissure.

of a fault may vary from a mere chink into which the point of a knife could hardly be inserted up to a band of broken rock many yards wide. But in these latter cases we may usually suspect that so great a breadth of fractured materials has been produced not by a single fault but by a series of closely adjoining and parallel faults.

Faults are sometimes vertical, but are generally inclined. The largest faults, that is. those which have the greatest

C

B

FIG. 28.-Section of a vertical and Inclined fault.

vertical displacement, slope at high angles, while those of only a few feet or yards may be inclined as low as 18° or 20°. The inclination of a fault from the vertical is called its hade. In fig. 28, for example, the fault between A and C being vertical has no hade, but that between C and B hades at an angle of 70° from the vertical to the right hand. The amount of displacement is represented as the same in both instances, so that the level of the bed a is raised between the two faults at C above the uniform horizon which it retains beyond them.

That faults are vertical displacements of parts of the earth's crust is most clearly shown when they traverse stratified rocks, for the regular lines of bedding and the originally flat position of these rocks afford a measure of the disturbance. Accordingly we may consider here the effects of faults as they traverse (1) horizontal, (2) inclined, or (3) undulating strata.

FIG. 29.-Measurement of the throw of a fault.

for the fault is vertical; consequently there is no lateral displacement. In fig. 29, however, where the fault hades considerably, there is a lateral shift of the bed, the end a being 150 yards to the left of b. In this example the lateral shift is half as much again as the vertical. It is obvious that a fault of this kind must seriously affect the value of a coal-field; for while the coal-seam might be worked up to a on the one side and to b on the other, there would be a space of 150 yards of barren ground between these two points where the seam never could be found. The lower the angle of. hade the greater the breadth of such barren ground. Hence the more nearly vertical the lines of fault, the better for the coal-fields.

In the vast majority of cases faults hade in the direction of downthrow, in other words, they slope away from the side which has risen. Consequently the mere inspection of a fault in any natural or artificial section suffices in most cases to show which side has been elevated. In mining operations the knowledge of this rule is invaluable, for it decides whether a coal seam, dislocated by a fault, is to be sought for by going up or down. In fig. 29, for example, a miner working from the right and meeting with the fault al b, would know from its hading towards him that he must ascend to find the coal. On the other hand were he to work from the left and catch the fault at a, he would see that it would be necessary to descend. According to this rule a normal fault never brings one part of a bed below another part, so as to be capable of being pierced twice by the same vertical shaft. Exceptional cases, however, where the hade is reversed, do occasionally appear. In fig. 30 a series of strata, 1 to 11, are represented as folded in an inverted anticline, and broken through by a fault along the axis. the portion on the right side having been pushed up.

[graphic]

1. In the above section (fig. 28) two faults are supposed to traverse a set of horizontal strata, and to displace them in opposite directions. Hence the portion between them appears as if it had been pushed up, or as if the part on either side had slipped down. The amount of vertical displacement is measured, from the end of any given stratum, say a, on one side of the fault, to its corresponding end on the other side. Suppose, for example, that the black band in fig. 29 represents a known stratum such as a seam of coal, which, having been explored in underground operations, is known to be cut by a fault at a depth of a hundred yards below the surface at A, and to lie 200 yards deep on the other side of the fault below B. The amount of displacement is the vertical distance between the two severed ends a and b. This is termed the throw of a fault. From these two sections (figs. 28 and 29) we see that the horizontal distance to which the two ends of a fauited stratum may be separated does not

[ocr errors]

Fio. 80.-Inverted anticline and reversed fault.

The effect of the movement has been to make the ends of the beds on that side overlie higher beds on the other side. A shaft would thus pierce the same stratum twice. Instances of reversed faults are chiefly met with in much dis turbed districts, such as mountain chains, where the rocks have been affected by great undulations and corrugations. But instances on a small scale, like that in fig. 31, may now and then be encountered even in lowland districts, where no great disturbance has taken place.

2. Faults traversing inclined strata usually group

them

selves into two series, one running in the same general tion would continue with every increase of inclination in direction as the dip of the strata, the other approximating the strata till among vertical beds there would be no heave at all.

[graphic][merged small][merged small][merged small][merged small][merged small]

to the trend of the strike. They are accordingly classified as dip-faults and strike-faults. They are not always to be sharply marked off from each other, for the dip-faults will often be observed to deviate considerably from the normal direction of dip, and the strike-faults from the prevalent strike, so that in such cases they pass into each other.

A dip-fault produces at the surface the effect of a lateral shift of the strata. This effect increases in proportion as the angle of dip lessens. It ceases altogether when the beds are vertical. Fig. 32 may be taken as a plan of a dip-fault

FIG. 33.-Section along the line of a fault in strata dipping at 25°. Strike-faults, where they exactly coincide with the strike, may sometimes remove the outcrop of some strata by never

[blocks in formation]
[graphic][merged small][merged small][ocr errors][merged small][merged small][merged small]

A

traversing a series of strata which dip northwards at 25°. The beds on the east side look as if they had been pushed horizontally southwards. That this apparent horizontal displacement is due really to a vertical movement, and to the subsequent planing down of the surface by denuding agents, will be clear if we consider what must be the effect of the vertical ascent or descent of the inclined beds on one side of a dislocation. Take the bed a in fig. 32, and suppose it to be still unbroken by the fault. It will then run in a straight east and west line. When the fault takes place, the part on the west side is pushed up, or, what comes to the same, that on the east side is let down. horizontal plane cutting the dislocated stratum will show the portion on the west side lying to the north of that on the east side of the fracture. The effect of denudation has usually been practically to produce such a plane, and thus to exhibit an apparently lateral shift. This surface displacement has been termed the heave of a fault. Its dependence upon the angle of dip of the strata may be seen by a comparison of figs. 33 and 34. In the former figure the bed a, once prolonged above the present surface (marked by the horizontal line), is represented as having dropped from db to ec, the angle of inclination being 25°. The heave amounts to the horizontal distance between b and e. But if the angle should rise to 60°, as in fig. 34, though the amount of throw or vertical displacement remains the same, we see that the heave or horizontal shift diminishes to about a quarter of what it is in fig. 33. This diminu

[ocr errors]
[merged small][merged small][merged small][graphic][merged small][merged small]

course have the effect of bringing the dislocated ends of the beds against the line of dislocation. In fig. 37, for in

[blocks in formation]
[merged small][ocr errors][merged small][ocr errors][ocr errors]
[merged small][merged small][merged small][merged small][merged small][ocr errors]

FIG. 37.-Plan of strata traversed by a diminishing strike-fault. stance, which represents in plan another strike fault, we see that the amount of throw is diminishing towards the left so as to allow lower beds to successively appear, until, at the extreme left side of the ground, the fault merely brings one part of the same bed (No. 5) against another part.

3. Their effects become more complicated where faults traverse undulating and contorted strata. Sometimes we can distinctly trace an undulation as the result of a fault. In the flat limestone beds shown in fig. 38, for example,

[merged small][ocr errors][merged small]

S

[merged small][merged small][merged small][ocr errors]

FIG. 40.-An anticline (A) and syncline (S), dislocated by a fank.

observed to be the case here. Beginning at the upper side of the diagram, which may be called north, we notice that the bed aa, dipping towards the lower side or south at 60°, is truncated by the fault at u, and that the portion on the upthrow side is shifted forwards or southward. Crossing the syncline we meet with the same bed, and as the upthrow of the fault still continues on the same side we must go some way southwards on the downthrow side before we meet with its continuation, On the southern slope of the anticline the same bed once more appears, and again is

[graphic]

FIG. 38.-Curving of strata on one side of a fault.

there can be no doubt that the gentle depression from & to e would not have taken place but for the existence of the fault ab. But in all countries where the rocks have been thrown into odds and corrugations these structures are traversed by faults. It then often happens that the same fault appears to be alternately a downthrow on opposite sides. Let us suppose a series of gently rolling strata to be cut by a transverse fault as in the diagram in fig. 39.

60°

u

S

FIG. 41.-Section along the upcast side of the fault in fig. 40.

shifted forwards as before. upcast side (uu) of the fault would give the structure reA section along the left or presented in fig. 41; while one along the downcast side

[graphic]

d

S

FIG. 39. Diagram of gently undulating strata cut by a faut, with alternate throw in opposite directions.

At each of the two ridges on the near side of the fault the effect is an upthrow, while in the intervening valley it is a downthrow. On the opposite side of the fault each of these effects is reversed. It rarely happens, however, that a fault makes any such visible crack at the surface. rocks have all been worn down so much that it is usually only by careful examination of their dip that the existence of faults can be determined.

The

The influence of faults upon curvatures may be illustrated by a plan and sections of a dislocated anticline and syncline, which will also show clearly how the apparently lateral displacement of outcrop produced by dip-faults is due to vertical movement. Fig. 40 represents a plan of strata thrown into an anticlinal fold AA and a synclinal fold SS, and traversed by a fault FF, which is an upthrow to the

[blocks in formation]

folds narrow the anticlines but widen the synclines on the downthrow side, while they widen the anticlines and narrow the synclines on the upthrow side.

Dislocation may take place either by a single fault or as the combined effects of two or more. Where there is only one fault, as in fig. 43, one of its sides may be pushed up or let down, or there may be a simultaneous opposite movement on either side. In such cases, there must be a gradual dying out of the dislocation towards either end; and there will usuálly be one or more points where the displacement has reached a maximum. Sometimes, as shown in fig. 44, a fault with a considerable maximum throw (35 feet, yards, or fathoms, in the drawing) splits into minor faults at the terminations. Examples of this kind occur not infrequently in coal-work

ing ground is displaced. The maximum displacement in such an instance would be sought for towards b; in the direction e there would be no displacement at all.

It often happens that, by a succession of parallel and adjoining faults, a series of strata is so dislocated that a given stratum which may be near the surface on one side is carried down by a series of steps to some distance below. Excellent examples of these step-faults (fig. 47) are to be

[blocks in formation]
[blocks in formation]

faults, point downwards. In the accompanying section (fig. 49) of a portion of the thick coal of South Staffordshire, drawn to scale by Mr Johnson of Dudley (Records of Geol. Survey, vol. i. part 2, p. 313), the commencement of a trough-fault is shown in the centre of the figure.

The late Mr Jukes carefully described this interesting section, and showed that the coal must once have been more arched than now, and that on the cessation of the elevatory process the fractured pieces adjusted themselves to their new position by means of dislocations. The mass of higher beds (A) driven as a wedge into the coal, has hindered the bed from regaining its horizontality, and at the same time has caused the adjacent parts of the coal (BB) to be so crushed by the enormous pressure as to have been reduced to "a paste of coal dust and very small coal" (Memoir on South Staffordshire Coal-field, 2d ed., p. 194).

It will be observed that the hade of the faults is towards

FIG. 49.-Section of a faulted part of the thick coal of South Staffordshire.

the downthrow side, and that the wedged-shaped masses with broad bottoms have risen, while those with narrow bottoms and broad tops have sunk.

It has been already (ante, p. 261) pointed out that faults are traceable to the effects of elevation. The general hade or inclination of faults towards the side of downthrow was edition of the present work, atisfactorily explained by the late Mr Jukes in the last

[ocr errors]

Suppose," he says, "that in diagram fig. 50 we have a portion of the earth's crust, of which AB is the surface, and CD a plane acted on by some widespread force of expansion tending to bulge line EF, it is obvious that the expanding force will, on the side of upwards the part ABCD. If then a fracture takes place along the AC, have the widest base CF to act upon, while it will have a proportionately less mass to move in the part AECF, which grows gradually smaller towards the surface, than on the other side of the fault, where, with the smaller base FD, the mass FDBE continually grows larger towards the surface. The mass G will consequently be

« EelmineJätka »