The Student's Elements of Geology 8

The Student's Elements of Geology 8

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 masses 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 spaces a, b, while the

adjoining dark portions are parts of the original coal seam left as props, beds

of sandy clay or shale constituting 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 spaces.

 

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 incumbent rocks. In this case the yielding mass, 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

compass 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 illustrated 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 compass 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 shore 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 space 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 illustrating the structure of the Swiss Jura.)

 

It has been already seen, in describing the curved strata on the east coast of

Scotland, in Forfarshire and Berwickshire, 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 Jurassiques 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 BASSET.

 

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 compass, constituting what is termed a qua-

quaversal dip-- that is, turning every way.

 

There are endless variations in the figures described by the basset-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 pass 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 illustrate FAULTS.)

 

SYNCLINAL STRATA FORMING RIDGES.

 

(FIGURE 69. Section of carboniferous rocks of Lancashire. (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.)

 

(FIGURE 73. Folded strata.)

 

We have already explained, Figure 69, that stratified rocks have usually their

strata bent into parallel folds forming anticlinal and synclinal axes, a group

of several of these folds having often been subjected to a common movement, and

having acquired a uniform strike or direction. In some disturbed regions these

folds have been doubled back upon themselves in such a manner that it is often

difficult for an experienced geologist to determine correctly the relative age

of the beds by superposition. Thus, if we meet with the strata seen in the

section, Figure 72, we should naturally suppose that there were twelve distinct

beds, or sets of beds, No. 1 being the newest, and No. 12 the oldest of the

series. But this section may perhaps exhibit merely six beds, which have been

folded in the manner seen in Figure 73, so that each of them is twice repeated,

the position of one half being reversed, and part of No. 1, originally the

uppermost, having now become the lowest of the series.

 

These phenomena are observable on a magnificent scale in certain regions in

Switzerland, in precipices often more than 2000 feet in perpendicular height,

and there are flexures not inferior in dimensions in the Pyrenees. The upper

part of the curves seen in this diagram, Figure 73, and expressed in fainter

lines, has been removed by what is called denudation, to be afterwards

explained.

 

FRACTURES OF THE STRATA AND FAULTS.

 

Numerous rents may often be seen in rocks which appear to have been simply

broken, the fractured parts still remaining in contact; but we often find a

fissure, several inches or yards wide, intervening between the disunited

portions. These fissures are usually filled with fine earth and sand, or with

angular fragments of stone, evidently derived from the fracture of the

contiguous rocks.

 

The face of each wall of the fissure is often beautifully polished, as if

glazed, striated, or scored with parallel furrows and ridges, such as would be

produced by the continued rubbing together of surfaces of unequal hardness.

These polished surfaces are called by miners "slickensides." It is supposed that

the lines of the striae indicate the direction in which the rocks were moved.

During one of the minor earthquakes in Chili, in 1840, the brick walls of a

building were rent vertically in several places, and made to vibrate for several

minutes during each shock, after which they remained uninjured, and without any

opening, although the line of each crack was still visible. When all movement

had ceased, there were seen on the floor of the house, at the bottom of each

rent, small heaps of fine brick-dust, evidently produced by trituration.

 

(FIGURE 74. Faults. A B perpendicular, C D oblique to the horizon.)

 

(FIGURE 75. E F, fault or fissure filled with rubbish, on each side of which the

shifted strata are not parallel.)

 

It is not uncommon to find the mass of rock on one side of a fissure thrown up

above or down below the mass with which it was once in contact on the other

side. "This mode of displacement is called a fault, shift, slip, or throw." "The

miner," says Playfair, describing a fault, "is often perplexed, in his

subterranean journey, by a derangement in the strata, which changes at once all

those lines and bearings which had hitherto directed his course. When his mine

reaches a certain plane, which is sometimes perpendicular, as in A B, Figure 74,

sometimes oblique to the horizon (as in C D, ibid.), he finds the beds of rock

broken asunder, those on the one side of the plane having changed their place,

by sliding in a particular direction along the face of the others. In this

motion they have sometimes preserved their parallelism, as in Figure 74, so that

the strata on each side of faults A B, C D, continue parallel to one another; in

other cases, the strata on each side are inclined, as in a, b, c, d (Figure 75),

though their identity is still to be recognised by their possessing the same

thickness and the same internal characters." (Playfair, Illustration of Hutt.

Theory paragraph 42.)

 

In Coalbrook Dale, says Mr. Prestwich (Geological Transactions second series

volume 5 page 452.), deposits of sandstone, shale, and coal, several thousand

feet thick, and occupying an area of many miles, have been shivered into

fragments, and the broken remnants have been placed in very discordant

positions, often at levels differing several hundred feet from each other. The

sides of the faults, when perpendicular, are commonly several yards apart, and

are sometimes as much as 50 yards asunder, the interval being filled with broken

debris of the strata. In following the course of the same fault it is sometimes

found to produce in different places very unequal changes of level, the amount

of shift being in one place 300, and in another 700 feet, which arises from the

union of two or more faults. In other words, the disjointed strata have in

certain districts been subjected to renewed movements, which they have not

suffered elsewhere.

 

We may occasionally see exact counterparts of these slips, on a small scale, in

pits of loose sand and gravel, many of which have doubtless been caused by the

drying and shrinking of argillaceous and other beds, slight subsidences having

taken place from failure of support. Sometimes, however, even these small slips

may have been produced during earthquakes; for land has been moved, and its

level, relatively to the sea, considerably altered, within the period when much

of the alluvial sand and gravel now covering the surface of continents was

deposited.

 

I have already stated that a geologist must be on his guard, in a region of

disturbed strata, against inferring repeated alternations of rocks, when, in

fact, the same strata, once continuous, have been bent round so as to recur in

the same section, and with the same dip. A similar mistake has often been

occasioned by a series of faults.

 

(FIGURE 76. Apparent alternations of strata caused by vertical faults.)

 

If, for example, the dark line A H (Figure 76) represent the surface of a

country on which the strata a, b, c frequently crop out, an observer who is

proceeding from H to A might at first imagine that at every step he was

approaching new strata, whereas the repetition of the same beds has been caused

by vertical faults, or downthrows. Thus, suppose the original mass, A, B, C, D,

to have been a set of uniformly inclined strata, and that the different masses

under E F, F G, and G D sank down successively, so as to leave vacant the spaces

marked in the diagram by dotted lines, and to occupy those marked by the

continuous lines, then let denudation take place along the line A H, so that the

protruding masses indicated by the fainter lines are swept away-- a miner, who

has not discovered the faults, finding the mass a, which we will suppose to be a

bed of coal four times repeated, might hope to find four beds, workable to an

indefinite depth, but first, on arriving at the fault G, he is stopped suddenly

in his workings, for he comes partly upon the shale b, and partly on the

sandstone c; the same result awaits him at the fault F, and on reaching E he is again stopped by a wall composed of the rock d.