ELEVATION OF STRATA ABOVE THE SEA--HORIZONTAL AND INCLINED STRATIFICATION.
Why the position of marine strata, above the level of the sea, should
be referred to the rising up of the land, not to the going down of the
sea--Upheaval of extensive masses of horizontal strata--Inclined and
vertical stratification--Anticlinal and synclinal lines--Bent strata
in east of Scotland--Theory of folding by lateral movement--Creeps--Dip
and strike--Structure of the Jura--Various forms of outcrop--Rocks
broken by flexure--Inverted position of disturbed strata--Unconformable
stratification--Hutton and Playfair on the same--Fractures of
strata--Polished surfaces--Faults--Appearance of repeated alternations
produced by them--Origin of great faults.
_Land has been raised, not the sea lowered._--It has been already stated
that the aqueous rocks containing marine fossils extend over wide
continental tracts, and are seen in mountain chains rising to great heights
above the level of the sea. Hence it follows, that what is now dry land was
once under water. But if we admit this conclusion, we must imagine, either
that there has been a general lowering of the waters of the ocean, or that
the solid rocks, once covered by water, have been raised up bodily out of
the sea, and have thus become dry land. The earlier geologists, finding
themselves reduced to this alternative, embraced the former opinion,
assuming that the ocean was originally universal, and had gradually sunk
down to its actual level, so that the present islands and continents were
left dry. It seemed to them far easier to conceive that the water had gone
down, than that solid land had risen upwards into its present position. It
was, however, impossible to invent any satisfactory hypothesis to explain
the disappearance of so enormous a body of water throughout the globe, it
being necessary to infer that the ocean had once stood at whatever height
marine shells might be detected. It moreover appeared clear, as the science
of Geology advanced, that certain spaces on the globe had been alternately
sea, then land, then estuary, then sea again, and, lastly, once more
habitable land, having remained in each of these states for considerable
periods. In order to account for such phenomena, without admitting any
movement of the land itself, we are required to imagine several retreats
and returns of the ocean; and even then our theory applies merely to cases
where the marine strata composing the dry land are horizontal, leaving
unexplained those more common instances where strata are inclined, curved,
or placed on their edges, and evidently not in the position in which they
were first deposited.
Geologists, therefore, were at last compelled to have recourse to the other
alternative, namely, the doctrine that the solid land has been repeatedly
moved upwards or downwards, so as permanently to change its position
relatively to the sea. There are several distinct grounds for preferring
this conclusion. First, it will account equally for the position of those
elevated masses of marine origin in which the stratification remains
horizontal, and for those in which the strata are disturbed, broken,
inclined, or vertical. Secondly, it is consistent with human experience
that land should rise gradually in some places and be depressed in others.
Such changes have actually occurred in our own days, and are now in
progress, having been accompanied in some cases by violent convulsions,
while in others they have proceeded so insensibly, as to have been
ascertainable only by the most careful scientific observations, made at
considerable intervals of time. On the other hand, there is no evidence
from human experience of a lowering of the sea's level in any region, and
the ocean cannot sink in one place without its level being depressed all
over the globe.
These preliminary remarks will prepare the reader to understand the great
theoretical interest attached to all facts connected with the position of
strata, whether horizontal or inclined, curved or vertical.
Now the first and most simple appearance is where strata of marine origin
occur above the level of the sea in horizontal position. Such are the
strata which we meet with in the south of Sicily, filled with shells for
the most part of the same species as those now living in the Mediterranean.
Some of these rocks rise to the height of more than 2000 feet above the
sea. Other mountain masses might be mentioned, composed of horizontal
strata of high antiquity, which contain fossil remains of animals wholly
dissimilar from any now known to exist. In the south of Sweden, for
example, near Lake Wener, the beds of one of the oldest of the
fossiliferous deposits, namely that formerly called Transition, and now
Silurian, by geologists, occur in as level a position as if they had
recently formed part of the delta of a great river, and been left dry on
the retiring of the annual floods. Aqueous rocks of about the same age
extend for hundreds of miles over the lake-district of North America, and
exhibit in like manner a stratification nearly undisturbed. The Table
Mountain at the Cape of Good Hope is another example of highly elevated yet
perfectly horizontal strata, no less than 3500 feet in thickness, and
consisting of sandstone of very ancient date.
Instead of imagining that such fossiliferous rocks were always at their
present level, and that the sea was once high enough to cover them, we
suppose them to have constituted the ancient bed of the ocean, and that
they were gradually uplifted to their present height. This idea, however
startling it may at first appear, is quite in accordance, as before stated,
with the analogy of changes now going on in certain regions of the globe.
Thus, in parts of Sweden, and the shores and islands of the Gulf of
Bothnia, proofs have been obtained that the land is experiencing, and has
experienced for centuries, a slow upheaving movement. Playfair argued in
favour of this opinion in 1802; and in 1807, Von Buch, after his travels in
Scandinavia, announced his conviction that a rising of the land was in
progress. Celsius and other Swedish writers had, a century before, declared
their belief that a gradual change had, for ages, been taking place in the
relative level of land and sea. They attributed the change to a fall of the
waters both of the ocean and the Baltic. This theory, however, has now been
refuted by abundant evidence; for the alteration of relative level has
neither been universal nor every where uniform in quantity, but has
amounted, in some regions, to several feet in a century, in others to a few
inches; while in the southernmost part of Sweden, or the province of
Scania, there has been actually a loss instead of a gain of land, buildings
having gradually sunk below the level of the sea.[46-A]
It appears, from the observations of Mr. Darwin and others, that very
extensive regions of the continent of South America have been undergoing
slow and gradual upheaval, by which the level plains of Patagonia, covered
with recent marine shells, and the Pampas of Buenos Ayres, have been raised
above the level of the sea.[46-B] On the other hand, the gradual sinking of
the west coast of Greenland, for the space of more than 600 miles from
north to south, during the last four centuries, has been established by the
observations of a Danish naturalist, Dr. Pingel. And while these proofs of
continental elevation and subsidence, by slow and insensible movements,
have been recently brought to light, the evidence has been daily
strengthened of continued changes of level effected by violent convulsions
in countries where earthquakes are frequent. There the rocks are rent from
time to time, and heaved up or thrown down several feet at once, and
disturbed in such a manner, that the original position of strata may, in
the course of centuries, be modified to any amount.
It has also been shown by Mr. Darwin, that, in those seas where circular
coral islands and barrier reefs abound, there is a slow and continued
sinking of the submarine mountains on which the masses of coral are based;
while there are other areas of the South Sea, where the land is on the
rise, and where coral has been upheaved far above the sea-level.
It would require a volume to explain to the reader the various facts which
establish the reality of these movements of land, whether of elevation or
depression, whether accompanied by earthquakes or accomplished slowly and
without local disturbance. Having treated fully of these subjects in the
Principles of Geology[46-C], I shall assume, in the present work, that such
changes are part of the actual course of nature; and when admitted, they
will be found to afford a key to the interpretation of a variety of
geological appearances, such as the elevation of horizontal, inclined, or
disturbed marine strata, and the superposition of freshwater to marine
deposits, afterwards to be described. It will also appear, in the sequel,
how much light the doctrine of a continued subsidence of land may throw on
the manner in which a series of strata, formed in shallow water, may have
accumulated to a great thickness. The excavation of valleys also, and other
effects of _denudation_, of which I shall presently treat, can alone be
understood when we duly appreciate the proofs, now on record, of the
prolonged rising and sinking of land, throughout wide areas.
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.
[Illustration: 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 shingle
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
Forfarshire, extending over a country twenty miles in breadth, from the
foot of the Grampians to the sea near Arbroath.
The mass 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 principal 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 passing 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 passing 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.
[Illustration: Fig. 62. Section of Forfarshire, 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 space, 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 principally 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.
[Illustration: Fig. 63. Curved strata of slate near St. Abb's Head,
Berwickshire. (Sir J. Hall.)]
[Illustration: Fig. 64. Block section.]
An experiment was made by Sir James Hall, with a view of illustrating the
manner in which such strata, assuming 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.
[Illustration: Fig. 65. Experimental set-up.]
We may still more easily illustrate 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 analogous 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 incumbent 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 repetition, 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
circumstances. 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
illustration of this fact.--First, it may be stated generally, that the
excavation of coal at a considerable depth causes the mass of overlying
strata to sink down bodily, even when props are left to support the roof of
the mine. "In Yorkshire," 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 incumbent mass 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
accumulates on the surface, or a railway traverses a coal-field.
[Illustration: 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 masses 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 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 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]
[Illustration: 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
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 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 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.
[Illustration: 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.
[Illustration: 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 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 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
shore to be at an angle of 40 deg.. 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.
[Illustration: 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 deg.,
formed by the meeting of the hands, so as to give an angle of 45 deg., or
whether it would divide the space 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 compass 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.
[Illustration: Fig. 71. Section illustrating the structure of
the Swiss Jura.]
[Illustration: Fig. 72. Ground plan of the denuded ridge, fig. 71.]
[Illustration: Fig. 73. Transverse section.]
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 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 _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 each 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.
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.
[Illustration: Fig. 74. Slope of valley 40 deg., dip of strata 20 deg..]
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.
[Illustration: Fig. 75. Slope of valley 20 deg., dip of strata 50 deg..]
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 deg., and crossed by a valley, which declines in an opposite
direction at 20 deg..[57-A]
[Illustration: Fig. 76. Slope of valley 20 deg., dip of strata 20 deg.,
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 pass 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.
[Illustration: Fig. 77. Cross section.]
On following one of the anticlinal ridges of the Jura, before mentioned, A,
B, C, fig. 71., we often discover longitudinal cracks and sometimes large
fissures along the line where the flexure was greatest. Some of these, as
above stated, have been enlarged by denudation into valleys of considerable
width, as at C, fig. 71., which follow the line of strike, and which we may
suppose to have been hollowed out at the time when these rocks were still
beneath the level of the sea, or perhaps at the period of their gradual
emergence from beneath the waters. The existence of such cracks at the
point of the sharpest bending of solid strata of limestone is precisely
what we should have expected; but the occasional want of all similar signs
of fracture, even where the strain has been greatest, as at _a_, fig. 71.,
is not always easy to explain. 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 (p. 35.), and in part to the permeation of sea-water while
they were yet submerged.
[Illustration: Fig. 78. 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_, fig. 78., 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 calcedony and quartz.
[Illustration: Fig. 79. Cross section.
_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_, fig.
79.), while the continuity of the more pliable and ductile marls, _m m_,
has not been interrupted.
[Illustration: Fig. 80. Cross section.]
I shall conclude my remarks on bent strata by stating, that, in mountainous
regions like the Alps, it is often difficult for an experienced geologist
to determine correctly the relative age of beds by superposition, so often
have the strata been folded back upon themselves, the upper parts of the
curve having been removed by denudation. Thus, if we met with the strata
seen in the section fig. 80., 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 fig. 81., 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 often observable on a magnificent scale in
certain regions in Switzerland in precipices from 2000 to 3000 feet in
perpendicular height. In the Iselten Alp, in the valley of the Lutschine,
between Unterseen and Grindelwald, curves of calcareous shale are seen from
1000 to 1500 feet in height, in which the beds sometimes plunge down
vertically for a depth of 1000 feet and more, before they bend round again.
There are many flexures not inferior in dimensions in the Pyrenees, as
those near Gavarnie, at the base of Mont Perdu.
[Illustration: Fig. 81. Cross section.]
[Illustration: Fig. 82. Curved strata of the Iselten Alp.]
[Illustration: Fig. 83. Unconformable junction of old red sandstone and
Silurian schist at the Siccar Point, near St. Abb's Head, Berwickshire. See
also Frontispiece.]
_Unconformable stratification._--Strata are said to be unconformable,
when one series is so placed over another, that the planes of the
superior repose on the edges of the inferior (see fig. 83.). In this
case it is evident that a period had elapsed between the production of
the two sets of strata, and that, during this interval, the older
series had been tilted and disturbed. Afterwards the upper series was
thrown down in horizontal strata upon it. If these superior beds, as
_d_, _d_, fig. 83., are also inclined, it is plain that the lower
strata, _a_, _a_, have been twice displaced; first, before the
deposition of the newer beds, _d_, _d_, and a second time when these
same strata were thrown out of the horizontal position.
Playfair has remarked[60-A] that this kind of junction which we now call
unconformable had been described before the time of Hutton, but that he was
the first geologist who appreciated its importance, as illustrating the
high antiquity and great revolutions of the globe. He had observed that
where such contacts occur, the lowest beds of the newer series very
generally consist of a breccia or conglomerate consisting of angular and
rounded fragments, derived from the breaking up of the more ancient rocks.
On one occasion the Scotch geologist took his two distinguished pupils,
Playfair and Sir James Hall, to the cliffs on the east coast of Scotland,
near the village of Eyemouth, not far from St. Abb's Head, where the
schists of the Lammermuir range are undermined and dissected by the sea.
Here the curved and vertical strata, now known to be of Silurian age, and
which often exhibit a ripple-marked surface[60-B], are well exposed at the
headland called the Siccar Point, penetrating with their edges into the
incumbent beds of slightly inclined sandstone, in which large pieces of the
schist, some round and others angular, are united by an arenaceous cement.
"What clearer evidence," exclaims Playfair, "could we have had of the
different formation of these rocks, and of the long interval which
separated their formation, had we actually seen them emerging from the
bosom of the deep? We felt ourselves necessarily carried back to the time
when the schistus on which we stood was yet at the bottom of the sea, and
when the sandstone before us was only beginning to be deposited in the
shape of sand or mud, from the waters of a superincumbent ocean. An epoch
still more remote presented itself, when even the most ancient of these
rocks, instead of standing upright in vertical beds, lay in horizontal
planes at the bottom of the sea, and was not yet disturbed by that
immeasurable force which has burst asunder the solid pavement of the globe.
Revolutions still more remote appeared in the distance of this
extraordinary perspective. The mind seemed to grow giddy by looking so far
into the abyss of time; and while we listened with earnestness and
admiration to the philosopher who was now unfolding to us the order and
series of these wonderful events, we became sensible how much farther
reason may sometimes go than imagination can venture to follow."[60-C]
In the frontispiece of this volume the reader will see a view of this
classical spot, reduced from a large picture, faithfully sketched and
coloured from nature by the youngest son of the late Sir James Hall. It was
impossible, however, to do justice to the original sketch, in an
engraving, as the contrast of the red sandstone and the light fawn-coloured
vertical schists could not be expressed. From the point of view here
selected, the underlying beds of the perpendicular schist, _a_, are visible
at _b_ through a small opening in the fractured beds of the covering of red
sandstone, _d d_, while on the vertical face of the old schist at _a' a"_ a
conspicuous ripple-mark is displayed.
[Illustration: Fig. 84. Junction of unconformable strata near
Mons, in Belgium.]
It often happens that in the interval between the deposition of two sets of
unconformable strata, the inferior rock has not only been denuded, but
drilled by perforating shells. Thus, for example, at Autreppe and Gusigny,
near Mons, beds of an ancient (paleozoic) limestone, highly inclined, and
often bent, are covered with horizontal strata of greenish and whitish
marls of the Cretaceous formation. The lowest and therefore the oldest bed
of the horizontal series is usually the sand and conglomerate, _a_, in
which are rounded fragments of stone, from an inch to two feet in diameter.
These fragments have often adhering shells attached to them, and have been
bored by perforating mollusca. The solid surface of the inferior limestone
has also been bored, so as to exhibit cylindrical and pear-shaped cavities,
as at _c_, the work of saxicavous mollusca; and many rents, as at _b_,
which descend several feet or yards into the limestone, have been filled
with sand and shells, similar to those in the stratum _a_.
_Fractures of the strata and faults._--Numerous rents may often be seen in
rocks which appear to have been simply broken, the separated parts
remaining in the same places; 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, and not unfrequently 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, which happened about the year 1840, and was described
to me by an eye-witness, 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 brickdust, evidently produced by trituration.
[Illustration: Fig. 85. Faults. A B perpendicular, C D oblique
to the horizon.]
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 shift, slip, or
fault. "The miner," says Playfair, describing a fault, "is often perplexed,
in his subterraneous 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, fig. 85., 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 fig. 85., so that the strata on each
side of the 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_ (fig.
86.), though their identity is still to be recognized by their possessing
the same thickness, and the same internal characters."[62-A]
[Illustration: Fig. 86. E F, fault or fissure filled with rubbish, on each
side of which the shifted strata are not parallel.]
In Coalbrook Dale, says Mr. Prestwich[62-B], 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 separated several yards, but 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, in some cases,
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 fine 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.
[Illustration: Fig. 87. Apparent alternations of strata caused
by vertical faults.]
If, for example, the dark line A H (fig. 87.) 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, upon reaching the strata of
sandstone _c_, or on arriving at the line of fault F he comes partly
upon the shale _b_, and partly on the sandstone _c_, and on reaching E
he is again stopped by a wall composed of the rock _d_.
[Illustration: Fig. 88. Cross section.]
The very different levels at which the separated parts of the same strata
are found on the different sides of the fissure, in some faults, is truly
astonishing. One of the most celebrated in England is that called the
"ninety-fathom dike," in the coal-field of Newcastle. This name has been
given to it, because the same beds are ninety fathoms lower on the northern
than they are on the southern side. The fissure has been filled by a body
of sand, which is now in the state of sandstone, and is called the dike,
which is sometimes very narrow, but in other places more than twenty yards
wide.[64-A] The walls of the fissure are scored by grooves, such as would
have been produced if the broken ends of the rock had been rubbed along the
plane of the fault.[64-B] In the Tynedale and Craven faults, in the north
of England, the vertical displacement is still greater, and has extended in
a horizontal direction for a distance of thirty miles or more. Some
geologists consider it necessary to imagine that the upward or downward
movement in these cases was accomplished at a single stroke, and not by a
series of sudden but interrupted movements. This idea appears to have been
derived from a notion that the grooved walls have merely been rubbed in one
direction. But this is so far from being a constant phenomenon in faults,
that it has often been objected to the received theory respecting those
polished surfaces called "slickensides" (see above, p. 61.), that the striae
are not always parallel, but often curved and irregular. It has, moreover,
been remarked, that not only the walls of the fissure or fault, but its
earthy contents, sometimes present the same polished and striated faces.
Now these facts seem to indicate partial changes in the direction of the
movement, and some slidings subsequent to the first filling up of the
fissure. Suppose the mass of rock A, B, C, to overlie an extensive chasm _d
e_, formed at the depth of several miles, whether by the gradual
contraction in bulk of a melted mass passing into a solid or crystalline
state, or the shrinking of argillaceous strata, baked by a moderate heat,
or by the subtraction of matter by volcanic action, or any other cause.
Now, if this region be convulsed by earthquakes, the fissures _f g_, and
others at right angles to them, may sever the mass B from A and from C, so
that it may move freely, and begin to sink into the chasm. A fracture may
be conceived so clean and perfect as to allow it to subside at once to the
bottom of the subterranean cavity; but it is far more probable that the
sinking will be effected at successive periods during different
earthquakes, the mass always continuing to slide in the same direction
along the planes of the fissures _f g_, and the edges of the falling mass
being continually more broken and triturated at each convulsion. If, as is
not improbable, the circumstances which have caused the failure of support
continue in operation, it may happen that when the mass B has filled the
cavity first formed, its foundations will again give way under it, so that
it will fall again in the same direction. But, if the direction should
change, the fact could not be discovered by observing the slickensides,
because the last scoring would efface the lines of previous friction. In
the present state of our ignorance of the causes of subsidence, an
hypothesis which can explain the great amount of displacement in some
faults, on sound mechanical principles, by a succession of movements, is
far preferable to any theory which assumes each fault to have been
accomplished by a single upcast or downthrow of several thousand feet. For
we know that there are operations now in progress, at great depths in the
interior of the earth, by which both large and small tracts of ground are
made to rise above and sink below their former level, some slowly and
insensibly, others suddenly and by starts, a few feet or yards at a time;
whereas there are no grounds for believing that, during the last 3000 years
at least, any regions have been either upheaved or depressed, at a single
stroke, to the amount of several hundred, much less several thousand feet.
When some of the ancient marine formations are described in the sequel, it
will appear that their structure and organic contents point to the
conclusion, that the floor of the ocean was slowly sinking at the time of
their origin. The downward movement was very gradual, and in Wales and the
contiguous parts of England a maximum thickness of 32,000 feet (more than
six miles) of Carboniferous, Devonian, and Silurian rock was formed, whilst
the bed of the sea was all the time continuously and tranquilly
subsiding.[65-A] Whatever may have been the changes which the solid
foundation underwent, whether accompanied by the melting, consolidation,
crystallization, or desiccation of subjacent mineral matter, it is clear
from the fact of the sea having remained shallow all the while that the
bottom never sank down suddenly to the depth of many hundred feet at once.
It is by assuming such reiterated variations of level, each separately of
small vertical amount, but multiplied by time till they acquire importance
in the aggregate, that we are able to explain the phenomena of denudation,
which will be treated of in the next chapter. By such movements every
portion of the surface of the land becomes in its turn a line of coast, and
is exposed to the action of the waves and tides. A country which is
undergoing such movement is never allowed to settle into a state of
equilibrium, therefore the force of rivers and torrents to remove or
excavate soil and rocky masses is sustained in undiminished energy.
FOOTNOTES:
[46-A] In the first three editions of my Principles of Geology, I expressed
many doubts as to the validity of the alleged proofs of a gradual rise of
land in Sweden; but after visiting that country, in 1834, I retracted these
objections, and published a detailed statement of the observations which
led me to alter my opinion in the Phil. Trans. 1835, Part I. See also the
Principles, 4th and subsequent editions.
[46-B] See his Journal of a Naturalist in Voyage of the Beagle, and his
work on Coral Reefs.
[46-C] See chapters xxviii. to xxxi. inclusive.
[48-A] Edin. Trans. vol. vii. pl. 3.
[50-A] Proceedings of Geol. Soc. vol. iii. p. 148.
[53-A] See plan by M. Chevalier, Burat's D'Aubuisson, tom. ii. p. 334.
[55-A] See M. Thurmann's work, "Essai sur les Soulevemens Jurassiques
du Porrentruy, Paris, 1832," with whom I examined part of these
mountains in 1835.
[57-A] 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_.
[60-A] Biographical account of Dr. Hutton.
[60-B] See above, p. 49. and section.
[60-C] Playfair, ibid.; see his Works, Edin. 1822, vol. iv. p. 81.
[62-A] Playfair, Illust. of Hutt. Theory, Sec. 42.
[62-B] Geol. Trans. second series, vol. v. p. 452.
[64-A] Conybeare and Phillips, Outlines, &c. p. 376.
[64-B] Phillips, Geology, Lardner's Cyclop. p. 41.
[65-A] See the results of the "Geological Survey of Great Britain;"
Memoirs, vols. i. and ii., by Sir H. De la Beche, Mr. A. C. Ramsay,
and Mr. John Phillips.