at the bottom of each gallery, as at _a_, Fig. 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.
Fig. 60
Dip and Strike.—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 annexed diagram (Fig. 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.
Fig. 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
(Fig. 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°. 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.
Fig. 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 Fig. 62. It is thus easy to discover whether the lines of the
inclined beds bisect the angle of 90°, formed by the meeting of the
hands, so as to give an angle of 45°, or whether it would divide the
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.
Fig. 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 Fig. 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.[4]
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.
Fig. 64: Ground-plan of the denuded ridge C, Fig. 63. Fig. 65:
Transverse section.
Outcrop of Strata.—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. 64, and the
cross-section of the same by Fig. 65. The line D E, Fig. 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 quâ-quâversal 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.
Fig. 66: Slope of valley 40°, dip of strata 20°. Fig. 67: Slope of
valley 20°, dip of strata 50°. Fig. 68: Slope of valley 20°, dip of
strata 20°, in opposite directions.
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 Fig. 66), those formed by the newer beds
appearing in a superior position, and extending highest up the valley,
as A is seen above B.
Thirdly, if the dip of the beds be steeper than the slope of the
valley, then the V’s will point downward (see Fig. 67), and those
formed of the older beds will now appear uppermost, as B appears above
A.
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. 68), which exhibits strata rising at
an angle of 20°, and crossed by a valley, which declines in an opposite
direction at 20°.
These rules may often be of great practical utility; for the different
degrees of dip occurring in the two cases represented in Figs. 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 Fig. 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, Fig. 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.[5]
Section of carboniferous rocks of Lancashire. Section of carboniferous
rocks of Lancashire. (E. Hull.[6])
Synclinal Strata forming Ridges.—Although in many cases an anticlinal
axis forms a ridge, and a synclinal axis a valley, as in A B, Fig. 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,_
Fig. 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_,
Fig. 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
p. 62 and in part to the permeation of sea-water while they were yet
submerged.
Fig. 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_, Fig. 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.
Fig. 71: Bent and undulating gypseous 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.
71), while the continuity of the more pliable and ductile marls, _m,
m,_ has not been interrupted.
Fig. 72: Folded strata.
We have already explained, Fig. 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, Fig. 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 Fig. 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.
Fig. 73
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,
Fig. 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 striæ 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.
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, Fig. 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 Fig. 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_ (Fig. 75), though their
identity is still to be recognised by their possessing the same
thickness and the same internal characters.”[7]
Fig. 74: Faults.
Fig. 75: 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[8], 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
_débris_ 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.
Fig. 76: Apparent alternations of strata caused by vertical faults.
If, for example, the dark line A H (Fig. 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._
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
(540 feet) 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.[9] 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.[10] In the Tynedale and Craven faults, in the north of England,
the vertical displacement is still greater, and the fracture has
extended in a horizontal direction for a distance of thirty miles or
more.
Great Faults the Result of Repeated Movements.—It must not, however, be
supposed that faults generally consist of single linear rents; there
are usually a number of faults springing off from the main one, and
sometimes a long strip of country seems broken up into fragments by
sets of parallel and connecting transverse faults. Oftentimes a great
line of fault has been repeated, or the movements have been continued
through successive periods, so that, newer deposits having covered the
old line of displacement, the strata both newer and older have given
way along the old line of fracture. Some geologists have considered 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. They appear to have derived this idea
from a notion that the grooved walls have merely been rubbed in one
direction, which is far from being a constant phenomenon. Not only are
some sets of striæ not parallel to others, but the clay and rubbish
between the walls, when squeezed or rubbed, have been streaked in
different directions, the grooves which the harder minerals have
impressed on the softer being frequently curved and irregular.
Fig. 77: Faults and denuded coal-strata, Ashby de la Zouch.
The usual absence of protruding masses of rock forming precipices or
ridges along the lines of great faults has already been alluded to in
explaining Fig. 76, p. 89, and the same remarkable fact is well
exemplified in every coal-field which has been extensively worked. It
is in such districts that the former relation of the beds which have
been shifted is determinable with great accuracy. Thus in the
coal-field of Ashby de la Zouch, in Leicestershire (see Fig. 77), a
fault occurs, on one side of which the coal-beds _a, b, c, d_ must once
have risen to the height of 500 feet above the corresponding beds on
the other side. But the uplifted strata do not stand up 500 feet above
the general surface; on the contrary, the outline of the country, as
expressed by the line _z z_, is uniformly undulating, without any
break, and the mass indicated by the dotted outline must have been
washed away.[11]
The student may refer to Mr. Hull’s measurement of faults, observed in
the Lancashire coal-field, where the vertical displacement has amounted
to thousands of feet, and yet where all the superficial inequalities
which must have resulted from such movements have been obliterated by
subsequent denudation. In the same memoir proofs are afforded of there
having been two periods of vertical movement in the same fault—one, for
example, before, and another after, the Triassic epoch.[12]
The shifting of the beds by faults is often intimately connected with
those same foldings which constitute the anticlinal and synclinal axes
before alluded to, and there is no doubt that the subterranean causes
of both forms of disturbance are to a great extent the same. A fault in
Virginia, believed to imply a displacement of several thousand feet,
has been traced for more than eighty miles in the same direction as the
foldings of the Appalachian chain.[13] An hypothesis which attributes
such a change of position to 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.
It is certainly not easy to understand how in the subterranean regions
one mass of solid rock should have been folded up by a continued series
of movements, while another mass in contact, or only separated by a
line of fissure, has remained stationary or has perhaps subsided. But
every volcano, by the intermittent action of the steam, gases, and lava
evolved during an eruption, helps us to form some idea of the manner in
which such operations take place. For eruptions are repeated at
uncertain intervals throughout the whole or a large part of a
geological period, some of the surrounding and contiguous districts
remaining quite undisturbed. And in most of the instances with which we
are best acquainted the emission of lava, scoria, and steam is
accompanied by the uplifting of the solid crust. Thus in Vesuvius,
Etna, the Madeiras, the Canary Islands, and the Azores there is
evidence of marine deposits of recent and tertiary date having been
elevated to the height of a thousand feet, and sometimes more, since
the commencement of the volcanic explosions. There is, moreover, a
general tendency in contemporaneous volcanic vents to affect a linear
arrangement, extending in some instances, as in the Andes or the Indian
Archipelago, to distances equalling half the circumference of the
globe. Where volcanic heat, therefore, operates at such a depth as not
to obtain vent at the surface, in the form of an eruption, it may
nevertheless be conceived to give rise to upheavals, foldings, and
faults in certain linear tracts. And marine denudation, to be treated
of in the next chapter, will help us to understand why that which
should be the protruding portion of the faulted rocks is missing at the
surface.
Arrangement and Direction of Parallel Folds of Strata.—The possible
causes of the folding of strata by lateral movements have been
considered in a former part of this chapter. No European chain of
mountains affords so remarkable an illustration of the persistency of
such flexures for a great distance as the Appalachians before alluded
to, and none has been studied and described by many good observers with
more accuracy. The chain extends from north to south, or rather N.N.E.
to S.S.W., for nearly 1500 miles, with a breadth of 50 miles,
throughout which the Palæozoic strata have been so bent as to form a
series of parallel anticlinal and synclinal ridges and troughs,
comprising usually three or four principal and many smaller plications,
some of them forming broad and gentle arches, others narrower and
steeper ones, while some, where the bending has been greatest, have the
position of their beds inverted, as before shown in Fig. 73, p. 87.
The strike of the parallel ridges, after continuing in a straight line
for many hundred miles, is then found to vary for a more limited
distance as much as 30°, the folds wheeling round together in the new
direction and continuing to be parallel, as if they had all obeyed the
same movement. The date of the movements by which the great flexures
were brought about must, of course, be subsequent to the formation of
the uppermost part of the coal or the newest of the bent rocks, but the
disturbance must have ceased before the Triassic strata were deposited
on the denuded edges of the folded beds.
The manner in which the numerous parallel folds, all simultaneously
formed, assume a new direction common to the whole of them, and
sometimes varying at an angle of 30° from the normal strike of the
chain, shows what deviation from an otherwise uniform strike of the
beds may be experienced when the geographical area through which they
are traced is on so vast a scale.
The disturbances in the case here adverted to occurred between the
Carboniferous period and that of the Trias, and this interval is so
vast that they may have occupied a great lapse of time, during which
their parallelism was always preserved. But, as a rule, wherever after
a long geological interval the recurrence of lateral movements gives
rise to a new set of folds, the strike of these last is different.
Thus, for example, Mr. Hull has pointed out that three principal lines
of disturbance, all later than the Carboniferous period, have affected
the stratified rocks of Lancashire. The first of these, having an
E.N.E. direction, took place at the close of the Carboniferous period.
The next, running north and south, at the close of the Permian, and the
third, having a N.N.W. direction, at the close of the Jurassic
period.[14]
Fig. 78: Unconformable junction of old red sandstone and Silurian
schist at the Siccar Point, near St. Abb’s Head, Berwickshire.
Unconformability of Strata.— 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. 78). 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, _d, d,_ Fig. 78,
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 upraised out of the
sea, and thrown slightly out of the horizontal position.
Fig. 79: 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 (primary or
palæozoic) 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._
Overlapping Strata.—Strata are said to overlap when an upper bed
extends beyond the limits of a lower one. This may be produced in
various ways; as, for example, when alterations of physical geography
cause the arms of a river or channels of discharge to vary, so that
sediment brought down is deposited over a wider area than before, or
when the sea-bottom has been raised up and again depressed without
disturbing the horizontal position of the strata. In this case the
newer strata may rest for the most part conformably on the older, but,
extending farther, pass over their edges. Every intermediate state
between unconformable and over-lapping beds may occur, because there
may be every gradation between a slight derangement of position, and a
considerable disturbance and denudation of the older formation before
the newer beds come on.
[1] See “Principles of Geology,” 1867, p. 314.
[2] Edin. Trans., vol. vii, pl. 3.
[3] Proceedings of Geol. Soc., vol. iii, p. 148.
[4] Thurmann, “Essai sur les Soulèvemens Jurassiques de Porrentruy,”
Paris, 1832.
[5] 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._
[6] Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 324, 1868.
[7] Playfair, Illust. of Hutt. Theory, § 42.
[8] Geol. Trans., second series. vol. v, p. 452.
[9] Conybeare and Phillips Outlines, etc., p. 376.
[10] Phillips, Geology, Lardner’s Cyclop., p. 41.
[11] See Mammatt’s Geological Facts, etc., p. 90 and plate.
[12] Hull, Quart. Geol. Journ., vol. xxiv, p. 318, 1868.
[13] H. D. Rogers, Geol. of Pennsylvania, p. 897.
[14] Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 323.
CHAPTER VI.
DENUDATION
Denudation defined. — Its Amount more than equal to the entire Mass of
Stratified Deposits in the Earth’s Crust. — Subaërial Denudation. —
Action of the Wind. — Action of Running Water. — Alluvium defined. —
Different Ages of Alluvium. — Denuding Power of Rivers affected by Rise
or Fall of Land. — Littoral Denudation. — Inland Sea-Cliffs. —
Escarpments. — Submarine Denudation. — Dogger-bank. — Newfoundland
Bank. — Denuding Power of the Ocean during Emergence of Land.
Denudation, which has been occasionally spoken of in the preceding
chapters, is the removal of solid matter by water in motion, whether of
rivers or of the waves and currents of the sea, and the consequent
laying bare of some inferior rock. This operation has exerted an
influence on the structure of the earth’s crust as universal and
important as sedimentary deposition itself; for denudation is the
necessary antecedent of the production of all new strata of mechanical
origin. The formation of every new deposit by the transport of sediment
and pebbles necessarily implies that there has been, somewhere else, a
grinding down of rock into rounded fragments, sand, or mud, equal in
quantity to the new strata. All deposition, therefore, except in the
case of a shower of volcanic ashes, and the outflow of lava, and the
growth of certain organic formations, is the sign of superficial waste
going on contemporaneously, and to an equal amount, elsewhere. The gain
at one point is no more than sufficient to balance the loss at some
other. Here a lake has grown shallower, there a ravine has been
deepened. Here the depth of the sea has been augmented by the removal
of a sandbank during a storm, there its bottom has been raised and
shallowed by the accumulation in its bed of the same sand transported
from the bank.
When we see a stone building, we know that somewhere, far or near, a
quarry has been opened. The courses of stone in the building may be
compared to successive strata, the quarry to a ravine or valley which
has suffered denudation. As the strata, like the courses of hewn stone,
have been laid one upon another gradually, so the excavation both of
the valley and quarry have been gradual. To pursue the comparison still
farther, the superficial heaps of mud, sand, and gravel, usually called
alluvium, may be likened to the rubbish of a quarry which has been
rejected as useless by the workmen, or has fallen upon the road between
the quarry and the building, so as to lie scattered at random over the
ground.
But we occasionally find in a conglomerate large rounded pebbles of an
older conglomerate, which had previously been derived from a variety of
different rocks. In such cases we are reminded that, the same materials
having been used over and over again, it is not enough to affirm that
the entire mass of stratified deposits in the earth’s crust affords a
monument and measure of the denudation which has taken place, for in
truth the quantity of matter now extant in the form of stratified rock
represents but a fraction of the material removed by water and
redeposited in past ages.
Subaërial Denudation.—Denudation may be divided into subaërial, or the
action of wind, rain, and rivers; and submarine, or that effected by
the waves of the sea, and its tides and currents. With the operation of
the first of these we are best acquainted, and it may be well to give
it our first attention.
_Action of the Wind._—In desert regions where no rain falls, or where,
as in parts of the Sahara, the soil is so salt as to be without any
covering of vegetation, clouds of dust and sand attest the power of the
wind to cause the shifting of the unconsolidated or disintegrated rock.
In examining volcanic countries I have been much struck with the great
superficial changes brought about by this power in the course of
centuries. The highest peak of Madeira is about 6050 feet above the
sea, and consists of the skeleton of a volcanic cone now 250 feet high,
the beds of which once dipped from a centre in all directions at an
angle of more than 30°. The summit is formed of a dike of basalt with
much olivine, fifteen feet wide, apparently the remains of a column of
lava which once rose to the crater. Nearly all the scoriæ of the upper
part of the cone have been swept away, those portions only remaining
which were hardened by the contact or proximity of the dike. While I
was myself on this peak on January 25, 1854, I saw the wind, though it
was not stormy weather, removing sand and dust derived from the
decomposing scoriæ. There had been frost in the night, and some ice was
still seen in the crevices of the rock.
On the highest platform of the Grand Canary, at an elevation of 6000
feet, there is a cylindrical column of hard lava, from which the softer
matter has been carried away; and other similar remnants of the dikes
of cones of eruption attest the denuding power of the wind at points
where running water could never have exerted any influence. The waste
effected by wind aided by frost and snow, may not be trifling, even in
a single winter, and when multiplied by centuries may become
indefinitely great.
Fig. 80: Section through several eroded formations.
_Action of Running Water._—There are different classes of phenomena
which attest in a most striking manner the vast spaces left vacant by
the erosive power of water. I may allude, first, to those valleys on
both sides of which the same strata are seen following each other in
the same order, and having the same mineral composition and fossil
contents. We may observe, for example, several formations, as Nos. 1,
2, 3, 4, in the diagram (Fig. 80): No. 1, conglomerate, No. 2, clay,
No. 3, grit, and No. 4, limestone, each repeated in a series of hills
separated by valleys varying in depth. When we examine the subordinate
parts of these four formations, we find, in like manner, distinct beds
in each, corresponding, on the opposite sides of the valleys, both in
composition and order of position. No one can doubt that the strata
were originally continuous, and that some cause has swept away the
portions which once connected the whole series. A torrent on the side
of a mountain produces similar interruptions; and when we make
artificial cuts in lowering roads, we expose, in like manner,
corresponding beds on either side. But in nature, these appearances
occur in mountains several thousand feet high, and separated by
intervals of many miles or leagues in extent.
In the “Memoirs of the Geological Survey of Great Britain” (vol. i),
Professor Ramsay has shown that the missing beds, removed from the
summit of the Mendips, must have been nearly a mile in thickness; and
he has pointed out considerable areas in South Wales and some of the
adjacent counties of England, where a series of primary (or palæozoic)
strata, no less than 11,000 feet in thickness, have been stripped off.
All these materials have of course been transported to new regions, and
have entered into the composition of more modern formations. On the
other hand, it is shown by observations in the same “Survey,” that the
Palæozoic strata are from 20,000 to 30,000 feet thick. It is clear that
such rocks, formed of mud and sand, now for the most part consolidated,
are the monuments of denuding operations, which took place on a grand
scale at a very remote period in the earth’s history. For, whatever has
been given to one area must always have been borrowed from another; a
truth which, obvious as it may seem when thus stated, must be
repeatedly impressed on the student’s mind, because in many geological
speculations it is taken for granted that the external crust of the
earth has been always growing thicker in consequence of the
accumulation, period after period, of sedimentary matter, as if the new
strata were not always produced at the expense of pre-existing rocks,
stratified or unstratified. By duly reflecting on the fact that all
deposits of mechanical origin imply the transportation from some other
region, whether contiguous or remote, of an equal amount of solid
matter, we perceive that the stony exterior of the planet must always
have grown thinner in one place, whenever, by accessions of new strata,
it was acquiring thickness in another.
It is well known that generally at the mouths of large rivers, deltas
are forming and the land is encroaching upon the sea; these deltas are
monuments of recent denudation and deposition; and it is obvious that
if the mud, sand, and gravel were taken from them and restored to the
continents they would fill up a large part of the gullies and valleys
which are due to the excavating and transporting power of torrents and
rivers.
Alluvium.—Between the superficial covering of vegetable mould and the
subjacent rock there usually intervenes in every district a deposit of
loose gravel, sand, and mud, to which when it occurs in valleys the
name of alluvium has been popularly applied. The term is derived from
_alluvio_, an inundation, or _alluo_, to wash, because the pebbles and
sand commonly resemble those of a river’s bed or the mud and gravel
washed over low lands by a flood.
In the course of those changes in physical geography which may take
place during the gradual emergence of the bottom of the sea and its
conversion into dry land, any spot may either have been a sunken reef,
or a bay, or estuary, or sea-shore, or the bed of a river. The
drainage, moreover, may have been deranged again and again by
earthquakes, during which temporary lakes are caused by landslips, and
partial deluges occasioned by the bursting of the barriers of such
lakes. For this reason it would be unreasonable to hope that we should
ever be able to account for all the alluvial phenomena of each
particular country, seeing that the causes of their origin are so
various. Besides, the last operations of water have a tendency to
disturb and confound together all pre-existing alluviums. Hence we are
always in danger of regarding as the work of a single era, and the
effect of one cause, what has in reality been the result of a variety
of distinct agents, during a long succession of geological epochs. Much
useful instruction may therefore be gained from the exploration of a
country like Auvergne, where the superficial gravel of very different
eras happens to have been preserved and kept separate by sheets of
lava, which were poured out one after the other at periods when the
denudation, and probably the upheaval, of rocks were in progress. That
region had already acquired in some degree its present configuration
before any volcanoes were in activity, and before any igneous matter
was superimposed upon the granitic and fossiliferous formations. The
pebbles therefore in the older gravels are exclusively constituted of
granite and other aboriginal rocks; and afterwards, when volcanic vents
burst forth into eruption, those earlier alluviums were covered by
streams of lava, which protected them from intermixture with gravel of
subsequent date. In the course of ages, a new system of valleys was
excavated, so that the rivers ran at lower levels than those at which
the first alluviums and sheets of lava were formed. When, therefore,
fresh eruptions gave rise to new lava, the melted matter was poured out
over lower grounds; and the gravel of these plains differed from the
first or upland alluvium, by containing in it rounded fragments of
various volcanic rocks, and often fossil bones belonging to species of
land animals different from those which had previously flourished in
the same country and been buried in older gravels.
Fig. 81: Lavas of Auvergne resting on alluviums of different ages.
The annexed drawing (Fig. 81) will explain the different heights at
which beds of lava and gravel, each distinct from the other in
composition and age, are observed, some on the flat tops of hills, 700
or 800 feet high, others on the slope of the same hills, and the newest
of all in the channel of the existing river where there is usually
gravel alone, although in some cases a narrow strip of solid lava
shares the bottom of the valley with the river.
The proportion of extinct species of quadrupeds is more numerous in the
fossil remains of the gravel No. 1 than in that indicated as No. 2; and
in No. 3 they agree more closely, sometimes entirely, with those of the
existing fauna. The usual absence or rarity of organic remains in beds
of loose gravel and sand is partly owing to the friction which
originally ground down the rocks into small fragments, and partly to
the porous nature of alluvium, which allows the free percolation
through it of rain-water, and promotes the decomposition and removal of
fossil remains.
The loose transported matter on the surface of a large part of the land
now existing in the temperate and arctic regions of the northern
hemisphere, must be regarded as being in a somewhat exceptional state,
in consequence of the important part which ice has played in
comparatively modern geological times. This subject will be more
specially alluded to when we describe, in the eleventh chapter, the
deposits called “glacial.”
Denuding Power of Rivers affected by Rise or Fall of Land.—It has long
been a matter of common observation that most rivers are now cutting
their channels through alluvial deposits of greater depth and extent
than could ever have been formed by the present streams. From this fact
it has been inferred that rivers in general have grown smaller, or
become less liable to be flooded than formerly. It may be true that in
the history of almost every country the rivers have been both larger
and smaller than they are at the present moment. For the rainfall in
particular regions varies according to climate and physical geography,
and is especially governed by the elevation of the land above the sea,
or its distance from it and other conditions equally fluctuating in the
course of time. But the phenomenon alluded to may sometimes be
accounted for by oscillations in the level of the land, experienced
since the existing valleys originated, even where no marked diminution
in the quantity of rain and in the size of the rivers has occurred.
We know that many large areas of land are rising and others sinking,
and unless it could be assumed that both the upward and downward
movements are everywhere uniform, many of the existing hydrographical
basins ought to have the appearance of having been temporary lakes
first filled with fluviatile strata and then partially re-excavated.
Suppose, for example, part of a continent, comprising within it a large
hydrographical basin like that of the Mississippi, to subside several
inches or feet in a century, as the west coast of Greenland, extending
600 miles north and south, has been sinking for three or four
centuries, between the latitudes 60° and 69° N.[1] It will rarely
happen that the rate of subsidence will be everywhere equal, and in
many cases the amount of depression in the interior will regularly
exceed that of the region nearer the sea. Whenever this happens, the
fall of the waters flowing from the upland country will be diminished,
and each tributary stream will have less power to carry its sand and
sediment into the main river, and the main river less power to convey
its annual burden of transported matter to the sea. All the rivers,
therefore, will proceed to fill up partially their ancient channels,
and, during frequent inundations, will raise their alluvial plains by
new deposits. If then the same area of land be again upheaved to its
former height, the fall, and consequently the velocity, of every river
will begin to augment. Each of them will be less given to overflow its
alluvial plain; and their power of carrying earthy matter seaward, and
of scouring out and deepening their channels, will be sustained till,
after a lapse of many thousand years, each of them has eroded a new
channel or valley through a fluviatile formation of comparatively
modern date. The surface of what was once the river-plain at the period
of greatest depression, will then remain fringing the valley-sides in
the form of a terrace apparently flat, but in reality sloping down with
the general inclination of the river. Everywhere this terrace will
present cliffs of gravel and sand, facing the river. That such a series
of movements has actually taken place in the main valley of the
Mississippi and in its tributary valleys during oscillations of level,
I have endeavoured to show in my description of that country;[2] and
the fresh-water shells of existing species and bones of land
quadrupeds, partly of extinct races, preserved in the terraces of
fluviatile origin, attest the exclusion of the sea during the whole
process of filling up and partial re-excavation.
Littoral Denudation.—Part of the action of the waves between high and
low watermark must be included in subaërial denudation, more especially
as the undermining of cliffs by the waves is facilitated by
land-springs, and these often lead to the sliding down of great masses
of land into the sea. Along our coasts we find numerous submerged
forests, only visible at low water, having the trunks of the trees
erect and their roots attached to them and still spreading through the
ancient soil as when they were living. They occur in too many places,
and sometimes at too great a depth, to be explained by a mere change in
the level of the tides, although as the coasts waste away and alter in
shape, the height to which the tides rise and fall is always varying,
and the level of high tide at any given point may, in the course of
many ages, differ by several feet or even fathoms. It is this
fluctuation in the height of the tides, and the erosion and destruction
of the sea-coast by the waves, that makes it exceedingly difficult for
us in a few centuries, or even perhaps in a few thousand years, to
determine whether there is a change by subterranean movement in the
relative level of sea and land.
We often behold, as on the coasts of Devonshire and Pembrokeshire,
facts which appear to lead to opposite conclusions. In one place a
raised beach with marine littoral shells, and in another immediately
adjoining a submerged forest. These phenomena indicate oscillations of
level, and as the movements are very gradual, they must give repeated
opportunities to the breakers to denude the land which is thus again
and again exposed to their fury, although it is evident that the
submergence is sometimes effected in such a manner as to allow the
trees which border the coast not to be carried away.
Inland Sea-cliffs.—In countries where hard limestone rocks abound,
inland cliffs have often retained faithfully for ages the characters
which they acquired when they constituted the boundary of land and sea.
Thus, in the Morea, no less than three or even four ranges of cliffs
are well-preserved, rising one above the other at different distances
from the actual shore, the summit of the highest and oldest
occasionally attaining 1000 feet in elevation. A consolidated beach
with marine shells is usually found at the base of each cliff, and a
line of littoral caverns. These ranges of cliff probably imply pauses
in the process of upheaval when the waves and currents had time to
undermine and clear away considerable masses of rock.
But the beginner should be warned not to expect to find evidence of the
former sojourn of the sea on all those lands which we are nevertheless
sure have been submerged at periods comparatively modern; for
notwithstanding the enduring nature of the marks left by littoral
action on some rocks, especially limestones, we can by no means detect
sea-beaches and inland cliffs everywhere. On the contrary, they are,
upon the whole, extremely partial, and are often entirely wanting in
districts composed of argillaceous and sandy formations, which must,
nevertheless, have been upheaved at the same time, and by the same
intermittent movements, as the adjoining harder rocks.
Escarpments.—Besides the inland cliffs above alluded to which mark the
ancient limits of the sea, there are other abrupt terminations of rocks
of various kinds which resemble sea-cliffs, but which have in reality
been due to subaërial denudation. These have been called “escarpments,”
a term which it is useful to confine to the outcrop of particular
formations having a scarped outline, as distinct from cliffs due to
marine action.
I formerly supposed that the steep line of cliff-like slopes seen along
the outcrop of the chalk, when we follow the edge of the North or South
Downs, was due to marine action; but Professor Ramsay has shown[3] that
the present outline of the physical geography is more in favour of the
idea of the escarpments having been due to gradual waste since the
rocks were exposed in the atmosphere to the action of rain and rivers.
Mr. Whittaker has given a good summary of the grounds for ascribing
these apparent sea-cliffs to waste in the open air. 1. There is an
absence of all signs of ancient sea-beaches or littoral deposits at the
base of the escarpment. 2. Great inequality is observed in the level of
the base line. 3. The escarpments do not intersect, like sea-cliffs, a
series of distinct rocks, but are always confined to the boundary-line
of the same formation. 4. There are sometimes different contiguous and
parallel escarpments—those, for example, of the greensand and
chalk—which are so near each other, and occasionally so similar in
altitude, that we cannot imagine any existing archipelago if converted
into dry land to present a like outline.
The above theory is by no means inconsistent with the opinion that the
limits of the outcrop of the chalk and greensand which the escarpments
now follow, were originally determined by marine denudation. When the
south-east of England last emerged from beneath the level of the sea,
it was acted upon, no doubt, by the tide, waves, and currents, and the
chalk would form from the first a mass projecting above the more
destructible clay called Gault. Still the present escarpments so much
resembling sea-cliffs have no doubt, for reasons above stated, derived
their most characteristic features subsequently to emergence from
subaërial waste by rain and rivers.
Submarine Denudation.—When we attempt to estimate the amount of
submarine denudation, we become sensible of the disadvantage under
which we labour from our habitual incapacity of observing the action of
marine currents on the bed of the sea. We know that the agitation of
the waves, even during storms, diminishes at a rapid rate, so as to
become very insignificant at the depth of a few fathoms, and is quite
imperceptible at the depth of about sixteen fathoms; but when large
bodies of water are transferred by a current from one part of the ocean
to another, they are known to maintain at great depths such a velocity
as must enable them to remove the finer, and sometimes even the
coarser, materials of the rocks over which they flow. As the
Mississippi when more than 150 feet deep can keep open its channel and
even carry down gravel and sand to its delta, the surface velocity
being not more than two or three miles an hour, so a gigantic current,
like the Gulf Stream, equal in volume to many hundred Mississippis, and
having in parts a surface velocity of more than three miles, may act as
a propelling and abrading power at still greater depths. But the
efficacy of the sea as a denuding agent, geologically considered, is
not dependent on the power of currents to preserve at great depths a
velocity sufficient to remove sand and mud, because, even where the
deposition or removal of sediment is not in progress, the depth of
water does not remain constant throughout geological time. Every page
of the geological record proves to us that the relative levels of land
and sea, and the position of the ocean and of continents and islands,
has been always varying, and we may feel sure that some portions of the
submarine area are now rising and others sinking. The force of tidal
and other currents and of the waves during storms is sufficient to
prevent the emergence of many lands, even though they may be undergoing
continual upheaval. It is not an uncommon error to imagine that the
waste of sea-cliffs affords the measure of the amount of marine
denudation of which it probably constitutes an insignificant portion.
Dogger-bank.—That great shoal called the Dogger-bank, about sixty miles
east of the coast of Northumberland, and occupying an area about as
large as Wales, has nowhere a depth of more than ninety feet, and in
its shallower parts is less than forty feet under water. It might
contribute towards the safety of the navigation of our seas to form an
artificial island, and to erect a light-house on this bank; but no
engineer would be rash enough to attempt it, as he would feel sure that
the ocean in the first heavy gale would sweep it away as readily as it
does every temporary shoal that accumulates from time to time around a
sunk vessel on the same bank.[4]
No observed geographical changes in historical times entitle us to
assume that where upheaval may be in progress it proceeds at a rapid
rate. Three or four feet rather than as many yards in a century may
probably be as much as we can reckon upon in our speculations; and if
such be the case, the continuance of the upward movement might easily
be counteracted by the denuding force of such currents aided by such
waves as, during a gale, are known to prevail in the German Ocean. What
parts of the bed of the ocean are stationary at present, and what areas
may be rising or sinking, is a matter of which we are very ignorant, as
the taking of accurate soundings is but of recent date.
_Newfoundland Bank._—The great bank of Newfoundland may be compared in
size to the whole of England. This part of the bottom of the Atlantic
is surrounded on three sides by a rapidly deepening ocean, the bank
itself being from twenty to fifty fathoms (or from 120 to 300 feet)
under water. We are unable to determine by the comparison of different
charts made at distant periods, whether it is undergoing any change of
level, but if it be gradually rising we cannot anticipate on that
account that it will become land, because the breakers in an open sea
would exercise a prodigious force even on solid rock brought up to
within a few yards of the surface. We know, for example, that when a
new volcanic island rose in the Mediterranean in 1831, the waves were
capable in a few years of reducing it to a sunken rock.
In the same way currents which flow over the Newfoundland bank a great
part of the year at the rate of two miles an hour, and are known to
retain a considerable velocity to near the bottom, may carry away all
loose sand and mud, and make the emergence of the shoal impossible, in
spite of the accessions of mud, sand, and boulders derived occasionally
from melting icebergs which, coming from the northern glaciers, are
frequently stranded on various parts of the bank. They must often leave
at the bottom large erratic blocks which the marine currents may be
incapable of moving, but the same rocky fragments may be made to sink
by the undermining of beds consisting of finer matter on which the
blocks and gravel repose. In this way gravel and boulders may continue
to overspread a submarine bottom after the latter has been lowered for
hundreds of feet, the surface never having been able to emerge and
become land. It is by no means improbable that the annual removal of an
average thickness of half an inch of rock might counteract the ordinary
upheaval which large submarine areas are undergoing; and the real
enigma which the geologist has to solve is not the extensive denudation
of the white chalk or of our tertiary sands and clays, but the fact
that such incoherent materials have ever succeeded in lifting up their
heads above water in an open sea. Why were they not swept away during
storms into some adjoining abysses, the highest parts of each shoal
being always planed off down to the depth of a few fathoms? The
hardness and toughness of some rocks already exposed to windward and
acting as breakwaters may perhaps have assisted; nor must we forget the
protection afforded by a dense and unbroken covering of barnacles,
limpets, and other creatures which flourish most between high and low
water and shelter some newly risen coasts from the waves.
[1] Principles of Geology 7th ed., p. 506; 10th ed., vol. ii, p. 196.
[2] Second Visit to the United States, vol. i, chap. xxxiv.
[3] Physical Geography and Geology of Great Britain, p. 78, 1864.
[4] Principles, 10th ed., vol. i, p. 569.
CHAPTER VII.
JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE IN REMODELLING THE
EARTH’S CRUST.
How we obtain an Insight at the Surface, of the Arrangement of Rocks at
great Depths. — Why the Height of the successive Strata in a given
Region is so disproportionate to their Thickness. — Computation of the
average annual Amount of subaërial Denudation. — Antagonism of Volcanic
Force to the Levelling Power of running Water. — How far the Transfer
of Sediment from the Land to a neighbouring Sea-bottom may affect
Subterranean Movements. — Permanence of Continental and Oceanic Areas.
How we obtain an Insight at the Surface, of the Arrangement of Rocks at
Great Depths.— The reader has been already informed that, in the
structure of the earth’s crust, we often find proofs of the direct
superposition of marine to fresh-water strata, and also evidence of the
alternation of deep-sea and shallow-water formations. In order to
explain how such a series of rocks could be made to form our present
continents and islands, we have not only to assume that there have been
alternate upward and downward movements of great vertical extent, but
that the upheaval in the areas which we at present inhabit has, in
later geological times, sufficiently predominated over subsidence to
cause these portions of the earth’s crust to be land instead of sea.
The sinking down of a delta beneath the sea-level may cause strata of
fluviatile or even terrestrial origin, such as peat with trees proper
to marshes, to be covered by deposits of deep-sea origin. There is also
no end to the thickness of mud and sand which may accumulate in shallow
water, provided that fresh sediment is brought down from the wasting
land at a rate corresponding to that of the sinking of the bed of the
sea. The latter, again, may sometimes sink so fast that the earthy
matter, being intercepted in some new landward depression, may never
reach its former resting-place, where, the water becoming clear may
favour the growth of shells and corals, and calcareous rocks of organic
origin may thus be superimposed on mechanical deposits.
The succession of strata here alluded to would be consistent with the
occurrence of gradual downward and upward movements of the land and bed
of the sea without any disturbance of the horizontality of the several
formations. But the arrangement of rocks composing the earth’s crust
differs materially from that which would result from a mere series of
vertical movements. Had the volcanic forces been confined to such
movements, and had the stratified rocks been first formed beneath the
sea and then raised above it, without any lateral compression, the
geologist would never have obtained an insight into the monuments of
various ages, some of extremely remote antiquity.
What we have said in Chapter V of dip and strike, of the folding and
inversion of strata, of anticlinal and synclinal flexures, and in
Chapter VI of denudation at different periods, whether subaërial or
submarine, must be understood before the student can comprehend what
may at first seem to him an anomaly, but which it is his business
particularly to understand. I allude to the small height above the
level of the sea attained by strata often many miles in thickness, and
about the chronological succession of which, in one and the same
region, there is no doubt whatever. Had stratified rocks in general
remained horizontal, the waves of the sea would have been enabled
during oscillations of level to plane off entirely the uppermost beds
as they rose or sank during the emergence or submergence of the land.
But the occurrence of a series of formations of widely different ages,
all remaining horizontal and in conformable stratification, is
exceptional, and for this reason the total annihilation of the
uppermost strata has rarely taken place. We owe, indeed, to the side
way movements of _lateral compression_ those anticlinal and synclinal
curves of the beds already described (Fig. 55), which, together with
denudation, subaërial and submarine, enable us to investigate the
structure of the earth’s crust many miles below those points which the
miner can reach. I have already shown in Fig. 56, how, at St. Abb’s
Head, a series of strata of indefinite thickness may become vertical,
and then denuded, so that the edges of the beds alone shall be exposed
to view, the altitude of the upheaved ridges being reduced to a
moderate height above the sea-level; and it may be observed that
although the incumbent strata of Old Red Sandstone are in that place
nearly horizontal, yet these same newer beds will in other places be
found so folded as to present vertical strata, the edges of which are
abruptly cut off, as in 2, 3, 4 on the right-hand side of the diagram,
Fig. 55.
Why the Height of the Successive Strata in a given Region is so
Disproportionate to their Thickness.—We cannot too distinctly bear in
mind how dependent we are on the joint action of the volcanic and
aqueous forces, the one in
disturbing the original position of rocks, and the other in destroying
large portions of them, for our power of consulting the different pages
and volumes of those stony records of which the crust of the globe is
composed. Why, it may be asked, if the ancient bed of the sea has been
in many regions uplifted to the height of two or three miles, and
sometimes twice that altitude, and if it can be proved that some single
formations are of themselves two or three miles thick, do we so often
find several important groups resting one upon the other, yet attaining
only the height of a few hundred feet above the level of the sea?
The American geologists, after carefully studying the Allegheny or
Appalachian mountains, have ascertained that the older fossiliferous
rocks of that chain (from the Silurian to the Carboniferous inclusive)
are not less than 42,000 feet thick, and if they were now superimposed
on each other in the order in which they were thrown down, they ought
to equal in height the Himalayas with the Alps piled upon them. Yet
they rarely reach an altitude of 5000 feet, and their loftiest peaks
are no more than 7000 feet high. The Carboniferous strata forming the
highest member of the series, and containing beds of coal, can be shown
to be of shallow-water origin, or even sometimes to have originated in
swamps in the open air. But what is more surprising, the lowest part of
this great Palæozoic series, instead of having been thrown down at the
bottom of an abyss more than 40,000 feet deep, consists of sediment
(the Potsdam sandstone), evidently spread out on the bottom of a
shallow sea, on which ripple-marked sands were occasionally formed.
This vast thickness of 40,000 feet is not obtained by adding together
the maximum density attained by each formation in distant parts of the
chain, but by measuring the successive groups as they are exposed in a
very limited area, and where the denuded edges of the vertical strata
forming the parallel folds alluded to at page 87 “crop out” at the
surface. Our attention has been called by Mr. James Hall,
Palæontologist of New York, to the fact that these Palæozoic rocks of
the Appalachian chain, which are of such enormous density, where they
are almost entirely of mechanical origin, thin out gradually as they
are traced to the westward, where evidently the contemporaneous seas
allowed organic rocks to be formed by corals, echinoderms, and
encrinites in clearer water, and where, although the same successive
periods are represented, the total mass of strata from the Silurian to
the Carboniferous, instead of being 40,000 is only 4000 feet thick.
A like phenomenon is exhibited in every mountainous country, as, for
example, in the European Alps; but we need not go farther than the
north of England for its illustration. Thus in Lancashire and central
England the thickness of the Carboniferous formation, including the
Millstone Grit and Yoredale beds, is computed to be more than 18,000
feet; to this we may add the Mountain Limestone, at least 2000 feet in
thickness, and the overlying Permian and Triassic formations, 3000 or
4000 feet thick. How then does it happen that the loftiest hills of
Yorkshire and Lancashire, instead of being 24,000 feet high, never rise
above 3000 feet? For here, as before pointed out in the Alleghenies,
all the great thicknesses are sometimes found in close approximation
and in a region only a few miles in diameter. It is true that these
same sets of strata do not preserve their full force when followed for
indefinite distances. Thus the 18,000 feet of Carboniferous grits and
shales in Lancashire, before alluded to, gradually thin out, as Mr.
Hull has shown, as they extend southward, by attenuation or original
deficiency of sediment, and not in consequence of subsequent
denudation, so that when we have followed them for about 100 miles into
Leicestershire, they have dwindled away to a thickness of only 3000
feet. In the same region the Carboniferous limestone attains so unusual
a thickness—namely, more than 4000 feet—as to appear to compensate in
some measure for the deficiency of contemporaneous sedimentary rock.[1]
It is admitted that when two formations are unconformable their fossil
remains almost always differ considerably. The break in the continuity
of the organic forms seems connected with a great lapse of time, and
the same interval has allowed extensive disturbance of the strata, and
removal of parts of them by denudation, to take place. The more we
extend our investigations the more numerous do the proofs of these
breaks become, and they extend to the most ancient rocks yet
discovered. The oldest examples yet brought to light in the British
Isles are on the borders of Rosshire and Sutherlandshire, and have been
well described by Sir Roderick Murchison, by whom their chronological
relations were admirably worked out, and proved to be very different
from those which previous observers had imagined them to be. I had an
opportunity in the autumn of 1869 of verifying the splendid section
given in Fig. 82 by climbing in a few hours from the banks of Loch
Assynt to the summit of the mountain called Queenaig, 2673 feet high.
The formations 1, 2, 3, the Laurentian, Cambrian, and
Silurian, to be explained in Chapters XXV and XXVI, not only occur in
succession in this one mountain, but their unconformable junctions are
distinctly exposed to view.
Fig. 82: Unconformable Palæozoic stata, Sutherlandshire (Murchison).
To begin with the oldest set of rocks, No. 1; they consist chiefly of
hornblendic gneiss, and in the neighbouring Hebrides form whole
islands, attaining a thickness of thousands of feet, although they have
suffered such contortions and denudation that they seldom rise more
than a few hundred feet above the sea-level. In discordant
stratification upon the edges of this gneiss reposes No. 2, a group of
conglomerate and purple sandstone referable to the Cambrian (or
Longmynd) formation, which can elsewhere be shown to be characterised
by its peculiar organic remains. On this again rests No. 3, a lower
member of the important group called Silurian, an outlier of which, 3′,
caps the summit of Queenaig, attesting the removal by denudation of
rocks of the same age, which once extended from the great mass 3 to 3′.
Although this rock now consists of solid quartz, it is clear that in
its original state it was formed of fine sand, perforated by numerous
lob-worms or annelids, which left their burrows in the shape of tubular
hollows Fig. 563 of _Arenicolites_), hundreds, nay thousands, of which
I saw as I ascended the mountain.
Fig. 83: Diagrammatic section of the same groups near Queenaig
(Murchison).
In Queenaig we only behold this single quartzose member of the Silurian
series, but in the neighbouring country (see Fig. 83) it is seen to the
eastward to be followed by limestones, 3_a_, and schists, 3_b_,
presenting numerous folds, and becoming more and more metamorphic and
crystalline, until at length, although very different in age and
strike, they much resemble in appearance the group No. 1. It is very
seldom that in the same country one continuous formation, such as the
Silurian, is, as in this case, more fossiliferous and less altered by
volcanic heat in its older than in its newer strata, and still more
rare to find an underlying and unconformable group like the Cambrian
retaining its original condition of a conglomerate and sandstone more
perfectly than the overlying formation. Here also we may remark in
regard to the origin of these Cambrian rocks that they were evidently
produced at the expense of the underlying Laurentian, for the rounded
pebbles occurring in them are identical in composition and texture with
that crystalline gneiss which constitutes the contorted beds of the
inferior formation No. 1. When the reader has studied the chapter on
metamorphism, and has become aware how much modification by heat,
pressure, and chemical action is required before the conversion of
sedimentary into crystalline strata can be brought about, he will
appreciate the insight which we thus gain into the date of the changes
which had already been effected in the Laurentian rocks long before the
Cambrian pebbles of quartz and gneiss were derived from them. The
Laurentian is estimated by Sir William Logan to amount in Canada to
30,000 feet in thickness. As to the Cambrian, it is supposed by Sir
Roderick Murchison that the fragment left in Sutherlandshire is about
3500 feet thick, and in Wales and the borders of Shropshire this
formation may equal 10,000 feet, while the Silurian strata No. 3,
difficult as it may be to measure them in their various foldings to the
eastward, where they have been invaded by intrusive masses of granite,
are supposed many times to surpass the Cambrian in volume and density.
But although we are dealing here with stratified rocks, each of which
would be several miles in thickness, if they were fully represented,
the whole of them do not attain the elevation of a single mile above
the level of the sea.
Computation of the Average Annual Amount of Subaërial Denudation.—The
geology of the district above alluded to may assist our imagination in
conceiving the extent to which groups of ancient rocks, each of which
may in their turn have formed continents and oceanic basins, have been
disturbed, folded, and denuded even in the course of a few out of many
of those geological periods to which our imperfect records relate. It
is not easy for us to overestimate the effects which causes in every
day action must produce when the multiplying power of time is taken
into account.
Attempts were made by Manfredi in 1736, and afterwards by Playfair in
1802, to calculate the time which it would require to enable the rivers
to deliver over the whole of the land into the basin of the ocean. The
data were at first too imperfect and vague to allow them even to
approximate to safe conclusions. But in our own time similar
investigations have been renewed with more prospect of success, the
amount brought down by many large rivers to the sea having been more
accurately ascertained. Mr. Alfred Tylor, in 1850, inferred that the
quantity of detritus now being distributed over the sea-bottom would,
at the end of 10,000 years, cause an elevation of the sea-level to the
extent of at least three inches.[2] Subsequently Mr. Croll, in 1867,
and again, with more exactness, in 1868, deduced from the latest
measurement of the sediment transported by European and American rivers
the rate of subaërial denudation to which the surface of large
continents is exposed, taking especially the hydrographical basin of
the Mississippi as affording the best available measure of the average
waste of the land. The conclusion arrived at in his able memoir,[3] was
that the whole terrestrial surface is denuded at the rate of one foot
in 6000 years and this opinion was simultaneously enforced by his
fellow-labourer, Mr. Geikie, who, being jointly engaged in the same
line of inquiry, published a luminous essay on the subject in 1868.
The student, by referring to my “Principles of Geology,”[4] may see
that Messrs. Humphrey and Abbot, during their survey of the
Mississippi, attempted to make accurate measurements of the proportion
of sediment carried down annually to the sea by that river, including
not only the mud held in suspension, but also the sand and gravel
forced along the bottom.
It is evident that when we know the dimensions of the area which is
drained, and the annual quantity of earthy matter taken from it and
borne into the sea, we can affirm how much on an average has been
removed from the general surface in one year, and there seems no danger
of our overrating the mean rate of waste by selecting the Mississippi
as our example, for that river drains a country equal to more than half
the continent of Europe, extends through twenty degrees of latitude,
and therefore through regions enjoying a great variety of climate, and
some of its tributaries descend from mountains of great height. The
Mississippi is also more likely to afford us a fair test of ordinary
denudation, because, unlike the St. Lawrence and its tributaries, there
are no great lakes in which the fluviatile sediment is thrown down and
arrested in its way to the sea. In striking a general average we have
to remember that there are large deserts in which there is scarcely any
rainfall, and tracts which are as rainless as parts of Peru, and these
must not be neglected as counterbalancing others, in the tropics, where
the quantity of rain is in excess. If then, argues Mr. Geikie, we
assume that the Mississippi is lowering the surface of the great basin
which it drains at the rate of one foot in 6000 years, 10 feet in
60,000 years, 100 feet in 600,000 years, and 1000 feet in 6,000,000
years, it would not require more than about 4,500,000 years to wear
away the whole of the North American continent if its mean height is
correctly estimated by Humboldt at 748 feet. And if the mean height of
all the land now above the sea throughout the globe is 1000 feet, as
some geographers believe, it would only require six million years to
subject a mass of rock equal in volume to the whole of the land to the
action of subaërial denudation. It may be objected that the annual
waste is partial, and not equally derived from the general surface of
the country, inasmuch as plains, water-sheds, and level ground at all
heights remain comparatively unaltered; but this, as Mr. Geikie has
well pointed out, does not affect our estimate of the sum total of
denudation. The amount remains the same, and if we allow too little for
the loss from the surface of table-lands we only increase the
proportion of the loss sustained by the sides and bottoms of the
valleys, and _vice versa._[5]
Antagonism of Volcanic Force to the Levelling Power of Running
Water.—In all these estimates it is assumed that the entire quantity of
land above the sea-level remains on an average undiminished in spite of
annual waste. Were it otherwise the subaërial denudation would be
continually lessened by the diminution of the height and dimensions of
the land exposed to waste. Unfortunately we have as yet no accurate
data enabling us to measure the action of that force by which the
inequalities of the surface of the earth’s crust may be restored, and
the height of the continents and depth of the seas made to continue
unimpaired. I stated in 1830 in the “Principles of Geology,”[6] that
running water and volcanic action are two antagonistic forces; the one
labouring continually to reduce the whole of the land to the level of
the sea, the other to restore and maintain the inequalities of the
crust on which the very existence of islands and continents depends. I
stated, however, that when we endeavour to form some idea of the
relation of these destroying and renovating forces, we must always bear
in mind that it is not simply by upheaval that subterranean movements
can counteract the levelling force of running water. For whereas the
transportation of sediment from the land to the ocean would raise the
general sea-level, the subsidence of the sea-bottom, by increasing its
capacity, would check this rise and prevent the submergence of the
land. I have, indeed, endeavoured to show that unless we assume that
there is, on the whole, more subsidence than upheaval, we must suppose
the diameter of the planet to be always increasing, by that quantity of
volcanic matter which is annually poured out in the shape of lava or
ashes, whether on the land or in the bed of the sea, and which is
derived from the interior of the earth. The abstraction of this matter
causes, no doubt, subterranean vacuities and a corresponding giving way
of the surface; if it were not so, the average density of parts of the
interior would be always lessening and the size of the planet
increasing.[7]
Our inability to estimate the amount or direction of the movements due
to volcanic power by no means renders its efficacy as a land-preserving
force in past times a mere matter of conjecture. The student will see
in Chapter XXIV that we have proofs of Carboniferous forests hundreds
of miles in extent which grew on the lowlands or deltas near the sea,
and which subsided and gave place to other forests, until in some
regions fluviatile and shallow-water strata with occasional seams of
coal were piled one over the other, till they attained a thickness of
many thousand feet. Such accumulations, observed in Great Britain and
America on opposite sides of the Atlantic, imply the long-continued
existence of land vegetation, and of rivers draining a former continent
placed where there is now deep sea.
It will be also seen in Chapter XXV that we have evidence of a rich
terrestrial flora, the Devonian, even more ancient than the
Carboniferous; while on the other hand, the later Triassic, Oolitic,
Cretaceous, and successive Tertiary periods have all supplied us with
fossil plants, insects, or terrestrial mammalia; showing that, in spite
of great oscillations of level and continued changes in the position of
land and sea, the volcanic forces have maintained a due proportion of
dry land. We may appeal also to fresh-water formations, such as the
Purbeck and Wealden, to prove that in the Oolitic and Neocomian eras
there were rivers draining ancient lands in Europe in times when we
know that other spaces, now above water, were submerged.
How far the Transfer of Sediment from the Land to a Neighbouring
Sea-bottom may affect Subterranean Movements.—Little as we understand
at present the laws which govern the distribution of volcanic heat in
the interior and crust of the globe, by which mountain chains, high
table-lands, and the abysses of the ocean are formed, it seems clear
that this heat is the prime mover on which all the grander features in
the external configuration of the planet depend.
It has been suggested that the stripping off by denudation of dense
masses from one part of a continent and the delivery of the same into
the bed of the ocean must have a decided effect in causing changes of
temperature in the earth’s crust below, or, in other words, in causing
the subterranean isothermals to shift their position. If this be so,
one part of the crust may be made to rise, and another to sink, by the
expansion and contraction of the rocks, of which the temperature is
altered.
I cannot, at present, discuss this subject, of which I have treated
more fully elsewhere,[8] but may state here that I believe this
transfer of sediment to play a very subordinate part in modifying those
movements on which the configuration of the earth’s crust depends. In
order that strata of shallow-water origin should be able to attain a
thickness of several thousand feet, and so come to exert a considerable
downward pressure, there must have been first some independent and
antecedent causes at work which have given rise to the incipient
shallow receptacle in which the sediment began to accumulate. The same
causes there continuing to depress the sea-bottom, room would be made
for fresh accessions of sediment, and it would only be by a long
repetition of the depositing process that the new matter could acquire
weight enough to affect the temperature of the rocks far below, so as
to increase or diminish their volume.
Permanence of Continental and Oceanic Areas.—If the thickness of more
than 40,000 feet of sedimentary strata before alluded to in the
Appalachians proves a preponderance of downward movements in Palæozoic
times in a district now forming the eastern border of North America, it
also proves, as before hinted, the continued existence and waste of
some neighbouring continent, probably formed of Laurentian rocks, and
situated where the Atlantic now prevails. Such an hypothesis would be
in perfect harmony with the conclusions forced upon us by the study of
the present configuration of our continents, and the relation of their
height to the depth of the oceanic basins; also to the considerable
elevation and extent sometimes reached by drift containing shells of
recent species, and still more by the fact of sedimentary strata,
several thousand feet thick, as those of central Sicily, or such as
flank the Alps and Apennines, containing fossil Mollusca sometimes
almost wholly identical with species still living.
I have remarked elsewhere[9] that upward and downward movements of 1000
feet or more would turn much land into sea and sea into land in the
continental areas and their borders, whereas oscillations of equal
magnitude would have no corresponding effect in the bed of the ocean
generally, believed as it is to have a mean depth of 15,000 feet, and
which, whether this estimate be correct or not, is certainly of great
profundity. Subaërial denudation would not of itself lessen the area of
the land, but would tend to fill up with sediment seas of moderate
depth adjoining the coast. The coarser matter falls to the bottom near
the shore in the first still water which it reaches, and whenever the
sea-bottom on which this matter has been thrown is slightly elevated,
it becomes land, and an upheaval of a thousand feet causes it to attain
the mean elevation of continents in general.
Suppose, therefore, we had ascertained that the triturating power of
subaërial denudation might in a given time—in three, or six, or a
greater number of millions of years—pulverise a volume of rock equal in
dimensions to all the present land, we might yet find, could we revisit
the earth at the end of such a period, that the continents occupied
very much the same position which they held before; we should find the
rivers employed in carrying down to the sea the very same mud, sand,
and pebbles with which they had been charged in our own time, the
superficial alluvial matter as well as a great thickness of sedimentary
strata would inclose shells, all or a great part of which we should
recognise as specifically identical with those already known to us as
living. Every geologist is aware that great as have been the
geographical changes in the northern hemisphere since the commencement
of the Glacial Period, there having been submergence and re-emergence
of land to the extent of 1000 feet vertically, and in the temperate
latitudes great vicissitudes of climate, the marine mollusca have not
changed, and the same drift which had been carried down to the sea at
the beginning of the period is now undergoing a second transportation
in the same direction.
As when we have measured a fraction of time in an hour-glass we have
only to reverse the position of our chronometer and we make the same
sand measure over again the duration of a second equal period, so when
the volcanic force has remoulded the form of a continent and the
adjoining sea-bottom, the same materials are made to do duty a second
time. It is true that at each oscillation of level the solid rocks
composing the original continent suffer some fresh denudation, and do
not remain unimpaired like the wooden and glass framework of the
hour-glass, still the wear and tear suffered by the larger area exposed
to subaërial denudation consists either of loose drift or of
sedimentary strata, which were thrown down in seas near the land, and
subsequently upraised, the same continents and oceanic basins remaining
in existence all the while.
From all that we know of the extreme slowness of the upward and
downward movements which bring about even slight geographical changes,
we may infer that it would require a long succession of geological
periods to cause the submarine and supramarine areas to change places,
even if the ascending movements in the one region and the descending in
the other were continuously in one direction. But we have only to
appeal to the structure of the Alps, where there are so many shallow
and deep water formations of various ages crowded into a limited area,
to convince ourselves that mountain chains are the result of great
oscillations of level. High land is not produced simply by uniform
upheaval, but by a predominance of elevatory over subsiding movements.
Where the ocean is extremely deep it is because the sinking of the
bottom has been in excess, in spite of interruptions by upheaval.
Yet persistent as may be the leading features of land and sea on the
globe, they are not immutable. Some of the finest mud is doubtless
carried to indefinite distances from the coast by marine currents, and
we are taught by deep-sea dredgings that in clear water at depths
equalling the height of the Alps organic beings may flourish, and their
spoils slowly accumulate on the bottom. We also occasionally obtain
evidence that submarine volcanoes are pouring out ashes and streams of
lava in mid-ocean as well as on land (see Principles, vol. ii, p. 64),
and that wherever mountains like Etna, Vesuvius, and the Canary Islands
are now the site of eruptions, there are signs of accompanying
upheaval, by which beds of ashes full of recent marine shells have been
uplifted many hundred feet. We need not be surprised, therefore, if we
learn from geology that the continents and oceans were not always
placed where they now are, although the imagination may well be
overpowered when it endeavours to contemplate the quantity of time
required for such revolutions.
We shall have gained a great step if we can approximate to the number
of millions of years in which the average aqueous denudation going on
upon the land would convey seaward a quantity of matter equal to the
average volume of our continents, and this might give us a gauge of the
minimum of volcanic force necessary to counteract such levelling power
of running water; but to discover a relation between these great
agencies and the rate at which species of organic beings vary, is at
present wholly beyond the reach of our computation, though perhaps it
may not prove eventually to transcend the powers of man.
[1] Hull, Quart. Geol. Journ., vol. xxiv, p. 322, 1868.
[2] Tylor, Phil. Mag., 4th series, p. 268, 1850.
[3] Croll, Phil. Mag., 1868, p. 381.
[4] Vol. i, p. 442, 1867.
[5] Trans. Geol. Soc. Glasgow, vol. iii, p. 169.
[6] 1st ed., chap. x, p. 167, 1830; see also 10th ed., vol. i, chap.
xv, p. 327, 1867.
[7] Principles, vol. ii, p. 237; also 1st ed., p. 447, 1830.
[8] Principles, vol. ii, p. 229, 1868.
[9] Principles, vol. i, p. 265, 1867.
CHAPTER VIII.
CHRONOLOGICAL CLASSIFICATION OF ROCKS.
Aqueous, Plutonic, volcanic, and metamorphic Rocks considered
chronologically. — Terms Primary, Secondary, and Tertiary; Palæozoic,
Mesozoic, and Cainozoic explained. — On the different Ages of the
aqueous Rocks. — Three principal Tests of relative Age: Superposition,
Mineral Character, and Fossils. — Change of Mineral Character and
Fossils in the same continuous Formation. — Proofs that distinct
Species of Animals and Plants have lived at successive Periods. —
Distinct Provinces of indigenous Species. — Great Extent of single
Provinces. — Similar Laws prevailed at successive Geological Periods. —
Relative Importance of mineral and palæontological Characters. — Test
of Age by included Fragments. — Frequent Absence of Strata of
intervening Periods. — Tabular Views of fossiliferous Strata.
Chronology of Rocks.— In the first chapter it was stated that the four
great classes of rocks, the aqueous, the volcanic, the Plutonic, and
the metamorphic, would each be considered not only in reference to
their mineral characters, and mode of origin, but also to their
relative age. In regard to the aqueous rocks, we have already seen that
they are stratified, that some are calcareous, others argillaceous or
siliceous, some made up of sand, others of pebbles; that some contain
fresh-water, others marine fossils, and so forth; but the student has
still to learn which rocks, exhibiting some or all of these characters,
have originated at one period of the earth’s history, and which at
another.
To determine this point in reference to the fossiliferous formations is
more easy than in any other class, and it is therefore the most
convenient and natural method to begin by establishing a chronology for
these strata, and then to refer as far as possible to the same
divisions, the several groups of Plutonic, volcanic, and metamorphic
rocks. Such a system of classification is not only recommended by its
greater clearness and facility of application, but is also best fitted
to strike the imagination by bringing into one view the contemporaneous
revolutions of the inorganic and organic creations of former times. For
the sedimentary formations are most readily distinguished by the
different species of fossil animals and plants which they inclose, and
of which one assemblage after another has flourished and then
disappeared from the earth in succession.
In the present work, therefore, the four great classes of rocks, the
aqueous, Plutonic, volcanic, and metamorphic, will form four parallel,
or nearly parallel, columns in one chronological table. They will be
considered as four sets of monuments relating to four contemporaneous,
or nearly contemporaneous, series of events. I shall endeavour, in a
subsequent chapter on the Plutonic rocks, to explain the manner in
which certain masses belonging to each of the four classes of rocks may
have originated simultaneously at every geological period, and how the
earth’s crust may have been continually remodelled, above and below, by
aqueous and igneous causes, from times indefinitely remote. In the same
manner as aqueous and fossiliferous strata are now formed in certain
seas or lakes, while in other places volcanic rocks break out at the
surface, and are connected with reservoirs of melted matter at vast
depths in the bowels of the earth, so, at every era of the past,
fossiliferous deposits and superficial igneous rocks were in progress
contemporaneously with others of subterranean and Plutonic origin, and
some sedimentary strata were exposed to heat, and made to assume a
crystalline or metamorphic structure.
It can by no means be taken for granted, that during all these changes
the solid crust of the earth has been increasing in thickness. It has
been shown, that so far as aqueous action is concerned, the gain by
fresh deposits, and the loss by denudation, must at each period have
been equal (see above, Chap. VI, p. 96); and in like manner, in the
inferior portion of the earth’s crust, the acquisition of new
crystalline rocks, at each successive era, may merely have
counterbalanced the loss sustained by the melting of materials
previously consolidated. As to the relative antiquity of the
crystalline foundations of the earth’s crust, when compared to the
fossiliferous and volcanic rocks which they support, I have already
stated, in the first chapter, that to pronounce an opinion on this
matter is as difficult as at once to decide which of the two, whether
the foundations or superstructure of an ancient city built on wooden
piles may be the oldest. We have seen that, to answer this question, we
must first be prepared to say whether the work of decay and restoration
had gone on most rapidly above or below; whether the average duration
of the piles has exceeded that of the buildings, or the contrary. So
also in regard to the relative age of the superior and inferior
portions of the earth’s crust; we cannot hazard even a conjecture on
this point, until we know whether, upon an average, the power of water
above, or that of heat below, is most efficacious in giving new forms
to solid matter.
The early geologists gave to all the crystalline and non-fossiliferous
rocks the name of Primitive or Primary, under the idea that they were
formed anterior to the appearance of life upon the earth, while the
aqueous or fossiliferous strata were termed Secondary, and alluviums or
other superficial deposits, Tertiary. The meaning of these terms, has,
however, been gradually modified with advancing knowledge, and they are
now used to designate three great chronological divisions under which
all geological formations can be classed, each of them being
characterised by the presence of distinctive groups of organic remains
rather than by any mechanical peculiarities of the strata themselves.
If, therefore, we retain the term “primary,” it must not be held to
designate a set of crystalline rocks some of which have been proved to
be even of Tertiary age, but must be applied to all rocks older than
the secondary formations. Some geologists, to avoid misapprehension,
have introduced the term Palæozoic for primary, from _palaion,_
“ancient,” and _zoon,_ “an organic being,” still retaining the terms
secondary and tertiary; Mr. Phillips, for the sake of uniformity, has
proposed Mesozoic, for secondary, from _mesos,_ “middle,” etc.; and
Cainozoic, for tertiary, from _kainos,_ “recent,” etc.; but the terms
primary, secondary, and tertiary have the claim of priority in their
favour, and are of corresponding value.
It may perhaps be suggested that some metamorphic strata, and some
granites, may be anterior in date to the oldest of the primary
fossiliferous rocks. This opinion is doubtless true, and will be
discussed in future chapters; but I may here observe, that when we
arrange the four classes of rocks in four parallel columns in one table
of chronology, it is by no means assumed that these columns are all of
equal length; one may begin at an earlier period than the rest, and
another may come down to a later point of time, and we may not be yet
acquainted with the most ancient of the primary fossiliferous beds, or
with the newest of the hypogene.
For reasons already stated, I proceed first to treat of the aqueous or
fossiliferous formations considered in chronological order or in
relation to the different periods at which they have been deposited.
There are three principal tests by which we determine the age of a
given set of strata; first, superposition; secondly, mineral character;
and, thirdly, organic remains. Some aid can occasionally be derived
from a fourth kind of proof, namely, the fact of one deposit including
in it fragments of a pre-existing rock, by which the relative ages of
the two may, even in the absence of all other evidence, be determined.
Superposition.—The first and principal test of the age of one aqueous
deposit, as compared to another, is relative position. It has been
already stated, that, where strata are horizontal, the bed which lies
uppermost is the newest of the whole, and that which lies at the bottom
the most ancient. So, of a series of sedimentary formations, they are
like volumes of history, in which each writer has recorded the annals
of his own times, and then laid down the book, with the last written
page uppermost, upon the volume in which the events of the era
immediately preceding were commemorated. In this manner a lofty pile of
chronicles is at length accumulated; and they are so arranged as to
indicate, by their position alone, the order in which the events
recorded in them have occurred.
In regard to the crust of the earth, however, there are some regions
where, as the student has already been informed, the beds have been
disturbed, and sometimes extensively thrown over and turned upside
down. (See p. 73, p. 87.) But an experienced geologist can rarely be
deceived by these exceptional cases. When he finds that the strata are
fractured, curved, inclined, or vertical, he knows that the original
order of superposition must be doubtful, and he then endeavours to find
sections in some neighbouring district where the strata are horizontal,
or only slightly inclined. Here, the true order of sequence of the
entire series of deposits being ascertained, a key is furnished for
settling the chronology of those strata where the displacement is
extreme.
Mineral Character.—The same rocks may often be observed to retain for
miles, or even hundreds of miles, the same mineral peculiarities, if we
follow the planes of stratification, or trace the beds, if they be
undisturbed, in a horizontal direction. But if we pursue them
vertically, or in any direction transverse to the planes of
stratification, this uniformity ceases almost immediately. In that case
we can scarcely ever penetrate a stratified mass for a few hundred
yards without beholding a succession of extremely dissimilar rocks,
some of fine, others of coarse grain, some of mechanical, others of
chemical origin; some calcareous, others argillaceous, and others
siliceous. These phenomena lead to the conclusion that rivers and
currents have dispersed the same sediment over wide areas at one
period, but at successive periods have been charged, in the same
region, with very different kinds of matter. The first observers were
so astonished at the vast spaces over which they were able to follow
the same homogeneous rocks in a horizontal direction, that they came
hastily to the opinion, that the whole globe had been environed by a
succession of distinct aqueous formations, disposed round the nucleus
of the planet, like the concentric coats of an onion. But, although, in
fact, some formations may be continuous over districts as large as half
of Europe, or even more, yet most of them either terminate wholly
within narrower limits, or soon change their lithological character.
Sometimes they thin out gradually, as if the supply of sediment had
failed in that direction, or they come abruptly to an end, as if we had
arrived at the borders of the ancient sea or lake which served as their
receptacle. It no less frequently happens that they vary in mineral
aspect and composition, as we pursue them horizontally. For example, we
trace a limestone for a hundred miles, until it becomes more
arenaceous, and finally passes into sand, or sandstone. We may then
follow this sandstone, already proved by its continuity to be of the
same age, throughout another district a hundred miles or more in
length.
Organic Remains.—This character must be used as a criterion of the age
of a formation, or of the contemporaneous origin of two deposits in
distant places, under very much the same restrictions as the test of
mineral composition.
First, the same fossils may be traced over wide regions, if we examine
strata in the direction of their planes, although by no means for
indefinite distances. Secondly, while the same fossils prevail in a
particular set of strata for hundreds of miles in a horizontal
direction, we seldom meet with the same remains for many fathoms, and
very rarely for several hundred yards, in a vertical line, or a line
transverse to the strata. This fact has now been verified in almost all
parts of the globe, and has led to a conviction that at successive
periods of the past, the same area of land and water has been inhabited
by species of animals and plants even more distinct than those which
now people the antipodes, or which now co-exist in the arctic,
temperate, and tropical zones. It appears that from the remotest
periods there has been ever a coming in of new organic forms, and an
extinction of those which pre-existed on the earth; some species having
endured for a longer, others for a shorter, time; while none have ever
reappeared after once dying out. The law which has governed the
succession of species, whether we adopt or reject the theory of
transmutation, seems to be expressed in the verse of the poet:—
Natura il fece, e poi ruppe la stampa. ARIOSTO.
Nature made him, and then broke the die.
And this circumstance it is, which confers on fossils their highest
value as chronological tests, giving to each of them, in the eyes of
the geologist, that authority which belongs to contemporary medals in
history.
The same cannot be said of each peculiar variety of rock; for some of
these, as red marl and red sandstone, for example, may occur at once at
the top, bottom, and middle of the entire sedimentary series;
exhibiting in each position so perfect an identity of mineral aspect as
to be undistinguishable. Such exact repetitions, however, of the same
mixtures of sediment have not often been produced, at distant periods,
in precisely the same parts of the globe; and even where this has
happened, we are seldom in any danger of confounding together the
monuments of remote eras, when we have studied their imbedded fossils
and their relative position.
Zoological Provinces.—It was remarked that the same species of organic
remains cannot be traced horizontally, or in the direction of the
planes of stratifications for indefinite distances. This might have
been expected from analogy; for when we inquire into the present
distribution of living beings, we find that the habitable surface of
the sea and land may be divided into a considerable number of distinct
provinces, each peopled by a peculiar assemblage of animals and plants.
In the “Principles of Geology,” I have endeavoured to point out the
extent and probable origin of these separate divisions; and it was
shown that climate is only one of many causes on which they depend, and
that difference of longitude as well as latitude is generally
accompanied by a dissimilarity of indigenous species.
As different seas, therefore, and lakes are inhabited, at the same
period, by different aquatic animals and plants, and as the lands
adjoining these may be peopled by distinct terrestrial species, it
follows that distinct fossils will be imbedded in contemporaneous
deposits. If it were otherwise—if the same species abounded in every
climate, or in every part of the globe where, so far as we can
discover, a corresponding temperature and other conditions favourable
to their existence are found—the identification of mineral masses of
the same age, by means of their included organic contents, would be a
matter of still greater certainty.
Nevertheless, the extent of some single zoological provinces,
especially those of marine animals, is very great; and our geological
researches have proved that the same laws prevailed at remote periods;
for the fossils are often identical throughout wide spaces, and in
detached deposits, consisting of rocks varying entirely in their
mineral nature.
The doctrine here laid down will be more readily understood, if we
reflect on what is now going on in the Mediterranean. That entire sea
may be considered as one zoological province; for although certain
species of testacea and zoophytes may be very local, and each region
has probably some species peculiar to it, still a considerable number
are common to the whole Mediterranean. If, therefore, at some future
period, the bed of this inland sea should be converted into land, the
geologist might be enabled, by reference to organic remains, to prove
the contemporaneous origin of various mineral masses scattered over a
space equal in area to half of Europe.
Deposits, for example, are well known to be now in progress in this sea
in the deltas of the Po, Rhone, Nile, and other rivers, which differ as
greatly from each other in the nature of their sediment as does the
composition of the mountains which their drain. There are also other
quarters of the Mediterranean, as off the coast of Campania, or near
the base of Etna, in Sicily, or in the Grecian Archipelago, where
another class of rocks is now forming; where showers of volcanic ashes
occasionally fall into the sea, and streams of lava overflow its
bottom; and where, in the intervals between volcanic eruptions, beds of
sand and clay are frequently derived from the waste of cliffs, or the
turbid waters of rivers. Limestones, moreover, such as the Italian
travertins, are here and there precipitated from the waters of mineral
springs, some of which rise up from the bottom of the sea. In all these
detached formations, so diversified in their lithological characters,
the remains of the same shells, corals, crustacea, and fish are
becoming inclosed; or, at least, a sufficient number must be common to
the different localities to enable the zoologist to refer them all to
one contemporaneous assemblage of species.
There are, however, certain combinations of geographical circumstances
which cause distinct provinces of animals and plants to be separated
from each other by very narrow limits; and hence it must happen that
strata will be sometimes formed in contiguous regions, differing widely
both in mineral contents and organic remains. Thus, for example, the
testacea, zoophytes, and fish of the Red Sea are, as a group, extremely
distinct from those inhabiting the adjoining parts of the
Mediterranean, although the two seas are separated only by the narrow
isthmus of Suez. Calcareous formations have accumulated on a great
scale in the Red Sea in modern times, and fossil shells of existing
species are well preserved therein; and we know that at the mouth of
the Nile large deposits of mud are amassed, including the remains of
Mediterranean species. It follows, therefore, that if at some future
period the bed of the Red Sea should be laid dry, the geologist might
experience great difficulties in endeavouring to ascertain the relative
age of these formations, which, although dissimilar both in organic and
mineral characters, were of synchronous origin.
But, on the other hand, we must not forget that the north-western
shores of the Arabian Gulf, the plains of Egypt, and the Isthmus of
Suez, are all parts of one province of _terrestrial_ species. Small
streams, therefore, occasional land- floods, and those winds which
drift clouds of sand along the deserts, might carry down into the Red
Sea the same shells of fluviatile and land testacea which the Nile is
sweeping into its delta, together with some remains of terrestrial
plants and the bones of quadrupeds, whereby the groups of strata before
alluded to might, notwithstanding the discrepancy of their mineral
composition and _marine_ organic fossils, be shown to have belonged to
the same epoch.
Yet, while rivers may thus carry down the same fluviatile and
terrestrial spoils into two or more seas inhabited by different marine
species, it will much more frequently happen that the coexistence of
terrestrial species of distinct zoological and botanical provinces will
be proved by the identity of the marine beings which inhabited the
intervening space. Thus, for example, the land quadrupeds and shells of
the valley of the Mississippi, of central America, and of the West
India islands differ very considerably, yet their remains are all
washed down by rivers flowing from these three zoological provinces
into the Gulf of Mexico.
In some parts of the globe, at the present period, the line of
demarkation between distinct provinces of animals and plants is not
very strongly marked, especially where the change is determined by
temperature, as it is in seas extending from the temperate to the
tropical zone, or from the temperate to the arctic regions. Here a
gradual passage takes place from one set of species to another. In like
manner the geologist, in studying particular formations of remote
periods, has sometimes been able to trace the gradation from one
ancient province to another, by observing carefully the fossils of all
the intermediate places. His success in thus acquiring a knowledge of
the zoological or botanical geography of very distant eras has been
mainly owing to this circumstance, that the mineral character has no
tendency to be affected by climate. A large river may convey yellow or
red mud into some part of the ocean, where it may be dispersed by a
current over an area several hundred leagues in length, so as to pass
from the tropics into the temperate zone. If the bottom of the sea be
afterwards upraised, the organic remains imbedded in such yellow or red
strata may indicate the different animals or plants which once
inhabited at the same time the temperate and equatorial regions.
It may be true, as a general rule, that groups of the same species of
animals and plants may extend over wider areas than deposits of
homogeneous composition; and if so, palæontological characters will be
of more importance in geological classification than the test of
mineral composition; but it is idle to discuss the relative value of
these tests, as the aid of both is indispensable, and it fortunately
happens, that where the one criterion fails, we can often avail
ourselves of the other.
Test by included Fragments of older Rocks.—It was stated, that proof
may sometimes be obtained of the relative date of two formations by
fragments of an older rock being included in a newer one. This evidence
may sometimes be of great use, where a geologist is at a loss to
determine the relative age of two formations from want of clear
sections exhibiting their true order of position, or because the strata
of each group are vertical. In such cases we sometimes discover that
the more modern rock has been in part derived from the degradation of
the older. Thus, for example, we may find chalk in one part of a
country, and in another strata of clay, sand, and pebbles. If some of
these pebbles consist of that peculiar flint, of which layers more or
less continuous are characteristic of the chalk, and which include
fossil shells, sponges, and foraminifera of cretaceous species, we may
confidently infer that the chalk was the oldest of the two formations.
Chronological Groups.—The number of groups into which the fossiliferous
strata may be separated are more or less numerous, according to the
views of classification which different geologists entertain; but when
we have adopted a certain system of arrangement, we immediately find
that a few only of the entire series of groups occur one upon the other
in any single section or district.
The thinning out of individual strata was before described (p. 42).But
let the diagram (Fig. 84) represent seven fossiliferous groups, instead
of as many strata. It will then be seen that in the middle all the
superimposed formations are present; but in consequence of some of them
thinning out, No. 2 and No. 5 are absent at one extremity of the
section, and No. 4 at the other.
Fig. 84: Seven fossiliferous groups.
In another diagram (Fig. 85), a real section of the geological
formations in the neighbourhood of Bristol and the Mendip Hills is
presented to the reader, as laid down on a true scale by Professor
Ramsay, where the newer groups 1, 2, 3, 4 rest unconformably on the
formations 5, 6, 7 and 8. At the southern end of the line of section we
meet with the beds No. 3 (the New Red Sandstone) resting immediately on
Nos. 7 and 8, while farther north as at Dundry Hill in Somersetshire,
we behold eight groups superimposed one upon the other, comprising all
the strata from the inferior Oolite, No. 1, to the coal and
carboniferous limestone. The limited horizontal extension of the groups
1 and 2 is owing to denudation, as these formations end abruptly, and
have left outlying patches to attest the fact of their having
originally covered a much wider area.
Section South of Bristol.
In order, therefore, to establish a chronological succession of
fossiliferous groups, a geologist must begin with a single section in
which several sets of strata lie one upon the other. He must then trace
these formations, by attention to their mineral character and fossils,
continuously, as far as possible, from the starting-point. As often as
he meets with new groups, he must ascertain by superposition their age
relatively to those first examined, and thus learn how to intercalate
them in a tabular arrangement of the whole.
By this means the German, French, and English geologists have
determined the succession of strata throughout a great part of Europe,
and have adopted pretty generally the following groups, almost all of
which have their representatives in the British Islands.
Abridged General Table of Fossiliferous Strata.
TABULAR VIEW OF THE FOSSILIFEROUS STRATA,
SHOWING THE ORDER OF SUPERPOSITION OR CHRONOLOGICAL SUCCESSION OF THE
PRINCIPAL GROUPS DESCRIBED IN THIS WORK.
POST-TERTIARY
EXAMPLES
POST-TERTIARY
EXAMPLES
POST-
TERTIARY 1.
RECENT
Shells and mammals, all of living species. British
Clyde marine strata, with canoes (p.
146).
Foreign
Danish kitchen middens (p.
146).
Lacustrine mud, with remains of Swiss lake-dwellings (p. 148).
Marine strata inclosing Temple of Serapis, at Puzzuoli (p. 146).
2.
POST-
PLIOCENE.
Shells, recent mammalia in part extinct. British
Loam of Brixham cave, with flint implements and bones of extinct
and living quadrupeds (p.
157)
Drift near Salisbury, with bones of mammoth, Spermophilus, and
stone implements (p. 161).
Glacial drift of Scotland, with marine shells and remains of
mammoth (p. 176.
Erratics of Pagham and Selsey Bill (p. 182).
Glacial drift of Wales, with marine fossil shells, about 1400
feet high, on Moel Tryfaen (p.
181).
Foreign
Dordogne caves of the reindeer period (p. 150).
Older valley-gravels of Amiens, with flint implements and bones
of extinct mammalia (p.
152).
Loess of Rhine (p. 154).
Ancient Nile-mud forming river-terraces (p. 154).
Loam and breccia of Liege caverns, with human remains (pp. 156, 157).
Australian cave breccias, with bones of extinct marsupials (p. 158).
Glacial drift of Northern Europe (p.
166, p. 174).
TERTIARY OR CAINOZOIC