The Student's Elements of Geology 7
The changes which fossil organic bodies have undergone since they were first
imbedded in rocks, throw much light on the consolidation of strata. Fossil
shells in some modern deposits have been scarcely altered in the course of
centuries, having simply lost a part of their animal matter. But in other cases
the shell has disappeared, and left an impression only of its exterior, or,
secondly, a cast of its interior form, or, thirdly, a cast of the shell itself,
the original matter of which has been removed. These different forms of
fossilisation may easily be understood if we examine the mud recently thrown out
from a pond or canal in which there are shells. If the mud be argillaceous, it
acquires consistency on drying, and on breaking open a portion of it we find
that each shell has left impressions of its external form. If we then remove the
shell itself, we find within a solid nucleus of clay, having the form of the
interior of the shell. This form is often very different from that of the outer
shell. Thus a cast such as a, Figure 51, commonly called a fossil screw, would
never be suspected by an inexperienced conchologist to be the internal shape of
the fossil univalve, b, Figure 51. Nor should we have imagined at first sight
that the shell a and the cast b, Figure 52, belong to one and the same fossil.
The reader will observe, in the last-mentioned figure (b, Figure 52), that an
empty space shaded dark, which the SHELL ITSELF once occupied, now intervenes
between the enveloping stone and the cast of the smooth interior of the whorls.
In such cases the shell has been dissolved and the component particles removed
by water percolating the rock. If the nucleus were taken out, a hollow mould
would remain, on which the external form of the shell with its tubercles and
striae, as seen in a, Figure 52, would be seen embossed. Now if the space
alluded to between the nucleus and the impression, instead of being left empty,
has been filled up with calcareous spar, flint, pyrites, or other mineral, we
then obtain from the mould an exact cast both of the external and internal form
of the original shell. In this manner silicified casts of shells have been
formed; and if the mud or sand of the nucleus happen to be incoherent, or
soluble in acid, we can then procure in flint an empty shell, which in shape is
the exact counterpart of the original. This cast may be compared to a bronze
statue, representing merely the superficial form, and not the internal
organisation; but there is another description of petrifaction by no means
uncommon, and of a much more wonderful kind, which may be compared to certain
anatomical models in wax, where not only the outward forms and features, but the
nerves, blood-vessels, and other internal organs are also shown. Thus we find
corals, originally calcareous, in which not only the general shape, but also the
minute and complicated internal organisation is retained in flint.
(FIGURE 53. Section of a tree from the coal-measures, magnified (Witham),
showing texture of wood.)
Such a process of petrifaction is still more remarkably exhibited in fossil
wood, in which we often perceive not only the rings of annual growth, but all
the minute vessels and medullary rays. Many of the minute cells and fibres of
plants, and even those spiral vessels which in the living vegetable can only be
discovered by the microscope, are preserved. Among many instances, I may mention
a fossil tree, seventy-two feet in length, found at Gosforth, near Newcastle, in
sandstone strata associated with coal. By cutting a transverse slice so thin as
to transmit light, and magnifying it about fifty-five times, the texture, as
seen in Figure 53, is exhibited. A texture equally minute and complicated has
been observed in the wood of large trunks of fossil trees found in the
Craigleith quarry near Edinburgh, where the stone was not in the slightest
degree siliceous, but consisted chiefly of carbonate of lime, with oxide of
iron, alumina, and carbon. The parallel rows of vessels here seen are the rings
of annual growth, but in one part they are imperfectly preserved, the wood
having probably decayed before the mineralising matter had penetrated to that
portion of the tree.
In attempting to explain the process of petrifaction in such cases, we may first
assume that strata are very generally permeated by water charged with minute
portions of calcareous, siliceous, and other earths in solution. In what manner
they become so impregnated will be afterwards considered. If an organic
substance is exposed in the open air to the action of the sun and rain, it will
in time putrefy, or be dissolved into its component elements, consisting usually
of oxygen, hydrogen, nitrogen, and carbon. These will readily be absorbed by the
atmosphere or be washed away by rain, so that all vestiges of the dead animal or
plant disappear. But if the same substances be submerged in water, they
decompose more gradually; and if buried in earth, still more slowly; as in the
familiar example of wooden piles or other buried timber. Now, if as fast as each
particle is set free by putrefaction in a fluid or gaseous state, a particle
equally minute of carbonate of lime, flint, or other mineral, is at hand ready
to be precipitated, we may imagine this inorganic matter to take the place just
before left unoccupied by the organic molecule. In this manner a cast of the
interior of certain vessels may first be taken, and afterwards the more solid
walls of the same may decay and suffer a like transmutation. Yet when the whole
is lapidified, it may not form one homogeneous mass of stone or metal. Some of
the original ligneous, osseous, or other organic elements may remain mingled in
certain parts, or the lapidifying substance itself may be differently coloured
at different times, or so crystallised as to reflect light differently, and thus
the texture of the original body may be faithfully exhibited.
The student may perhaps ask whether, on chemical principles, we have any ground
to expect that mineral matter will be thrown down precisely in those spots where
organic decomposition is in progress? The following curious experiments may
serve to illustrate this point: Professor Goppert of Breslau, with a view of
imitating the natural process of petrifaction, steeped a variety of animal and
vegetable substances in waters, some holding siliceous, others calcareous,
others metallic matter in solution. He found that in the period of a few weeks,
or sometimes even days, the organic bodies thus immersed were mineralised to a
certain extent. Thus, for example, thin vertical slices of deal, taken from the
Scotch fir (Pinus sylvestris), were immersed in a moderately strong solution of
sulphate of iron. When they had been thoroughly soaked in the liquid for several
days they were dried and exposed to a red-heat until the vegetable matter was
burnt up and nothing remained but an oxide of iron, which was found to have
taken the form of the deal so exactly that casts even of the dotted vessels
peculiar to this family of plants were distinctly visible under the microscope.
The late Dr. Turner observes, that when mineral matter is in a "nascent state,"
that is to say, just liberated from a previous state of chemical combination, it
is most ready to unite with other matter, and form a new chemical compound.
Probably the particles or atoms just set free are of extreme minuteness, and
therefore move more freely, and are more ready to obey any impulse of chemical
affinity. Whatever be the cause, it clearly follows, as before stated, that
where organic matter newly imbedded in sediment is decomposing, there will
chemical changes take place most actively.
An analysis was lately made of the water which was flowing off from the rich mud
deposited by the Hooghly River in the Delta of the Ganges after the annual
inundation. This water was found to be highly charged with carbonic acid holding
lime in solution. (Piddington Asiatic Researches volume 18 page 226.) Now if
newly-deposited mud is thus proved to be permeated by mineral matter in a state
of solution, it is not difficult to perceive that decomposing organic bodies,
naturally imbedded in sediment, may as readily become petrified as the
substances artificially immersed by Professor Goppert in various fluid mixtures.
It is well known that the waters of all springs are more or less charged with
earthy, alkaline, or metallic ingredients derived from the rocks and mineral
veins through which they percolate. Silex is especially abundant in hot springs,
and carbonate of lime is almost always present in greater or less quantity. The
materials for the petrifaction of organic remains are, therefore, usually at
hand in a state of chemical solution wherever organic remains are imbedded in
new strata.
CHAPTER V.
ELEVATION OF STRATA ABOVE THE SEA.-- HORIZONTAL AND INCLINED STRATIFICATION.
Why the Position of Marine Strata, above the Level of the Sea, should be
referred to the rising up of the Land, not to the going down of the Sea.
Strata of Deep-sea and Shallow-water Origin alternate.
Also Marine and Fresh-water Beds and old Land Surfaces.
Vertical, inclined, and folded Strata.
Anticlinal and Synclinal Curves.
Theories to explain Lateral Movements.
Creeps in Coal-mines.
Dip and Strike.
Structure of the Jura.
Various Forms of Outcrop.
Synclinal Strata forming Ridges.
Connection of Fracture and Flexure of Rocks.
Inverted Strata.
Faults described.
Superficial Signs of the same obliterated by Denudation.
Great Faults the Result of repeated Movements.
Arrangement and Direction of parallel Folds of Strata.
Unconformability.
Overlapping Strata.
LAND HAS BEEN RAISED, NOT THE SEA LOWERED.
It has been already stated that the aqueous rocks containing marine fossils
extend over wide continental tracts, and are seen in mountain chains rising to
great heights above the level of the sea (Chapter 1). Hence it follows, that
what is now dry land was once under water. But if we admit this conclusion, we
must imagine, either that there has been a general lowering of the waters of the
ocean, or that the solid rocks, once covered by water, have been raised up
bodily out of the sea, and have thus become dry land. The earlier geologists,
finding themselves reduced to this alternative, embraced the former opinion,
assuming that the ocean was originally universal, and had gradually sunk down to
its actual level, so that the present islands and continents were left dry. It
seemed to them far easier to conceive that the water had gone down, than that
solid land had risen upward into its present position. It was, however,
impossible to invent any satisfactory hypothesis to explain the disappearance of
so enormous a body of water throughout the globe, it being necessary to infer
that the ocean had once stood at whatever height marine shells might be
detected. It moreover appeared clear, as the science of geology advanced, that
certain spaces on the globe had been alternately sea, then land, then estuary,
then sea again, and, lastly, once more habitable land, having remained in each
of these states for considerable periods. In order to account for such phenomena
without admitting any movement of the land itself, we are required to imagine
several retreats and returns of the ocean; and even then our theory applies
merely to cases where the marine strata composing the dry land are horizontal,
leaving unexplained those more common instances where strata are inclined,
curved, or placed on their edges, and evidently not in the position in which
they were first deposited.
Geologists, therefore, were at last compelled to have recourse to the doctrine
that the solid land has been repeatedly moved upward or downward, so as
permanently to change its position relatively to the sea. There are several
distinct grounds for preferring this conclusion. First, it will account equally
for the position of those elevated masses of marine origin in which the
stratification remains horizontal, and for those in which the strata are
disturbed, broken, inclined, or vertical. Secondly, it is consistent with human
experience that land should rise gradually in some places and be depressed in
others. Such changes have actually occurred in our own days, and are now in
progress, having been accompanied in some cases by violent convulsions, while in
others they have proceeded so insensibly as to have been ascertainable only by
the most careful scientific observations, made at considerable intervals of
time. On the other hand, there is no evidence from human experience of a rising
or lowering of the sea's level in any region, and the ocean can not be raised or
depressed in one place without its level being changed all over the globe.
These preliminary remarks will prepare the reader to understand the great
theoretical interest attached to all facts connected with the position of
strata, whether horizontal or inclined, curved or vertical.
Now the first and most simple appearance is where strata of marine origin occur
above the level of the sea in horizontal position. Such are the strata which we
meet with in the south of Sicily, filled with shells for the most part of the
same species as those now living in the Mediterranean. Some of these rocks rise
to the height of more than 2000 feet above the sea. Other mountain masses might
be mentioned, composed of horizontal strata of high antiquity, which contain
fossil remains of animals wholly dissimilar from any now known to exist. In the
south of Sweden, for example, near Lake Wener, the beds of some of the oldest
fossiliferous deposits, called Silurian and Cambrian by geologists, occur in as
level a position as if they had recently formed part of the delta of a great
river, and been left dry on the retiring of the annual floods. Aqueous rocks of
equal antiquity extend for hundreds of miles over the lake-district of North
America, and exhibit in like manner a stratification nearly undisturbed. The
Table Mountain at the Cape of Good Hope is another example of highly elevated
yet perfectly horizontal strata, no less than 3500 feet in thickness, and
consisting of sandstone of very ancient date.
Instead of imagining that such fossiliferous rocks were always at their present
level, and that the sea was once high enough to cover them, we suppose them to
have constituted the ancient bed of the ocean, and to have been afterwards
uplifted to their present height. This idea, however startling it may at first
appear, is quite in accordance, as before stated, with the analogy of changes
now going on in certain regions of the globe. Thus, in parts of Sweden, and the
shores and islands of the Gulf of Bothnia, proofs have been obtained that the
land is experiencing, and has experienced for centuries, a slow upheaving
movement. (See "Principles of Geology" 1867 page 314.)
It appears from the observations of Mr. Darwin and others, that very extensive
regions of the continent of South America have been undergoing slow and gradual
upheaval, by which the level plains of Patagonia, covered with recent marine
shells, and the Pampas of Buenos Ayres, have been raised above the level of the
sea. On the other hand, the gradual sinking of the west coast of Greenland, for
the space of more than 600 miles from north to south, during the last four
centuries, has been established by the observations of a Danish naturalist, Dr.
Pingel. And while these proofs of continental elevation and subsidence, by slow
and insensible movements, have been recently brought to light, the evidence has
been daily strengthened of continued changes of level effected by violent
convulsions in countries where earthquakes are frequent. There the rocks are
rent from time to time, and heaved up or thrown down several feet at once, and
disturbed in such a manner as to show how entirely the original position of
strata may be modified in the course of centuries.
Mr. Darwin has also inferred that, in those seas where circular coral islands
and barrier reefs abound, there is a slow and continued sinking of the submarine
mountains on which the masses of coral are based; while there are other areas of
the South Sea where the land is on the rise, and where coral has been upheaved
far above the sea-level.
ALTERNATIONS OF MARINE AND FRESH-WATER STRATA.
It has been shown in the third chapter that there is such a difference between
land, fresh-water, and marine fossils as to enable the geologist to determine
whether particular groups of strata were formed at the bottom of the ocean or in
estuaries, rivers, or lakes. If surprise was at first created by the discovery
of marine corals and shells at the height of several miles above the sea-level,
the imagination was afterwards not less startled by observing that in the
successive strata composing the earth's crust, especially if their total
thickness amounted to thousands of feet, they comprised in some parts formations
of shallow-sea as well as of deep-sea origin; also beds of brackish or even of
purely fresh-water formation, as well as vegetable matter or coal accumulated on
ancient land. In these cases we as frequently find fresh-water beds below a
marine set or shallow-water under those of deep-sea origin as the reverse. Thus,
if we bore an artesian well below London, we pass through a marine clay, and
there reach, at the depth of several hundred feet, a shallow-water and
fluviatile sand, beneath which comes the white chalk originally formed in a deep
sea. Or if we bore vertically through the chalk of the North Downs, we come,
after traversing marine chalky strata, upon a fresh-water formation many
hundreds of feet thick, called the Wealden, such as is seen in Kent and Surrey,
which is known in its turn to rest on purely marine beds. In like manner, in
various parts of Great Britain we sink vertical shafts through marine deposits
of great thickness, and come upon coal which was formed by the growth of plants
on an ancient land-surface sometimes hundreds of square miles in extent.
VERTICAL, INCLINED, AND CURVED STRATA.
(FIGURE 54. Vertical conglomerate and sandstone.)
It has been stated that marine strata of different ages are sometimes found at a
considerable height above the sea, yet retaining their original horizontality;
but this state of things is quite exceptional. As a general rule, strata are
inclined or bent in such a manner as to imply that their original position has
been altered.
(FIGURE 55. Section of Forfarshire, from N.W. to S.E., from the foot of the
Grampians to the sea at Arbroath (volcanic or trap rocks omitted). Length of
section twenty miles.
From S.E. (left) Sea: Whiteness, Arbroath: Strata a, 2, 3: Leys Mill: Strata 4:
Sidlaw Hills. Viney R.: Strata B: Pitmuies: Strata 4: Position and nature of the
rocks below No. 4 unknown: Turin: Findhaven: Strata 3, 2, A: Valley of
Strathmore: Strata 1, 2, 3: W. Ogle: Strata 4 and Clay-Slate: to N.W. (right).)
The most unequivocal evidence of such a change is afforded by their standing up
vertically, showing their edges, which is by no means a rare phenomenon,
especially in mountainous countries. Thus we find in Scotland, on the southern
skirts of the Grampians, beds of pudding-stone alternating with thin layers of
fine sand, all placed vertically to the horizon. When Saussure first observed
certain conglomerates in a similar position in the Swiss Alps, he remarked that
the pebbles, being for the most part of an oval shape, had their longer axes
parallel to the planes of stratification (see Figure 54). From this he inferred
that such strata must, at first, have been horizontal, each oval pebble having
settled at the bottom of the water, with its flatter side parallel to the
horizon, for the same reason that an egg will not stand on either end if
unsupported. Some few, indeed, of the rounded stones in a conglomerate
occasionally afford an exception to the above rule, for the same reason that in
a river's bed, or on a shingle beach, some pebbles rest on their ends or edges;
these having been shoved against or between other stones by a wave or current,
so as to assume this position.
ANTICLINAL AND SYNCLINAL CURVES.
Vertical strata, when they can be traced continuously upward or downward for
some depth, are almost invariably seen to be parts of great curves, which may
have a diameter of a few yards, or of several miles. I shall first describe two
curves of considerable regularity, which occur in Forfarshire, extending over a
country twenty miles in breadth, from the foot of the Grampians to the sea near
Arbroath.
The mass of strata here shown may be 2000 feet in thickness, consisting of red
and white sandstone, and various coloured shales, the beds being distinguishable
into four principal groups, namely, No. 1, red marl or shale; No. 2, red
sandstone, used for building; No. 3, conglomerate; and No. 4, grey paving-stone,
and tile-stone, with green and reddish shale, containing peculiar organic
remains. A glance at the section (Figure 55.) will show that each of the
formations 2, 3, 4 are repeated thrice at the surface, twice with a southerly,
and once with a northerly inclination or DIP, and the beds in No. 1, which are
nearly horizontal, are still brought up twice by a slight curvature to the
surface, once on each side of A. Beginning at the north-west extremity, the
tile-stones and conglomerates, No. 4 and No. 3, are vertical, and they generally
form a ridge parallel to the southern skirts of the Grampians. The superior
strata, Nos. 2 and 1, become less and less inclined on descending to the valley
of Strathmore, where the strata, having a concave bend, are said by geologists
to lie in a "trough" or "basin." Through the centre of this valley runs an
imaginary line A, called technically a "synclinal line," where the beds, which
are tilted in opposite directions, may be supposed to meet. It is most important
for the observer to mark such lines, for he will perceive by the diagram that,
in travelling from the north to the centre of the basin, he is always passing
from older to newer beds; whereas, after crossing the line A, and pursuing his
course in the same southerly direction, he is continually leaving the newer, and
advancing upon older strata. All the deposits which he had before examined begin
then to recur in reversed order, until he arrives at the central axis of the
Sidlaw hills, where the strata are seen to form an arch, or SADDLE, having an
ANTICLINAL line, B, in the centre. On passing this line, and continuing towards
the S.E., the formations 4, 3, and 2, are again repeated, in the same relative
order of superposition, but with a southerly dip. At Whiteness (see Figure 55)
it will be seen that the inclined strata are covered by a newer deposit, a, in
horizontal beds. These are composed of red conglomerate and sand, and are newer
than any of the groups, 1, 2, 3, 4, before described, and rest UNCONFORMABLY
upon strata of the sandstone group, No. 2.
An example of curved strata, in which the bends or convolutions of the rock are
sharper and far more numerous within an equal space, has been well described by
Sir James Hall. (Edinburgh Transactions volume 7 plate 3.) It occurs near St.
Abb's Head, on the east coast of Scotland, where the rocks consist principally
of a bluish slate, having frequently a ripple-marked surface. The undulations of
the beds reach from the top to the bottom of cliffs from 200 to 300 feet in
height, and there are sixteen distinct bendings in the course of about six
miles, the curvatures being alternately concave and convex upward.
FOLDING BY LATERAL MOVEMENT.
(FIGURE 56. Curved strata of slate near St. Abb's Head, Berwickshire. (Sir J.
Hall.)
(FIGURE 57. Curved strata in line of cliff.)
(FIGURE 58. Folded cloths imitating bent strata.)
An experiment was made by Sir James Hall, with a view of illustrating the manner
in which such strata, assuming them to have been originally horizontal, may have
been forced into their present position. A set of layers of clay were placed
under a weight, and their opposite ends pressed towards each other with such
force as to cause them to approach more nearly together. On the removal of the
weight, the layers of clay were found to be curved and folded, so as to bear a
miniature resemblance to the strata in the cliffs. We must, however, bear in
mind that in the natural section or sea-cliff we only see the foldings
imperfectly, one part being invisible beneath the sea, and the other, or upper
portion, being supposed to have been carried away by DENUDATION, or that action
of water which will be explained in the next chapter. The dark lines in the plan
(Figure 57) represent what is actually seen of the strata in the line of cliff
alluded to; the fainter lines, that portion which is concealed beneath the sea-
level, as also that which is supposed to have once existed above the present
surface.
We may still more easily illustrate the effects which a lateral thrust might
produce on flexible strata, by placing several pieces of differently coloured
cloths upon a table, and when they are spread out horizontally, cover them with
a book. Then apply other books to each end, and force them towards each other.
The folding of the cloths (see Figure 58) will imitate those of the bent strata;
the incumbent book being slightly lifted up, and no longer touching the two
volumes on which it rested before, because it is supported by the tops of the
anticlinal ridges formed by the curved cloths. In like manner there can be no
doubt that the squeezed strata, although laterally condensed and more closely
packed, are yet elongated and made to rise upward, in a direction perpendicular
to the pressure.
Whether the analogous flexures in stratified rocks have really been due to
similar sideway movements is a question which we can not decide by reference to
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