AQUEOUS ROCKS--THEIR COMPOSITION AND FORMS OF STRATIFICATION.
Mineral composition of strata--Arenaceous
rocks--Argillaceous--Calcareous--Gypsum--Forms of
stratification--Original horizontality--Thinning out--Diagonal
arrangement--Ripple mark.
In pursuance of the arrangement explained in the last chapter, we shall
begin by examining the aqueous or sedimentary rocks, which are for the most
part distinctly stratified, and contain fossils. We may first study them
with reference to their mineral composition, external appearance, position,
mode of origin, organic contents, and other characters which belong to them
as aqueous formations, independently of their age, and we may afterwards
consider them chronologically or with reference to the successive
geological periods when they originated.
I have already given an outline of the data which led to the belief that
the stratified and fossiliferous rocks were originally deposited under
water; but, before entering into a more detailed investigation, it will be
desirable to say something of the ordinary materials of which such strata
are composed. These may be said to belong principally to three divisions,
the arenaceous, the argillaceous, and the calcareous, which are formed
respectively of sand, clay, and carbonate of lime. Of these, the
arenaceous, or sandy masses, are chiefly made up of siliceous or flinty
grains; the argillaceous, or clayey, of a mixture of siliceous matter,
with a certain proportion, about a fourth in weight, of aluminous earth;
and, lastly, the calcareous rocks or limestones consist of carbonic
acid and lime.
_Arenaceous or siliceous rocks._--To speak first of the sandy division:
beds of loose sand are frequently met with, of which the grains consist
entirely of silex, which term comprehends all purely siliceous minerals, as
quartz and common flint. Quartz is silex in its purest form; flint usually
contains some admixture of alumine and oxide of iron. The siliceous grains
in sand are usually rounded, as if by the action of running water.
Sandstone is an aggregate of such grains, which often cohere together
without any visible cement, but more commonly are bound together by a
slight quantity of siliceous or calcareous matter, or by iron or clay.
Pure siliceous rocks may be known by not effervescing when a drop of
nitric, sulphuric, or other acid is applied to them, or by the grains not
being readily scratched or broken by ordinary pressure. In nature there is
every intermediate gradation, from perfectly loose sand, to the hardest
sandstone. In _micaceous sandstones_ mica is very abundant; and the thin
silvery plates into which that mineral divides, are often arranged in
layers parallel to the planes of stratification, giving a slaty or
laminated texture to the rock.
When sandstone is coarse-grained, it is usually called _grit_. If the
grains are rounded, and large enough to be called pebbles, it becomes a
_conglomerate_, or _pudding-stone_, which may consist of pieces of one or
of many different kinds of rock. A conglomerate, therefore, is simply
gravel bound together by a cement.
_Argillaceous rocks._--Clay, strictly speaking, is a mixture of silex or
flint with a large proportion, usually about one fourth, of alumine, or
argil; but, in common language, any earth which possesses sufficient
ductility, when kneaded up with water, to be fashioned like paste by the
hand, or by the potter's lathe, is called a _clay_; and such clays vary
greatly in their composition, and are, in general, nothing more than mud
derived from the decomposition or wearing down of various rocks. The purest
clay found in nature is porcelain clay, or kaolin, which results from the
decomposition of a rock composed of felspar and quartz, and it is almost
always mixed with quartz.[11-A] _Shale_ has also the property, like clay,
of becoming plastic in water: it is a more solid form of clay, or
argillaceous matter, condensed by pressure. It usually divides into
irregular laminae.
One general character of all argillaceous rocks is to give out a peculiar,
earthy odour when breathed upon, which is a test of the presence of
alumine, although it does not belong to pure alumine, but, apparently, to
the combination of that substance with oxide of iron.[11-B]
_Calcareous rocks._--This division comprehends those rocks which, like
chalk, are composed chiefly of lime and carbonic acid. Shells and corals
are also formed of the same elements, with the addition of animal matter.
To obtain pure lime it is necessary to calcine these calcareous substances,
that is to say, to expose them to heat of sufficient intensity to drive off
the carbonic acid, and other volatile matter, without vitrifying or melting
the lime itself. White chalk is often pure carbonate of lime; and this
rock, although usually in a soft and earthy state, is sometimes
sufficiently solid to be used for building, and even passes into a
_compact_ stone, or a stone of which the separate parts are so minute as
not to be distinguishable from each other by the naked eye.
Many limestones are made up entirely of minute fragments of shells and
coral, or of calcareous sand cemented together. These last might be called
"calcareous sandstones;" but that term is more properly applied to a rock
in which the grains are partly calcareous and partly siliceous, or to
quartzose sandstones, having a cement of carbonate of lime.
The variety of limestone called "oolite" is composed of numerous small
egg-like grains, resembling the roe of a fish, each of which has usually a
small fragment of sand as a nucleus, around which concentric layers of
calcareous matter have accumulated.
Any limestone which is sufficiently hard to take a fine polish is called
_marble_. Many of these are fossiliferous; but statuary marble, which is
also called saccharine limestone, as having a texture resembling that of
loaf-sugar, is devoid of fossils, and is in many cases a member of the
metamorphic series.
_Siliceous limestone_ is an intimate mixture of carbonate of lime and
flint, and is harder in proportion as the flinty matter predominates.
The presence of carbonate of lime in a rock may be ascertained by applying
to the surface a small drop of diluted sulphuric, nitric, or muriatic
acids, or strong vinegar; for the lime, having a greater chemical affinity
for any one of these acids than for the carbonic, unites immediately with
them to form new compounds, thereby becoming a sulphate, nitrate, or
muriate of lime. The carbonic acid, when thus liberated from its union with
the lime, escapes in a gaseous form, and froths up or effervesces as it
makes its way in small bubbles through the drop of liquid. This
effervescence is brisk or feeble in proportion as the limestone is
pure or impure, or, in other words, according to the quantity of foreign
matter mixed with the carbonate of lime. Without the aid of this test,
the most experienced eye cannot always detect the presence of carbonate
of lime in rocks.
The above-mentioned three classes of rocks, the siliceous, argillaceous,
and calcareous, pass continually into each other, and rarely occur in a
perfectly separate and pure form. Thus it is an exception to the general
rule to meet with a limestone as pure as ordinary white chalk, or with clay
as aluminous as that used in Cornwall for porcelain, or with sand so
entirely composed of siliceous grains as the white sand of Alum Bay in the
Isle of Wight, or sandstone so pure as the grit of Fontainebleau, used for
pavement in France. More commonly we find sand and clay, or clay and marl,
intermixed in the same mass. When the sand and clay are each in
considerable quantity, the mixture is called _loam_. If there is much
calcareous matter in clay it is called _marl_; but this term has
unfortunately been used so vaguely, as often to be very ambiguous. It has
been applied to substances in which there is no lime; as, to that red loam
usually called red marl in certain parts of England. Agriculturists were in
the habit of calling any soil a marl, which, like true marl, fell to pieces
readily on exposure to the air. Hence arose the confusion of using this
name for soils which, consisting of loam, were easily worked by the plough,
though devoid of lime.
_Marl slate_ bears the same relation to marl which shale bears to clay,
being a calcareous shale. It is very abundant in some countries, as in the
Swiss Alps. Argillaceous or marly limestone is also of common occurrence.
There are few other kinds of rock which enter so largely into the
composition of sedimentary strata as to make it necessary to dwell here on
their characters. I may, however, mention two others,--magnesian limestone
or dolomite, and gypsum. _Magnesian limestone_ is composed of carbonate of
lime and carbonate of magnesia; the proportion of the latter amounting in
some cases to nearly one half. It effervesces much more slowly and feebly
with acids than common limestone. In England this rock is generally of a
yellowish colour; but it varies greatly in mineralogical character, passing
from an earthy state to a white compact stone of great hardness.
_Dolomite_, so common in many parts of Germany and France, is also a
variety of magnesian limestone, usually of a granular texture.
_Gypsum._--Gypsum is a rock composed of sulphuric acid, lime, and water. It
is usually a soft whitish-yellow rock, with a texture resembling that of
loaf-sugar, but sometimes it is entirely composed of lenticular crystals.
It is insoluble in acids, and does not effervesce like chalk and dolomite,
because it does not contain carbonic acid gas, or fixed air, the lime being
already combined with sulphuric acid, for which it has a stronger affinity
than for any other. Anhydrous gypsum is a rare variety, into which water
does not enter as a component part. Gypseous marl is a mixture of gypsum
and marl. Alabaster is a granular and compact variety of gypsum found in
masses large enough to be used in sculpture and architecture. It is
sometimes a pure snow-white substance, as that of Volterra in Tuscany, well
known as being carved for works of art in Florence and Leghorn. It is a
softer stone than marble, and more easily wrought.
_Forms of stratification._--A series of strata sometimes consists of one of
the above rocks, sometimes of two or more in alternating beds. Thus, in the
coal districts of England, for example, we often pass through several beds
of sandstone, some of finer, others of coarser grain, some white, others of
a dark colour, and below these, layers of shale and sandstone or beds of
shale, divisible into leaf-like laminae, and containing beautiful
impressions of plants. Then again we meet with beds of pure and impure
coal, alternating with shales and sandstones, and underneath the whole,
perhaps, are calcareous strata, or beds of limestone, filled with corals
and marine shells, each bed distinguishable from another by certain
fossils, or by the abundance of particular species of shells or zoophytes.
This alternation of different kinds of rock produces the most distinct
stratification; and we often find beds of limestone and marl, conglomerate
and sandstone, sand and clay, recurring again and again, in nearly regular
order, throughout a series of many hundred strata. The causes which may
produce these phenomena are various, and have been fully discussed in my
treatise on the modern changes of the earth's surface.[14-A] It is there
seen that rivers flowing into lakes and seas are charged with sediment,
varying in quantity, composition, colour, and grain according to the
seasons; the waters are sometimes flooded and rapid, at other periods low
and feeble; different tributaries, also, draining peculiar countries and
soils, and therefore charged with peculiar sediment, are swollen at
distinct periods. It was also shown that the waves of the sea and
currents undermine the cliffs during wintry storms, and sweep away
the materials into the deep, after which a season of tranquillity
succeeds, when nothing but the finest mud is spread by the movements
of the ocean over the same submarine area.
It is not the object of the present work to give a description of these
operations, repeated as they are, year after year, and century after
century; but I may suggest an explanation of the manner in which some
micaceous sandstones have originated, those in which we see innumerable
thin layers of mica dividing layers of fine quartzose sand. I observed the
same arrangement of materials in recent mud deposited in the estuary of La
Roche St. Bernard in Brittany, at the mouth of the Loire. The surrounding
rocks are of gneiss, which, by its waste, supplies the mud: when this dries
at low water, it is found to consist of brown laminated clay, divided by
thin seams of mica. The separation of the mica in this case, or in that of
micaceous sandstones, may be thus understood. If we take a handful of
quartzose sand, mixed with mica, and throw it into a clear running stream,
we see the materials immediately sorted by the water, the grains of quartz
falling almost directly to the bottom, while the plates of mica take a much
longer time to reach the bottom, and are carried farther down the stream.
At the first instant the water is turbid, but immediately after the flat
surfaces of the plates of mica are seen alone reflecting a silvery light,
as they descend slowly, to form a distinct micaceous lamina. The mica is
the heavier mineral of the two; but it remains longer suspended, owing to
its great extent of surface. It is easy, therefore, to perceive that where
such mud is acted upon by a river or tidal current, the thin plates of mica
will be carried farther, and not deposited in the same places as the
grains of quartz; and since the force and velocity of the stream varies
from time to time, layers of mica or of sand will be thrown down
successively on the same area.
_Original horizontality._--It has generally been said that the upper and
under surfaces of strata, or the planes of stratification, as they are
termed, are parallel. Although this is not strictly true, they make an
approach to parallelism, for the same reason that sediment is usually
deposited at first in nearly horizontal layers. The reason of this
arrangement can by no means be attributed to an original evenness or
horizontality in the bed of the sea; for it is ascertained that in those
places where no matter has been recently deposited, the bottom of the ocean
is often as uneven as that of the dry land, having in like manner its
hills, valleys, and ravines. Yet if the sea should sink, or the water be
removed near the mouth of a large river where a delta has been forming, we
should see extensive plains of mud and sand laid dry, which, to the eye,
would appear perfectly level, although, in reality, they would slope gently
from the land towards the sea.
This tendency in newly-formed strata to assume a horizontal position arises
principally from the motion of the water, which forces along particles of
sand or mud at the bottom, and causes them to settle in hollows or
depressions, where they are less exposed to the force of a current than
when they are resting on elevated points. The velocity of the current and
the motion of the superficial waves diminish from the surface downwards,
and are least in those depressions where the water is deepest.
[Illustration: Fig. 1. Cross section.]
A good illustration of the principle here alluded to may be sometimes seen
in the neighbourhood of a volcano, when a section, whether natural or
artificial, has laid open to view a succession of various-coloured layers
of sand and ashes, which have fallen in showers upon uneven ground. Thus
let A B (fig. 1.) be two ridges, with an intervening valley. These original
inequalities of the surface have been gradually effaced by beds of sand and
ashes _c_, _d_, _e_, the surface at e being quite level. It will be seen
that although the materials of the first layers have accommodated
themselves in a great degree to the shape of the ground A B, yet each bed
is thickest at the bottom. At first a great many particles would be carried
by their own gravity down the steep sides of A and B, and others would
afterwards be blown by the wind as they fell off the ridges, and would
settle in the hollow, which would thus become more and more effaced as the
strata accumulated from _c_ to _e_. This levelling operation may perhaps be
rendered more clear to the student by supposing a number of parallel
trenches to be dug in a plain of moving sand, like the African desert, in
which case the wind would soon cause all signs of these trenches to
disappear, and the surface would be as uniform as before. Now, water in
motion can exert this levelling power on similar materials more easily
than air, for almost all stones lose in water more than a third of the
weight which they have in air, the specific gravity of rocks being in
general as 2-1/2 when compared to that of water, which is estimated at 1.
But the buoyancy of sand or mud would be still greater in the sea, as the
density of salt water exceeds that of fresh.
Yet, however uniform and horizontal may be the surface of new deposits in
general, there are still many disturbing causes, such as eddies in the
water, and currents moving first in one and then in another direction,
which frequently cause irregularities. We may sometimes follow a bed of
limestone, shale, or sandstone, for a distance of many hundred yards
continuously; but we generally find at length that each individual stratum
thins out, and allows the beds which were previously above and below it to
meet. If the materials are coarse, as in grits and conglomerates, the same
beds can rarely be traced many yards without varying in size, and often
coming to an end abruptly. (See fig. 2.)
[Illustration: Fig. 2. Section of strata of sandstone, grit,
and conglomerate.]
[Illustration: Fig. 3. Section of sand at Sandy Hill, near Biggleswade,
Bedfordshire. Height 20 feet. (Greensand formation.)]
_Diagonal or Cross Stratification._--There is also another phenomenon of
frequent occurrence. We find a series of larger strata, each of which is
composed of a number of minor layers placed obliquely to the general planes
of stratification. To this diagonal arrangement the name of "false or cross
stratification" has been given. Thus in the annexed section (fig. 3.) we
see seven or eight large beds of loose sand, yellow and brown, and the
lines _a_, _b_, _c_, mark some of the principal planes of stratification,
which are nearly horizontal. But the greater part of the subordinate laminae
do not conform to these planes, but have often a steep slope, the
inclination being sometimes towards opposite points of the compass. When
the sand is loose and incoherent, as in the case here represented, the
deviation from parallelism of the slanting laminae cannot possibly be
accounted for by any re-arrangement of the particles acquired during the
consolidation of the rock. In what manner then can such irregularities be
due to original deposition? We must suppose that at the bottom of the sea,
as well as in the beds of rivers, the motions of waves, currents, and
eddies often cause mud, sand, and gravel to be thrown down in heaps on
particular spots, instead of being spread out uniformly over a wide area.
Sometimes, when banks are thus formed, currents may cut passages through
them, just as a river forms its bed. Suppose the bank A (fig. 4.) to be
thus formed with a steep sloping side, and the water being in a tranquil
state, the layer of sediment No. 1. is thrown down upon it, conforming
nearly to its surface. Afterwards the other layers, 2, 3, 4, may be
deposited in succession, so that the bank B C D is formed. If the current
then increases in velocity, it may cut away the upper portion of this mass
down to the dotted line _e_ (fig. 4.), and deposit the materials thus
removed farther on, so as to form the layers 5, 6, 7, 8. We have now the
bank B C D E (fig. 5.), of which the surface is almost level, and on which
the nearly horizontal layers, 9, 10, 11, may then accumulate. It was shown
in fig. 3. that the diagonal layers of successive strata may sometimes have
an opposite slope. This is well seen in some cliffs of loose sand on the
Suffolk coast. A portion of one of these is represented in fig. 6., where
the layers, of which there are about six in the thickness of an inch, are
composed of quartzose grains. This arrangement may have been due to the
altered direction of the tides and currents in the same place.
[Illustration: Fig. 4. Cross section.]
[Illustration: Fig. 5. Cross section.]
[Illustration: Fig. 6. Cliff between Mismer and Dunwich.]
[Illustration: Fig. 7. Section from Monte Calvo to the sea by the valley of
Magnan, near Nice.
A. Dolomite and sandstone. (Greensand formation?)
_a_, _b_, _d_. Beds of gravel and sand.
_c._ Fine marl and sand of St. Madeleine, with marine shells.]
The description above given of the slanting position of the minor layers
constituting a single stratum is in certain cases applicable on a much
grander scale to masses several hundred feet thick, and many miles in
extent. A fine example may be seen at the base of the Maritime Alps near
Nice. The mountains here terminate abruptly in the sea, so that a depth of
many hundred fathoms is often found within a stone's throw of the beach,
and sometimes a depth of 3000 feet within half a mile. But at certain
points, strata of sand, marl, or conglomerate, intervene between the shore
and the mountains, as in the annexed fig. 7., where a vast succession of
slanting beds of gravel and sand may be traced from the sea to Monte Calvo,
a distance of no less than 9 miles in a straight line. The dip of these
beds is remarkably uniform, being always southward or towards the
Mediterranean, at an angle of about 25 deg.. They are exposed to view in nearly
vertical precipices, varying from 200 to 600 feet in height, which bound
the valley through which the river Magnan flows. Although in a general
view, the strata appear to be parallel and uniform, they are nevertheless
found, when examined closely, to be wedge-shaped, and to thin out when
followed for a few hundred feet or yards, so that we may suppose them to
have been thrown down originally upon the side of a steep bank, where a
river or alpine torrent discharged itself into a deep and tranquil sea, and
formed a delta, which advanced gradually from the base of Monte Calvo to a
distance of 9 miles from the original shore. If subsequently this part of
the Alps and bed of the sea were raised 700 feet, the coast would acquire
its present configuration, the delta would emerge, and a deep channel might
then be cut through it by a river.
It is well known that the torrents and streams, which now descend from the
alpine declivities to the shore, bring down annually, when the snow melts,
vast quantities of shingle and sand, and then, as they subside, fine mud,
while in summer they are nearly or entirely dry; so that it may be safely
assumed, that deposits like those of the valley of the Magnan, consisting
of coarse gravel alternating with fine sediment, are still in progress at
many points, as, for instance, at the mouth of the Var. They must advance
upon the Mediterranean in the form of great shoals terminating in a steep
talus; such being the original mode of accumulation of all coarse
materials conveyed into deep water, especially where they are composed in
great part of pebbles, which cannot be transported to indefinite distances
by currents of moderate velocity. By inattention to facts and inferences of
this kind, a very exaggerated estimate has sometimes been made of the
supposed depth of the ancient ocean. There can be no doubt, for example,
that the strata _a_, fig. 7., or those nearest to Monte Calvo, are older
than those indicated by _b_, and these again were formed before _c_; but
the vertical depth of gravel and sand in any one place cannot be proved to
amount even to 1000 feet, although it may perhaps be much greater, yet
probably never exceeding at any point 3000 or 4000 feet. But were we to
assume that all the strata were once horizontal, and that their present dip
or inclination was due to subsequent movements, we should then be forced to
conclude, that a sea 9 miles deep had been filled up with alternate layers
of mud and pebbles thrown down one upon another.
In the locality now under consideration, situated a few miles to the west
of Nice, there are many geological data, the details of which cannot be
given in this place, all leading to the opinion, that when the deposit of
the Magnan was formed, the shape and outline of the alpine declivities and
the shore greatly resembled what we now behold at many points in the
neighbourhood. That the beds, a, b, c, d, are of comparatively modern date
is proved by this fact, that in seams of loamy marl intervening between the
pebbly beds are fossil shells, half of which belong to species now living
in the Mediterranean.
[Illustration: Fig. 8. Slab of ripple-marked (new red) sandstone
from Cheshire.]
_Ripple mark._--The ripple mark, so common on the surface of sandstones of
all ages (see fig. 8.), and which is so often seen on the sea-shore at low
tide, seems to originate in the drifting of materials along the bottom of
the water, in a manner very similar to that which may explain the inclined
layers above described. This ripple is not entirely confined to the beach
between high and low water mark, but is also produced on sands which are
constantly covered by water. Similar undulating ridges and furrows may also
be sometimes seen on the surface of drift snow and blown sand. The
following is the manner in which I once observed the motion of the air to
produce this effect on a large extent of level beach, exposed at low tide
near Calais. Clouds of fine white sand were blown from the neighbouring
dunes, so as to cover the shore, and whiten a dark level surface of sandy
mud, and this fresh covering of sand was beautifully rippled. On levelling
all the small ridges and furrows of this ripple over an area of several
yards square, I saw them perfectly restored in about ten minutes, the
general direction of the ridges being always at right angles to that of the
wind. The restoration began by the appearance here and there of small
detached heaps of sand, which soon lengthened and joined together, so as to
form long sinuous ridges with intervening furrows. Each ridge had one side
slightly inclined, and the other steep; the lee-side being always steep, as
_b, c,--d, e_; the windward-side a gentle slope, as _a, b,--c, d_, fig. 9.
When a gust of wind blew with sufficient force to drive along a cloud of
sand, all the ridges were seen to be in motion at once, each encroaching on
the furrow before it, and, in the course of a few minutes, filling the
place which the furrows had occupied. The mode of advance was by the
continual drifting of grains of sand up the slopes _a b_ and _c d_, many of
which grains, when they arrived at _b_ and _d_, fell over the scarps _b c_
and _d e_, and were under shelter from the wind; so that they remained
stationary, resting, according to their shape and momentum, on different
parts of the descent, and a few only rolling to the bottom. In this manner
each ridge was distinctly seen to move slowly on as often as the force of
the wind augmented. Occasionally part of a ridge, advancing more rapidly
than the rest, overtook the ridge immediately before it, and became
confounded with it, thus causing those bifurcations and branches which are
so common, and two of which are seen in the slab, fig. 8. We may observe
this configuration in sandstones of all ages, and in them also, as now on
the sea-coast, we may often detect two systems of ripples interfering with
each other; one more ancient and half effaced, and a newer one, in which
the grooves and ridges are more distinct, and in a different direction.
This crossing of two sets of ripples arises from a change of wind, and the
new direction in which the waves are thrown on the shore.
[Illustration: Fig. 9. Sketch of ripples.]
The ripple mark is usually an indication of a sea-beach, or of water
from 6 to 10 feet deep, for the agitation caused by waves even during
storms extends to a very slight depth. To this rule, however, there are
some exceptions, and recent ripple marks have been observed at the depth
of 60 or 70 feet. It has also been ascertained that currents or large
bodies of water in motion may disturb mud and sand at the depth of 300
or even 450 feet.[21-A]
FOOTNOTES:
[11-A] The kaolin of China consists of 71.15 parts of silex, 15.86 of
alumine, 1.92 of lime, and 6.73 of water (W. Phillips, Mineralogy, p. 33.);
but other porcelain clays differ materially, that of Cornwall being
composed, according to Boase of nearly equal parts of silica and alumine,
with 1 per cent. of magnesia. (Phil. Mag. vol. x. 1837.)
[11-B] See W. Phillips's Mineralogy, "Alumine."
[14-A] Consult Index to Principles of Geology, "Stratification,"
"Currents," "Deltas," "Water," &c.
[21-A] Siau. Edin. New Phil. Journ. vol. xxxi.; and Darwin, Volc.
Islands, p. 134.