CHEMICAL EROSION.—Attention is invited first to the specimens numbered
1, 2, 3, and 4. No. 1 is a sound, fresh piece of the rather common rock,
diabase; and those who are acquainted with minerals will recognize that
the light-colored grains in the rock are feldspar, and the dark, augite.
This specimen came from a depth in the quarry, and has not been exposed
to the action of the weather.
The second specimen differs from the first, apparently, as much as
possible; and yet, except in being somewhat finer grained, it was
originally of precisely similar composition and appearance. In fact, it
is a portion of the same rock, but a _weathered_ portion. In this we can
no longer recognize the feldspar and augite as such, but both these
minerals are very much changed, while in the place of a strong, hard
rock we have an incoherent friable mass, which is, externally at least,
easily crushed to powder; and with the next step in the weathering, as
we may readily observe in the natural ledges, the rock is completely
disintegrated, forming a loose earth or soil.
We have two examples of such natural powders in the specimens numbered 3
and 4; and by washing these (especially the finer one, No. 4) with
water, we can prove that they consist of an impalpable substance which
we may call clay, and angular grains which we may call sand. The
sand-grains are really portions of the feldspar not yet entirely changed
to clay.
Thus we learn that the result of the exposure of this hard rock to the
weather is that it is reduced to the condition of sand and clay. What we
mean especially by the weather are _moisture_ and certain constituents
of the air, particularly _carbon dioxide_.
The action of the weather on the rocks is almost entirely chemical. With
a very few exceptions, the principal minerals of which rocks are
composed, such as feldspar, hornblende, augite, and mica, are silicates,
_i.e._, consist of silicic acid or silica combined with various bases,
especially aluminum, magnesium, iron, calcium, potassium, and sodium.
Now the silica does not hold all these bases with equal strength; but
carbon dioxide, in the presence of moisture, is able to take the sodium,
potassium, calcium, and magnesium away from the silica in the form of
carbonates, which, being soluble, are carried away by the rain-water.
The silicate of aluminum, with more or less iron, takes on water at the
same time, and remains behind as a soft, impalpable powder, which is
common clay.
In the case of our diabase, continued exposure to the weather would
reduce the whole mass to clay. But other rocks contain grains of quartz,
a hard mineral which cannot be decomposed, and it always forms sand.
Certain classes of rocks, too, such as the limestones and some
iron-ores, are completely dissolved by water holding carbon dioxide in
solution, and nothing is left to form soil, except usually a small
proportion of insoluble impurities like sand or clay.
Let us see next how these agents of decay get at the rocks. Neither
water nor air can penetrate the solid rock or mineral to any
considerable extent, so that practically the action is limited to
surfaces, and whatever multiplies surfaces must favor decomposition.
First, we have the upper surface of the rock where it is bare, but more
especially where it is covered with soil, for there it is always wet.
All rocks are naturally divided by joints into blocks, which are
frequently more or less regular, and often of quite small size. Water
and air penetrate into these cracks and decompose the surfaces of the
blocks, and thus the field of their operations is enormously extended.
These rock-blocks sometimes show very beautifully the progress of the
decomposing agents from the outside inward by concentric layers or
shells of rotten material, which, in the larger blocks, often envelop a
nucleus of the unaltered rock.
It is interesting to observe, too, that these concentric lines of decay
cut off the angles of the original blocks, so that the undecomposed
nucleus, when it is found, is approximately spherical instead of
cuboidal. Both these points are well illustrated by specimen No. 2; for
although now nearly spherical, it was originally perfectly angular, and
has become rounded by the peeling off, in concentric layers, of the
decomposed material, and in most cases several of these layers are
distinctly visible, like the coats of an onion. But by stripping these
off we should discover, in all the larger balls at least, a solid,
spheroidal nucleus, while in the smaller balls the decomposition has
penetrated to the centre.
In the rocks also we find many imperfect joints and minute cracks. In
cold countries these are extended and widened by the expansive power of
freezing water, and thus the surfaces of decomposition become constantly
greater.
Nearly all rocks suffer this chemical decomposition when exposed to the
weather, but in some the decay goes on much faster than in others.
Diabase is one of the rocks which decay most readily; while granite is,
among common rocks, one of those that resist decay most effectually.
The caverns which are so large and numerous in most limestone countries
are a splendid example of the solvent action of meteoric waters, being
formed entirely by the dissolving out of the limestone by the water
circulating through the joint cracks. The process must go on with
extreme slowness at first, when the joints are narrow, and more rapidly
as they are widened and more water is admitted. We get some idea, too,
of the magnitude of the results accomplished by these silent and
unobtrusive agencies when we reflect that almost all the loose earth and
soil covering the solid rocks are simply the insoluble residue which
carbon dioxide and water cannot remove. In low latitudes, where a warm
climate accelerates the decay of the rocks, the soil is usually from 50
to 300 feet deep.
MECHANICAL EROSION.—_On the edge of the land._—Let us trace next the
_mechanical_ action of water and air upon the land. First we will
consider the _edge_ of the land, where it is washed by the waves of the
sea. Whoever has been on the shore must have noticed that the sand along
the water’s edge is kept in constant motion by the ebb and flow of the
surf.
Where the beach is composed of gravel or shingle the motion is evident
to the _ear_ as well as the eye; and when the surf is strong, the
rattling and grinding of the pebbles as they are rolled up and down the
beach develops into a roar.
The constant shifting of the grains of sand, pebbles, and stones is, of
course, attended by innumerable collisions, which are the cause of the
noise. Now it is practically impossible, as we may easily prove by
experiment, to knock or rub two pieces of stone together, at least so as
to produce much noise, without abrading their surfaces; small particles
are detached, and sand and dust are formed.
That this abrasion actually occurs in the case of the moving sand is
most beautifully shown by the sandblast. We are to conclude, then, that
every time a pebble, large or small, is rolled up or down the beach it
becomes smaller, and some sand and dust or clay are formed which are
carried off by the water.
But what are the pebbles originally? This question is not difficult. A
little observation on the beach shows us that the pebbles are not all
equally round and smooth, but many are more or less angular. And we soon
see that it is possible to select a series showing all gradations
between the most perfectly rounded forms and angular fragments of rock
that are only slightly abraded on the corners. The three principal
members of such a series are shown in specimens 5, 6, and 7 from the
beach on Marblehead Neck; but equally instructive specimens can be
obtained at many other points on our coast. It is also observable that
the well-rounded pebbles are much smaller on the average than the
angular blocks.
From these facts we draw the legitimate inference that the pebbles were
all originally angular, and that the same abrasion which diminishes
their size makes them round and smooth.
A little reflection, too, shows that the rounding of the angular
fragments is a natural and necessary result of their mutual collisions;
for the angles are at the same time their weakest and most exposed
points, and must wear off faster than the flat or concave surfaces.
Having traced each pebble back to a larger angular rock-fragment, the
question arises, Whence come these angular blocks?
Behind our gravel-beach, or at its end, we have usually a cliff of
rocks. As we approach this it is distinctly observable that the angular
pebbles are more numerous, larger, and more angular; and a little
observation shows that these are simply the blocks produced by jointing,
and that the cliff is entirely composed of them. In other words, our
cliff is a mass of natural masonry, which chemical agencies, the frost,
and the sea are gradually disintegrating and removing. As soon as the
blocks are brought within reach of the surf their mutual collisions make
them rounder and smaller; and small round pebbles, sand, and clay are
the final result.
For a more complete account of the formation of pebbles, teachers are
referred to the first or introductory number of this series of guides,
by Prof. Hyatt, “About Pebbles.”
Where the waves can drive the shingle directly against the base of the
cliff, this is gradually ground away in the same manner as the loose
stones themselves, sometimes forming a cavern of considerable depth, but
always leaving a smooth, hard surface, which is very characteristic, and
contrasts strongly with the upper portion of the cliff, which is acted
on only by the rain and frost. A good example of such a pebble-carved
cliff may be seen behind the beach on the sea-ward side of Marblehead
Neck.
The sea acts within very narrow limits vertically, a few feet or a few
yards at most; but the coast-lines of the globe (including inland lakes
and seas) have an aggregate length of more than 150,000 miles. Hence it
is easy to see that the amount of solid rock ground to powder in the
mill of the ocean-beach annually must be very considerable.
MECHANICAL EROSION.—_On the surface of the land._—I next ask attention
to the _mechanical_ action of water upon the _surface_ of the land.
It is a familiar fact that after heavy rains the roadside rills carry
along much sand and clay (which we know have been produced by the
previous action of chemical forces), and also frequently small pebbles
or gravel. It is easy to show that in all important respects the rill
differs in size only from brooks and rivers; and the former afford us
fine models of the systems of valleys worn out during the lapse of ages
by rivers. The turbidity of rivers is often very evident, and in shallow
streams we can sometimes see the pebbles rolled along by the current.
Now here, just as on the beach, the collisions of rock-fragments are
attended by mutual abrasion, sand and clay are formed, and the fragments
become smaller and rounder. Our series of pebbles from the beach might
be matched perfectly among the river-gravel. In mountain streams
especially we may often observe that pebbles of a particular kind of
rock become more numerous, larger, and more angular as we proceed up
stream, until we reach the solid ledge from which they were derived,
showing the same gradation as the beach pebbles when followed back to
the parent cliff.
The pebbles, however, not only grind each other, but also the solid
rocks which form the bed of the streams in many places, and these are
gradually worn away. When the rocky bed is uneven and the water is
swift, pebbles collect in hollows where eddies are formed, by which they
are kept whirling and turning, and the hollow is deepened to a pot-hole,
while the pebbles, the river’s tools, are worn out at the same time.
By these observations we learn not only that running water carries away
sand and clay already formed, but that it also has great power of
grinding down hard rocks to sand and clay. Of course the pulverized rock
always moves in the same direction as the stream which carries it; and,
in a certain sense, all streams run in one direction, viz., toward the
sea. Therefore the constant tendency of the rain falling upon the land
is to break up the rocks by chemical and mechanical action and transport
the débris to the sea.
Rivers, as we all know, are continually uniting to form larger and
larger streams; and thus the drainage of a wide area sometimes, as in
the case of the Mississippi Valley, reaches the sea through a single
mouth. By careful measurements made at the mouth of the Mississippi it
has been shown that the 20,000,000,000,000 cubic feet of water
discharged into the Gulf of Mexico annually carries with it no less than
7,500,000,000 cubic feet of sand, clay, and dissolved mineral matter;
and this, spread over the whole Mississippi basin, would form a layer a
little more than 1/5000 of a foot in thickness. So that we may conclude
that the surface of the continent is being cut down on the average about
_one foot_ in _five thousand_ years.
We can only allude in passing to the very important geological action of
water in the solid state, as in glaciers and icebergs. The moisture
precipitated from the atmosphere, and falling as rain, makes ordinary
rivers; but falling in the form of snow in cold regions, where more snow
falls than is melted, the excess accumulates and is gradually compacted
to ice, which, like water, yields to the enormous pressure of its own
mass and flows toward lower levels. When the ice-river reaches the sea
it breaks off in huge blocks, which float away as icebergs. Moving ice,
like moving water, is a powerful agent of erosion; and the glacial marks
or scratches observable upon the ledges everywhere in the Northern
States and Canada attest the magnitude of the ice-action at a
comparatively recent period.
We have already noticed incidentally the powerful disintegrating action
of water where it freezes in the joints and pores of the rocks; and it
is probable that it thus facilitates the destruction of the rocks in
cold countries nearly as much as the higher temperature and greater
rain-fall do in warm countries.
Our observations up to this point show us that _erosion_, by which we
mean the breaking up by chemical and mechanical action of the rocks of
the land and the transportation of the débris into the sea, is one great
result accomplished by the inorganic aqueous agencies.
MECHANICAL DEPOSITION.—Next let us notice what becomes of all this vast
amount of clay, sand, and gravel after it is washed into the ocean. By
taking up a glass of turbid water from our roadside rill, and observing
that as soon as the water is undisturbed the sand and clay begin to
settle, we learn that the solid matter is held in suspension by the
motion of the water. But it does not remain in suspension long after
being washed into the sea, for otherwise the sea would, in the course of
time, become turbid for long distances from shore; and it is a
well-known fact that the sea-water is usually clear and free from
sensible turbidity close along shore and even near the mouths of large
rivers, while at a distance of only 50 or 100 miles we find the
transparency of the central ocean.
Putting these facts together, we see that the ocean, nothwithstanding
the ceaseless and often violent undulations of its surface, must be as a
whole a vast body of still water; and to the reflecting mind the almost
perfect tranquillity of the ocean is one of its most impressive
features. For it is in striking contrast, in this respect, with the more
mobile aerial ocean above it.
We have got hold, now, of two facts of great geological importance: (1)
The débris washed off the land by waves and rivers into the still water
of the ocean very soon settles to the bottom; and (2) it nearly all
settles on that part of the ocean-floor near the land.
And now we have in view the second great office of the inorganic aqueous
agencies,—deposition, the counterpart or complement of erosion.
The land is the great theatre of erosion and the sea of deposition; the
rocks which are constantly wasting away on the former are as constantly
renewed in the latter.
We will now observe the process of deposition a little more closely.
Each of these two bottles contains the same amount of fine yellow clay,
but in one the water is fresh, and in the other it is salt. At the
beginning of the lesson, as you may have observed, I brought the clay in
both bottles into suspension by violent agitation, and since then they
have remained undisturbed. The main point is that the salt water has
become quite clear, while the fresh water is still distinctly turbid,
showing that the salt favors the rapid deposition of the clay. At the
second lecture, a week later, these two bottles, yet undisturbed, were
exhibited, and the fresh water seen to be still sensibly turbid. The
fact is, the clay is not held in suspension wholly by the _motion_ of
the water; but, just as in the case of dust in the atmosphere, a small
portion of the medium is condensed around or adheres to each solid
particle, _i.e._, each clay particle in our experiment has an atmosphere
of water which moves with it and buoys it up. Now the effect of the salt
is to diminish the adhesion of the water to the particles, _i.e._, to
diminish their atmospheres, and consequently their buoyancy. The
diminished adhesion of the salt water is well shown by the smaller drops
which it forms on a glass rod.
The geological importance of this principle is very great; for it is
undoubtedly largely to the saltness of the sea that we owe its
transparency, and the fact that the fine, clayey sediment from the land,
like the coarse, is deposited near the shore.
This bottle of fresh water contains some fine gravel, coarse sand, fine
sand, and clay. By agitating the water, all this material is brought
into suspension. Now, suddenly placing the bottle in a state of rest, we
observe that the gravel falls to the bottom almost instantly, followed
quickly by the coarse sand, and very soon afterward by the fine sand;
and then there appears to be a pause, the fine particles of clay all
remain in suspension; but finally, when the water is quite motionless,
they begin to settle; they fall very slowly, however, and the water will
not be clear for hours.
This is a very instructive experiment. We learn from it:
First, that the power of the water to hold particles in suspension is
inversely proportional to the size of the particles;
Second, that all materials deposited in water are assorted according to
size;
Third, and this is one of the most important facts in geology, all
water-deposited sediments are arranged in horizontal layers, _i.e._, are
stratified. And we have now traced to its conclusion, though very
briefly, the process of the formation of one great division of
_stratified_ rocks,—the _mechanically-formed_ or _fragmental_ rocks.
These are so called because the clay, sand, and gravel are, in every
instance, fragments of pre-existing rocks; and because the formation,
transportation, and especially the _deposition_ of these fragments, are
the work chiefly or entirely of mechanical forces.
CHEMICAL DEPOSITION.—It is a well-known fact that the sea holds in
solution vast amounts of common salt as well as many other substances;
and analyses of river-waters show that dissolved minerals derived from
the chemical decomposition of the rocks of the land are being constantly
carried into the sea.
Portions of the sea which are cut off from the main body, and which are
gradually drying up, like the Great Salt Lake, Dead Sea, and Caspian
Sea, become saturated solutions of the various dissolved minerals, and
these are slowly deposited. This process is very nicely illustrated
along our shores in summer, where, during storms, salt-water spray is
thrown above the reach of the tides, and, collecting in hollows in the
rocks, gradually dries up, leaving behind a crust of salt.
When the sea lays down matter which it held in _suspension_, we call the
process _mechanical_ deposition, and the result is _mechanically_-formed
rocks.
But when it lays down matter which it held in _solution_, we call the
process _chemical_ deposition, and the result is _chemically_-formed
rocks.
The principal substances which the sea deposits chemically are common
salt, forming beds of rock-salt; sulphate of calcium, forming beds of
gypsum; carbonate of calcium, forming beds of limestone; and the double
carbonate of calcium and magnesium, forming beds of dolomite.
Inorganic deposition, like inorganic erosion, is both chemical and
mechanical.