THE HARDENING OF ROCKS
After the time when the great overflows of lava took place, spreading
over continents and sometimes seas, there was an era when the
explosions and outbursts began to diminish in violence, and the
world slowly settled down to conditions something like those which
we see in our own day. The seas were forming; there was rainfall and
summer and winter on the earth. The rains and the winds, the summer
heats and winter snows were more violent than now, and the volcanic
activity of which we have spoken was much more fierce than anything
of which mankind has any recollection. In the British Isles the
rainfall in a year averages something in the neighbourhood of thirty
inches. In some regions of the earth it is as much as four times
that amount, and deluges of fifteen inches have fallen in a day. But
in the era of which we have spoken deluging rains that were to be
measured in feet rather than inches fell incessantly. The air was
saturated with moisture, and it no sooner descended on the warm earth
than it steamed back to the clouds again. For reasons not unlike
these, nor unconnected with them, the great currents of air fed by
the constant transference of vapour from the earth to the skies, and
the condensation of the vapour to rain, falling again on the earth,
were greatly magnified. Thus the rocks of the earth, some of them
only cooling and not yet hardened, were subjected to "weathering"
of a kind of which it is hard to form any sufficient idea. The key
to all geology is that what is going on now on the earth is similar
to what always has been happening, (differing in degree rather than
in kind), and that consequently the rocks of millions of years ago
were washed by rivers down to the lower levels and were deposited
as sediment in streams, in lakes, and in the sea. Thus the age of
the "sedimentary rocks" began while the earth was still too warm to
preserve any vestiges of life.
Earthquakes much more violent than now and volcanic outbursts often
upset the steady order of things, but the earth was settling down.
During this settling-down process rocks, as we have seen, were being
formed by deposits; but they were very liable still to be invaded by
bursts of volcanic activity from the inner cauldron of the earth,
and they were very apt to be twisted out of their regular shape by
great earth movements. They were also liable to be baked by the
neighbourhood of the restless, unconfined molten rocks, nearer then
to the surface than now. Geologists call the great period of time
when all the rocks continually flowed out on to the surface of the
earth, and were, in fact, all molten before they solidified, the
Archæan Era (from a Greek word signifying the beginning). Next in
order to these rocks are those which were laid down in the agitated
times when the earth was still warm, and when the climate of the
earth might be described as a continual thunderstorm. In this period
earthquakes still had a great deal to do with the formation of the
rocks, but then, as now, the sea and lakes and oceans laid them down.
Geologists call this the Proterozoic Era. There are great masses
of these Proterozoic rocks in North America. In Arizona the three
periods of rock formation are sometimes visible together, and may,
indeed, be perceived in some of our photographs; the Archæan all
jumbled together being the lowest; Proterozoic lying crumpled or
tilted over them, and the later rocks resting more regularly on these
strata. In America, however, these separate ages of the Proterozoic
rocks can be identified, and each age is represented by rocks
thousands of feet in thickness. Three separate ages of rocks are
found in this great era in North America. It is not very important to
remember their names, which are merely those of the localities where
these great deposits are most marked, but it is important not to
forget that each of these depositions of rocks represents a period in
the earth's history older than the lifetime of a river or a lake and
as old as the lifetime of a continent. The lowest of these divisions
consists of rocks that are much altered by the heat of the rocks
below. The topmost division is hardly altered at all. In Scotland we
have similar rocks. The Torridonian sandstones, 8000 to 10,000 feet
thick, are believed to belong to this era. In France also, in Spain,
Germany, Finland, Sweden, India, and Brazil, the Proterozoic rocks
are found. In the lowermost of them are no signs that living things
ever existed, but in the upper ones f life begin to appear. We may
see in them to-day the first fossils. A fossil means literally a
thing dug up, and was a term applied at first to all kinds of mineral
substances taken out of the earth. We use the word now exclusively
for the remains of plants and animals embedded in any kind of rock.
In later chapters of this volume a good deal will have to be said
about fossils, and of the way in which they tell us the kind of life
that existed when they were first sunk in the rocks where now they
are found, and how also they give us information about the climate
and the distribution of land and sea, of lake and of river, in those
eras far "in the backward and the dark abyss of time."
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
The Pinnacled Castle-like Peaks of the Ramshorn Mountains Of Wyoming
The successive strata of sandstone are clearly evident in the
peaks.]
For the present, however, we may concern ourselves with the condition
of the earth and its rocks in Proterozoic times, observing merely
that the remains of animals which we find there are of an order
(Crustacea) which shows that life had progressed a good deal from
its earliest beginnings in the age when the rocks containing these
crustacea were laid down. After these rocks had been deposited
they were subjected to many influences of which we have only dim
conceptions. In a previous chapter we have compared the layers of
the undermost and harder rocks of the earth to the lines on a page
of this book as they would appear if the pages were crumpled up into
a ball. Sometimes the beds of solid rock have been so distorted that
they look like waves of the sea; sometimes they have been completely
overturned; hardly ever have they been suffered to lie down flat.
More than that has happened to them. Their very nature has been
changed. This was done partly by heat, partly by pressure, partly by
shock.
Let us consider the heat first. When a mass of erupted molten rock
forces its way through the earth's crust, it produces effects which
are quite easily recognised on the rocks it penetrates. Limestone
becomes hard and crystalline. Rocks with silica in them become glassy
and like quartz or other hard rocks which are sometimes polished
to make ornaments. Clayey strata become baked into hard brick-like
rocks. The changes are not altogether due to the heat. The eruption
of rocks is accompanied by steam at high pressure and with all sorts
of acids in the steam, so that chemical changes are also produced.
It has been supposed by Sir William Crookes that diamonds, which are
crystals of carbon, were produced by carbon being melted at some
enormous heat under great pressure. Given the requisite conditions
of heat and pressure, the parts of rocks which by their chemical
composition are susceptible to crystallisation will form into
enormous crystals--not unlike the intrusive rocks themselves. They
can, however, be readily distinguished from the shapes which the
intrusive rocks (like basalt) assume. The basalt rocks which form,
for example, the Giant's Causeway in Ireland were volcanic lavas.
Sometimes the lavas were masses which had solidified underground and
had been thrust up by pressure from below or have been exposed by the
weathering of the rocks above. Sometimes they have been lava poured
out on the surface. The black compact kinds most often are seen in
forms like columns. If these veins of volcanic rock have been thrust
up through a bed of coal, the coal is changed or "metamorphosed"
where the basalt has pierced it. Sometimes it becomes hard coal like
anthracite; sometimes it is changed into graphite--the black rock of
which pencils are frequently made.
Limestone pierced by basalt becomes marble. When sandstone is
discovered in contact with ancient volcanic rock it is found to have
lost its reddish colour, and to have become white, grey, green, or
black. It separates into crystals; it becomes glassy and hard. All
these instances are those of rocks which we can perceive to have been
altered by coming into contact with great heat. But there is another
kind of change of a very much more widespread character which can
be perceived among the most ancient of those rocks which we know
must have been first quietly laid down as sediments. It is sometimes
spoken of as "general metamorphism."
This widespreading change may extend over great regions and vast
extents of country. The most striking series of such rocks was first
described by Sir W. E. Logan, Director of the Canadian Geological
Survey; and he estimated the thickness of them at 30,000 feet. They
lie beneath all the unaltered rocks, and are (in North America)
the rocks which were the base or foundation of the North American
continent before the later sedimentary rocks were laid down on them.
They are called the Laurentian rocks, because they were first found
in the neighbourhood of the St. Lawrence River; but they exist in
many places besides Canada and North America; and the foundations
of Scandinavia and of the Hebrides are of the same texture and
material. Now this change or metamorphism does not appear to be the
same as that produced by the intrusion of hot eruptive rocks. Let us
take a simple instance. We have seen that limestone is changed by
heat into marble. Sometimes its fossils are preserved; sometimes they
completely disappear. Sometimes it is threaded by veins of harder
and more crystalline rocks. But in the case of the white marble of
Carrara, which was once a bed of coral, the change seems to have
taken place less violently, less suddenly, more gradually. The change
was due, therefore, not to violent heat suddenly applied, but to the
penetrating action of water, probably aided by sustained heat, and
certainly aided by pressure. When a rock is subjected to sufficient
pressure its very structure will alter; its original constituents
may be torn out of it, pressed out of it, filtered out of it, and
afterwards rearranged.
Once more let us call attention to the astounding effects which great
pressures can have. If pressure enough be applied iron can be made to
flow like treacle; and the pressure of two or three miles of strata
is enough to crumple or shear or tear any rock however hard. Now we
have shown that in the earth's long history some regions are always
being denuded of materials in order that these materials may be laid
down as sediments elsewhere. These movements may be compared to those
of a pair of scales, in which we are continually taking weight from
the scale pan that is weighted in order to put it into the scale that
is empty. The scale that is weighted is the land from which material
is being removed by the rains and the rivers; the scale that is
empty is the sea, in which the eroded material is laid down to form
beds and strata. These two scales are never quite balanced. But
suppose a time comes when we have taken all the material we can from
the weighted scale, so as to make the hitherto unweighted one the
heavier--what will happen? The newly weighted scale will inevitably
fall, and we shall have to begin to reverse our system of taking
from, and adding to, the scales. Similarly there will always come
a time when there will be a flow of the earth-mass from the areas
which have been receiving great loads of sediments, towards the areas
which have been robbed to supply them. Think for a moment how the
weight of a mountain set up in a plain might act if we can imagine
some giant force piling up the mountain higher and higher. The mere
weight of the mountain would tend to make it settle, and begin to
press outwards all round its base. If you find a difficulty in seeing
how this could be, imagine the mountain to be made of pitch. In such
a case we can quite easily realise how it would spread. Similarly
mountains, or even great plains and plateaux, of sediment built up
for millions of years in the oceans, would tend to spread; and they
would _spread towards the land which in the first place had supplied
them with materials_. At first, of course, the stiffness or rigidity
of the land would resist this spreading. But the masses thus built up
would become so great and so heavy in the course of millions of years
that no stiffness of the land could resist their spread. They would
begin to roll or slide towards the land; the heavier parts must
always roll towards the lighter. The action would be as resistless as
the slow moving onward motion of those masses of ice called glaciers;
or as the movement of the great ice plains in Greenland; or of those
ice plains which in Antarctic regions are always spreading towards
the sea.
There are two views of it. There is the outward pressure of the
regions where the sediments of rock are being laid down. There is the
inward pressure towards the regions which have lost soil. Sometimes
these two actions may conspire. A region where great denudation is
taking place may send its waste material towards the sea, where
it is deposited near the coast and not far from the highlands or
mountains or plateaux which founded the soil. The shallows near the
shore become a belt which is being loaded; the big mountains near by
are a belt which is unloading; and thus there are two strains set up
together. It is not hard to see the enormous crumpling effect which
this would produce on the lower strata one or two miles beneath the
surface of the sea and three or four miles below the topmost crests
of the mountains. These are circumstances which may not be common;
but the reader will find a quite sufficient explanation of some of
the crumpling, and many of the changes in composition and appearance
of the deep-sunk rocks, if he remembers the great pressure over them,
and the fact that the high regions may be supposed always to have a
tendency to slip towards the lowlands.