Chapter XII: Part IV: Dynamical Geology (1)

This section of the science includes the investigation of those processes of change which are at present in progress upon the earth, whereby modifications are made on the structure and composition of the crust, on the relations between the interior and the surface, as shown by volcanoes, earthquakes and other terrestrial disturbances, on the distribution of oceans and continents, on the outlines of the land, on the form and depth of the sea-bottom, on climate, and on the races of plants and animals by which the earth is tenanted. It brings before us, in short, the whole range of activities which it is the province of geology to study, and leads us to precise notions regarding their relations to each other and the results which they achieve. A knowledge of this branch of the subject is thus the essential groundwork of a true and fruitful acquaintance with the principles of geology, seeing that it necessitates a study of the present order of nature, and thus provides a key for the interpretation of the past.

The whole range of operations included within the scope of inquiry in this branch of the science may be regarded as a vast cycle of change, into which we may break at any point, and round which we may travel, only to find ourselves brought back to our starting-point. It is a matter of comparatively small moment at what part of the cycle we begin our inquiries. We shall always find that the changes we see in action have resulted from some that preceded, and give place to others which follow them.

At an early time in the earth's history, anterior to any of the periods of which a record remains in the visible rocks, the chief sources of geological action probably lay within the earth itself. If, as is generally supposed, the planet still retained a great store of its initial heat, it was doubtless the theatre of great chemical changes, giving rise, perhaps, to manifestations of volcanic energy somewhat like those which have so marvellously roughened the surface of the moon. As the outer layers of the globe cooled, and the disturbances due to internal heat and chemical action became less marked, the conditions would arise in which the materials for geological history were accumulated. The influence of the sun, which must always have operated, would then stand out more clearly, giving rise to that wide circle of superficial changes wherein variations of temperature and the circulation of air and water over the surface of the earth come into play.

In the pursuit of his inquiries into the past history and into the present _regime_ of the earth, the geologist must needs keep his mind ever open to the reception of evidence for kinds and especially for degrees of action which he had not before imagined. Human experience has been too short to allow him to assume that all the causes and modes of geological change have been definitively ascertained. On the earth itself there may remain for future discovery evidence of former operations by heat, magnetism, chemical change or otherwise, which may explain many of the phenomena with which geology has to deal. Of the influences, so many and profound, which the sun exerts upon our planet, we can as yet only perceive a little. Nor can we tell what other cosmical influences may have lent their aid in the evolution of geological changes.

Much useful information regarding many geological processes has been obtained from experimental research in laboratories and elsewhere, and much more may be confidently looked for from future extensions of this method of inquiry. The early experiments of Sir James Hall, already noticed, formed the starting-point for numerous subsequent researches, which have elucidated many points in the origin and history of rocks. It is true that we cannot hope to imitate those operations of nature which demand enormous pressures and excessively high temperatures combined with a long lapse of time. But experience has shown that in regard to a large number of processes, it is possible to imitate nature's working with sufficient accuracy to enable us to understand them, and so to modify and control the results as to obtain a satisfactory solution of some geological problems.

In the present state of our knowledge, all the geological energy upon and within the earth must ultimately be traced back to the primeval energy of the parent nebula or sun. There is, however, a certain propriety and convenience in distinguishing between that part of it which is due to the survival of some of the original energy of the planet and that part which arises from the present supply of energy received day by day from the sun. In the former case we have to deal with the interior of the earth, and its reaction upon the surface; in the latter, we deal with the surface of the earth and to some extent with its reaction on the interior. This distinction allows of a broad treatment of the subject under two divisions:

I. Hypogene or Plutonic Action: The changes within the earth caused by internal heat, mechanical movement and chemical rearrangements.

II. Epigene or Surface Action: The changes produced on the superficial parts of the earth, chiefly by the circulation of air and water set in motion by the sun's heat.

_DIVISION I.--HYPOGENE OR PLUTONIC ACTION_

In the discussion of this branch of the subject we must carry in our minds the conception of a globe still possessing a high internal temperature, radiating heat into space and consequently contracting in bulk. Portions of molten rocks from inside are from time to time poured out at the surface. Sudden shocks are generated by which destructive earthquakes are propagated through the diameter of the globe as well as to and along its surface. Wide geographical areas are pushed up or sink down. In the midst of these movements remarkable changes are produced upon the rocks of the crust; they are plicated, fractured, crushed, rendered crystalline and even fused.

(A) _Volcanoes and Volcanic Action._

This subject is discussed in the article VOLCANO, and only a general
view of its main features will be given here. Under the term volcanic
action (vulcanism, vulcanicity) are embraced all the phenomena
connected with the expulsion of heated materials from the interior of
the earth to the surface. A volcano may be defined as a conical hill
or mountain, built up wholly or mainly of materials which have been
ejected from below, and which have accumulated around the central vent
of eruption. As a rule its truncated summit presents a cup-shaped
cavity, termed the crater, at the bottom of which is the opening of
the main funnel or pipe whereby communication is maintained with the
heated interior. From time to time, however, in large volcanoes rents
are formed on the sides of the cone, whence steam and other hot
vapours and also streams of molten lava are poured forth. On such
rents smaller or parasitic cones are often formed, which imitate the
operations of the parent cone and, after repeated eruptions, may rise
to hills hundreds of feet in height. In course of centuries the result
of the constant outpouring of volcanic materials may be to build up a
large mountain like Etna, which towers above the sea to a height of
10,840 feet, and has some 200 minor cones along its flanks.

But all volcanic eruptions do not proceed from central orifices. In
Iceland it has been observed that, from fissures opened in the ground
and extending for long distances, molten material has issued in such
abundance as to be spread over the surrounding country for many miles,
while along the lines of fissure small cones or hillocks of
fragmentary material have accumulated round more active parts of the
rent. There is reason to believe that in the geological past this
fissure-type of eruption has repeatedly been developed, as well as the
more common form of central cones like Vesuvius or Etna.

In the operations of existing volcanoes only the superficial
manifestations of volcanic action are observable. But when the rocks
of the earth's crust are studied, they are found to enclose the relics
of former volcanic eruptions. The roots of ancient volcanoes have thus
been laid bare by geological revolutions; and some of the
subterranean phases of volcanic action are thereby revealed which are
wholly concealed in an active volcano. Hence to obtain as complete a
conception as possible of the nature and history of volcanic action,
regard must be had, not merely to modern volcanoes, but to the records
of ancient eruptions which have been preserved within the crust.

The substances discharged from volcanic vents consist of--(1) Gases
and vapours: which, dissolved in the molten magma of the interior,
take the chief share in volcanic activity. They include in greatest
abundance water-gas, which condenses into the clouds of steam so
conspicuous in volcanic eruptions. Hydrochloric acid and sulphuretted
hydrogen are likewise plentiful, together with many other substances
which, sublimed by the high internal temperature, take a solid form on
cooling at the surface. (2) Molten rock or lava: which ranges from the
extremely acid type of the obsidians and rhyolites with 70% or more of
silica, to the more basic and heavy varieties such as basalts and
leucite-lavas with much iron, and sometimes no more than 45% of
silica. The specific gravity of lavas varies between 2.37 and 3.22,
and the texture ranges from nearly pure glass, like obsidian, to a
coarse granitoid compound, as in some rhyolites. (3) Fragmentary
materials, which are sometimes discharged in enormous quantity and
dispersed over a wide extent of country, the finer particles being
transported by upper air-currents for hundreds of miles. These
materials arise either from the explosion of lava by the sudden
expansion of the dissolved vapours and gases, as the molten rock rises
to the surface, or from the breaking up and expulsion of portions of
the walls of the vent, or of the lava, which happens to have
solidified within these walls. They vary from the finest impalpable
dust and ashes, through increasing stages of coarseness up to huge
"bombs" torn from the upper surface of the molten rock in the vent,
and large blocks of already solidified lava, or of non-volcanic rock
detached from the sides of the pipe up which the eruptions take place.

Nothing is yet known as to the determining cause of any particular
volcanic eruption. Some vents, like that of Stromboli, in the
Mediterranean, are continually active, and have been so ever since man
has observed them. Others again have been only intermittently in
eruption, with intervals of centuries between their periods of
activity. We are equally in the dark as to what has determined the
sites on which volcanic action has manifested itself. There is reason,
indeed, to believe that extensive fractures of the terrestrial crust
have often provided passages up which the vapours, imprisoned in the
internal magma, have been able to make their way, accompanied by other
products. Where chains of volcanoes rise along definite lines, like
those of Sumatra, Java, and many other tracts both in the Old and the
New World, there appears to be little doubt that their linear
distribution should be attributed to this cause. But where a volcano
has appeared by itself, in a region previously exempt from volcanic
action, the existence of a contributing fissure cannot be so
confidently presumed. The study of certain ancient volcanoes, the
roots of which have been exposed by long denudation, has shown an
absence of any visible trace of their having availed themselves of
fractures in the crust. The inference has been drawn that volcanic
energy is capable of itself drilling an orifice through the crust,
probably at some weaker part, and ejecting its products at the
surface. The source of this energy is to be sought in the enormous
expansive force of the vapours and gases dissolved in the magma. They
are kept in solution by the enormous pressure within the earth; but as
the lava approaches the surface and this pressure is relieved these
dissolved vapours and gases rush out with explosive violence, blowing
the upper part of the lava column into dust, and allowing portions of
the liquid mass below to rise and escape, either from the crater or
from some fissure which the vigour of explosion has opened on the side
of the cone. So gigantic is the energy of these pent-up vapours, that,
after a long period of volcanic quiescence, they sometimes burst forth
with such violence as to blow off the whole of the upper part or even
one side of a large cone. The history of Vesuvius, and the great
eruptions of Krakatoa in 1883 and of Bandaizan in 1888 furnish
memorable examples of great volcanic convulsions. It has been observed
that such stupendous discharges of aeriform and fragmentary matter may
be attended with the emission of little or no lava. On the other hand,
some of the largest outflows of lava have been accompanied by
comparatively little fragmentary material. Thus, the great lava-floods
of Iceland in 1783 spread for 40 m. away from their parent fissure,
which was marked only by a line of little cones of slag.

The temperature of lava as it issues from underground has been
measured more or less satisfactorily, and affords an indication of
that existing within the earth. At Vesuvius it has been ascertained to
be more than 2000 deg. Fahr. At first the molten rock glows with a
white light, which rapidly reddens, and disappears under the rugged
brown and black crust that forms on the surface. Underneath this badly
conducting crust, the lava cools so slowly that columns of steam have
been noticed rising from its surface more than 80 years after its
eruption.

Considerable alteration in the topography of volcanic regions may be
produced by successive eruptions. The fragmentary materials are
sometimes discharged in such abundance as to cover the ground for many
miles around with a deposit of loose ashes, cinders and slag. Such a
deposit accumulating to a depth of many feet may completely bury
valleys and water-courses, and thus greatly affect the drainage. The
coarsest materials accumulate nearest to the vent that emits them. The
finer dust is not infrequently hurled forth with such an impetus as to
be carried for thousands of feet into the tracks of upper
air-currents, whereby it may be borne for hundreds of miles away from
the vent so as ultimately to fall to the ground in countries far
removed from any active volcano. Outflows of lava, from their greater
solidity and durability, produce still more serious and lasting
changes in the external features of the ground over which they flow.
As they naturally seek the lowest levels, they find their way into the
channels of streams. If they keep along the channels, they seal them
up under a mass of compact stone which the running water, if not
wholly diverted elsewhere, will take many long centuries to cut
through. If, on the other hand, the lava crosses a stream, it forms a
massive dam, above which the water is ponded back so as to form a
lake.

As the result of prolonged activity a volcanic cone is gradually built
up by successive outflows of lava and showers of dust and stones.
These materials are arranged in beds, or sheets, inclined outwards
from the central vent. On surrounding level ground the alternating
beds are flat. In course of time, deep gullies are cut on the outer
slopes of the cone by rain, and by the heavy showers that arise from
the condensation of the copious discharges of steam during eruptions.
Along the sides of these ravines instructive sections may be studied
of the volcanic strata. The larger rivers of some volcanic regions
have likewise eroded vast gorges in the more horizontal lavas and
ashes of the flatter country, and have thus laid bare stupendous
cliffs, along which the successive volcanic sheets can be seen piled
above each other for many hundred feet. On a small scale, some of
these features are well displayed among the rivers that drain the
volcanic tracts of central France; on a great scale, they are
presented in the course of the Snake river, and other streams that
traverse the great volcanic country of western North America. Similar
volcanic scenery has been produced in western Europe by the action of
denudation in dissecting the flat Tertiary lavas of Scotland, the
Faeroe Isles and Iceland.

Of special interest to the geologist are those volcanoes which have
taken their rise on the sea-bottom; for the volcanic intercalations
among the stratified formations of the earth's crust are almost
entirely of submarine origin. Many active volcanoes situated on
islands have begun their eruptions below sea-level. Both Vesuvius and
Etna sprang up on the floor of the Mediterranean sea, and have
gradually built up their cones into conspicuous parts of the dry land.
Examples of a similar history are to be found among the volcanic
islands of the Pacific Ocean. In some of these cases a movement of
elevation has carried the submarine lavas, tuffs and agglomerates
above sea-level, and has furnished opportunities of comparing these
materials with those of recent subaerial origin, and also with the
ancient records of submarine eruptions which have been preserved among
the stratified formations. From the evidence thus supplied, it can be
shown that the materials ejected from modern submarine volcanic vents
closely resemble those accumulated by subaerial volcanoes; that the
dust, ashes and stones become intermingled or interstratified with
coral-mud, or other non-volcanic deposit of the sea-bottom, that
vesicular lavas may be intercalated among them as on land, and that
between the successive sheets of volcanic origin, layers of limestone
may be laid down which are composed chiefly, or wholly, of the remains
of calcareous marine organisms.

Though active volcanoes are widely distributed over the globe, and are
especially abundant around the vast basin of the Pacific Ocean, they
afford an incomplete picture of the extent to which volcanic action
has displayed itself on the surface of our planet. When the rocks of
the land are attentively studied they disclose proofs of that action
in many districts where there is now no outward sign of it. Not only
so, but they reveal that volcanoes have been in eruption in some of
these districts during many different periods of the past, back to the
beginnings of geological history. The British Islands furnish a
remarkable example of such a series of ancient eruptions. From the
Cambrian period all through Palaeozoic times there rose at intervals
in that country a succession of volcanic centres from some of which
thousands of feet of lavas and tuffs were discharged. Again in older
Tertiary times the same region witnessed a stupendous outpouring of
basalt, the surviving relics of which are more than 3000 ft. thick,
and cover many hundreds of square miles. Similar evidence is supplied
in other countries both in the Old and the New world. Hence it is
proved that, in the geological past, volcanic action has been vigorous
at long intervals on the same sites during a vast series of ages,
though no active vents are to be seen there now. The volcanoes now
active form but a small proportion of the total number which has
appeared on the surface of the earth.

With regard to the cause of volcanic action much has been speculated,
but little can be confidently affirmed. That water in the form of
occluded gas plays the chief part in forcing the lava column up a
volcanic chimney, and in the violent explosions that accompany the
rise of the molten material, is generally admitted. But opinions
differ as to the source of this water. According to some
investigators, it should be regarded as in large measure of meteoric
origin, derived from the descent of rain into the earth, and its
absorption by the molten magma in the interior. Others, contending
that the supply so furnished, even if it could reach and be dissolved
in the magma, would yet be insufficient to furnish the prodigious
quantity of aqueous vapour discharged during an eruption, maintain
that the water belongs to the magma itself. They point to the admitted
fact that many substances, particularly metals in a state of fusion,
can absorb large quantities of vapours and gases without chemical
combination, and on cooling discharge them with eruptive phenomena
somewhat like those of volcanoes. This question must be regarded as
one of the still unsolved problems of geology.

(B) _Movements of the Earth's Crust._

Among the hypogene forces in geological dynamics an important place
must be assigned to movements of the terrestrial crust. Though the
expression "the solid earth" has become proverbial, it appears
singularly inappropriate in the light of the results obtained in
recent years by the use of delicate instruments of observation. With
the facilities supplied by these instruments (see SEISMOMETER), it has
been ascertained that the ground beneath our feet is subject to
continual slight tremors, and feeble pulsations of longer duration,
some of which may be due to daily or seasonal variations of
temperature, atmospheric pressure or other meteorological causes. The
establishment of self-recording seismometers all over the world has
led to the detection of many otherwise imperceptible shocks, over and
above the appreciable earth-waves propagated from earthquake centres
of disturbance. Moreover, it has been ascertained that some parts of
the surface of the land are slowly rising, while others are falling
with reference to the sea-level. From time to time the surface suffers
calamitous devastation from earthquakes, when portions of the crust
under great strain suddenly give way. Lastly, at intervals, probably
separated from each other by vast periods of time, the terrestrial
crust undergoes intense plication and fracture, and is consequently
ridged up into mountain-chains. No event of this kind has been
witnessed since man began to record his experiences. But from the
structure of mountains, as laid open by prolonged denudation, it is
possible to form a vivid conception of the nature and effects of these
most stupendous of all geological revolutions.

In considering this department of geological inquiry it will be
convenient to treat it under the following heads: (1) Slow depression
and upheaval; (2) Earthquakes; (3) Mountain-making; (4) Metamorphism
of rocks.

1. _Slow Depression and Upheaval._--On the west side of Japan the land
is believed to be sinking below the sea, for fields are replaced by
beaches of sand or shingle, while the depth of the sea off shore has
perceptibly increased. A subsidence of the south of Sweden has taken
place in comparatively recent times, for streets and foundations of
houses at successive levels are found below high-water mark. The west
coast of Greenland over an extent of more than 600 m. is sinking, and
old settlements are now submerged. Proofs of submergence of land are
furnished by "submerged forests," and beds of terrestrial peat now
lying at various depths below the level of the sea, of which many
examples have been collected along the shores of the British Isles,
Holland and France. Interesting evidence that the west of Europe now
stands at a lower level than it did at a late geological period is
supplied in the charts of the North Sea and Atlantic, which show that
the valleys of the land are prolonged under the sea. These valleys
have been eroded out of the rocks by the streams which flow in them,
and the depth of their submerged portions below the sea level affords
an indication of the extent of the subsidence.

The uprise of land has been detected in various parts of the world.
One of the most celebrated instances is that of the shores of the Gulf
of Bothnia, where, at Stockholm, the elevation, between the years 1774
and 1875, appears to have been 48 centimetres (18-1/2 in.) in a
century. But on the west side of Sweden, fronting the Skager Rak, the
coast, between the years 1820 and 1870, rose 30 centimetres, which is
at the rate of 60 centimetres, or nearly 2 ft. in a century. In the
region of the Great Lakes in the interior of Canada and the United
States it has been ascertained that the land is undergoing a slow tilt
towards the south-west, of which the mean rate appears to be rather
less than 6 in. in a century. If this rate of change should continue
the waters of Lake Michigan, owing to the progress of the tilt, will,
in some 500 or 600 years, submerge the city of Chicago, and eventually
the drainage of the lakes will be diverted into the basin of the
Mississippi. Proof of recent emergence of land is supplied by what are
called "raised beaches" or "strand-lines," that is, lines of former
shores marked by sheets of littoral deposits, or platforms cut by
shore-waves in rock and flanked by old sea-cliffs and lines of
sea-worn caves. Admirable examples of these features are to be seen
along the west coast of Europe from the south of England to the north
of Norway. These lines of old shores become fainter in proportion to
their antiquity. In Britain they occur at various heights, the
platforms at 25, 50 and 100 ft. being well marked.

The cause of these slow upward and downward movements of the crust of
the earth is still imperfectly understood. Upheaval might conceivably
be produced by an ascent of the internal magma, and the consequent
expansion of the overlying crust by heat; while depression might
follow any subsidence of the magma, or its displacement to another
district. If, as is generally believed, the globe is still
contracting, the shrinkage of the surface may cause both these
movements. Subsidence will be in excess, but between subsiding tracts
lateral thrust may suffice to push upward intervening more solid and
stable ground; but no solution of the problem yet proposed is wholly
satisfactory.

2. _Earthquakes._--As this subject is discussed in a separate article
it will be sufficient here to take note of its more important
geological bearings. It was for many centuries taken for granted that
earthquakes and volcanoes are due to a common cause. We have seen that
in classical antiquity they were looked on as the results of the
movements of wind imprisoned within the earth. Long after this notion
was discarded, and a more scientific appreciation of volcanic action
was reached, it was still thought that earthquakes should be regarded
as manifestations of the same source of energy as that which displays
itself in volcanic eruptions. It is true that earthquakes are frequent
in districts of active volcanoes, and they may undoubtedly be often
due there to the explosions of the magma, or to the rupture of rocks
caused by its ascent towards the surface. But such shocks are
comparatively local in their range and feeble in their effects. There
is now a general agreement that between the great world-shaking
earthquakes and volcanic phenomena, no immediate and intimate
relationship can be traced, though they may be connected in ways which
are not yet perceived. Some of the more recent great earthquakes on
land have proved that the waves of shock are produced by the sudden
rupture or collapse of rocks under great strain, either along lines of
previous fracture or of new rents in the terrestrial crust; and that
such ruptures may occur at a remote distance from any volcano. Thus
the recent disastrous San Francisco earthquake has been recognized to
have resulted from a slipping of ground along the line of an old
fault, which has been traced for a long distance in California
generally parallel to the coast. The position of this fault at the
surface has long been clearly followed by its characteristic
topography. After the earthquake these superficial features were found
to have been removed by the same cause that had originated them. For
some 300 m. on the track of this old fault-line a renewed slipping was
seen to have taken place along one or both sides, and the ground at
the surface was ruptured as well as displaced horizontally. Obviously,
the jar occasioned by the sudden and simultaneous subsidence of a
portion of the earth's crust several hundred miles long, must be far
more serious than could be produced by an earthquake radiating from a
single local volcanic focus.

From their disastrous effects on buildings and human lives, an
exaggerated importance has been imputed to earthquakes as agents of
geological change. Experience shows that even after a severe shock
which may have destroyed numerous towns and villages, together with
thousands of their inhabitants, the face of the country has suffered
scarcely any perceptible change, and that, in the course of a year or
two, when the ruined houses and prostrate trees have been cleared
away, little or no obvious trace of the catastrophe may remain. Among
the more enduring records of a great earthquake may be enumerated (a)
landslips, which lay bare hillsides, and sometimes pond back the
drainage of valleys so as to give rise to lakes; (b) alterations of
the topography, as in fissuring of the ground, or in the production of
inequalities whereby the drainage is affected; new valleys and new
lakes may thus be formed, while previously existing lakes may be
emptied; (c) permanent changes of level, either in an upward or
downward direction.

3. _Mountain-making._--This subject may be referred to here for the
striking evidence which it supplies of the importance of movements of
the earth's crust among geological processes. The structure of a great
mountain-chain such as the Alps proves that the crust of the earth has
been intensely plicated, crumpled and fractured. Vast piles of
sedimentary strata have been folded to such an extent as to occupy now
only half of their original horizontal extent. This compression in the
case of the Alps has been computed to amount to as much as 120,000
metres or 74 English miles, so that two points on the opposite sides
of that chain have been brought by so much nearer to each other than
they were originally before the movements. Besides such intense
plication, extensive rupturing of the crust has taken place in the
same range of mountains. Not only have the most ancient rocks been
squeezed up into the central axis of the chain, but huge slices of
them have been torn away from the main body, and thrust forward for
many miles, so as now actually to form the summits of mountains, which
are almost entirely composed of much younger formations. If these
colossal disturbances occurred rapidly, they would give rise to
cataclysms of inconceivable magnitude over the surface of the globe.
No record has been discovered of such accompanying devastation. But
whether sudden and violent, or prolonged and gradual, such stupendous
upturnings of the crust did undoubtedly take place, as is clearly
revealed in innumerable natural sections, which have been laid open by
the denudation of the crests and sides of the mountains.

4. _Metamorphism of Rocks_ (see METAMORPHISM).--During the movements
to which the crust of the earth has been subject, not only have the
rocks been folded and fractured, but they have likewise, in many
regions, acquired new internal structures, and have thus undergone a
process of "regional metamorphism." This rearrangement of their
substance has been governed by conditions which are probably not yet
all recognized, but among them we should doubtless include a high
temperature, intense pressure, mechanical movement resulting in
crushing, shearing and foliation, and the presence of water in their
pores. It is among igneous rocks that the progressive stages of
metamorphism can be most easily traced. Their definite original
structure and mineral composition afford a starting-point from which
the investigation may be begun and pursued. Where an igneous rock has
been invaded by metamorphic changes, it may be observed to have been
first broken down into separate lenticles, the cores of which may
still retain, with little or no alteration, the original
characteristic minerals and crystalline structure of the rock. Between
these lenticles, the intervening portions have been crushed down into
a powder or paste, which seems to have been squeezed round and past
them, and shows a laminated arrangement that resembles the
flow-structure in lavas. As the degree of metamorphism increases, the
lenticles diminish in size, and the intervening crushed and foliated
matrix increases in amount, until at last it may form the entire mass
of the rock. While the original minerals are thus broken down, new
varieties make their appearance. Of these, among the earliest to
present themselves are usually the micas, that impart their
characteristic silvery sheen to the surfaces of the folia along which
they spread. Younger felspars, as well as mica, are developed, and
there arise also sillimanite, garnet, andalusite and many others. The
texture becomes more coarsely crystalline, and the segregation of the
constituent minerals more definite along the lines of foliation. From
the finest silky phyllites a graduation may be traced through
successively coarser mica-schists, until we reach the almost granitic
texture of the coarsest gneisses.

Regional metamorphism has arisen in the heart of mountain-chains, and
in any other district where the deformation of the crust has been
sufficiently intense. There is another type of alteration termed
"contact-metamorphism," which is developed around masses of igneous
rock, especially where these have been intruded in large bosses among
stratified formations. It is particularly displayed around masses of
granite, where sandstones are found altered into quartzite, shales and
grits into schistose compounds, and where sometimes fossils are still
recognizable among the metamorphic minerals.

_DIVISION II.--EPIGENE OR SUPERFICIAL ACTION_

It is on the surface of the globe, and by the operation of agents working there, that at present the chief amount of visible geological change is effected. In considering this branch of inquiry, we are not involved in a preliminary difficulty regarding the very nature of the agencies as is the case in the investigation of plutonic action. On the contrary, the surface agents are carrying on their work under our very eyes. We can watch it in all its stages, measure its progress, and mark in many ways how accurately it represents similar changes which, for long ages previously, must have been effected by the same means. But in the systematic treatment of this subject we encounter a difficulty of another kind. We discover that while the operations to be discussed are numerous and readily observable, they are so interwoven into one great network that any separation of them under different subdivisions is sure to be more or less artificial and to convey an erroneous impression. While, therefore, under the unavoidable necessity of making use of such a classification of subjects, we must always bear in mind that it is employed merely for convenience, and that in nature superficial geological action must be continually viewed as a whole, since the work of each agent has constant reference to that of the others, and is not properly intelligible unless that connexion be kept in view.

The movements of the air; the evaporation from land and sea; the fall of rain, hail and snow; the flow of rivers and glaciers; the tides, currents and waves of the ocean; the growth and decay of organized existence, alike on land and in the depths of the sea;--in short, the whole circle of movement, which is continually in progress upon the surface of our planet, are the subjects now to be examined. It is desirable to adopt some general term to embrace the whole of this range of inquiry. For this end the word epigene (Gr. [Greek: epi], upon) has been suggested as a convenient term, and antithetical to hypogene (Gr. [Greek: hypo], under), or subterranean action.

A simple arrangement of this part of Geological Dynamics is in three sections:

A. _Air._--The influence of the atmosphere in destroying and forming rocks.

B. _Water._--The geological functions of the circulation of water through the air and between sea and land, and the action of the sea.

C. _Life._--The part taken by plants and animals in preserving, destroying or reproducing geological formations.

The words destructive, reproductive and conservative, employed in describing the operations of the epigene agents, do not necessarily imply that anything useful to man is destroyed, reproduced or preserved. On the contrary, the destructive action of the atmosphere may turn barren rock into rich soil, while its reproductive effects sometimes turn rich land into barren desert. Again, the conservative influence of vegetation has sometimes for centuries retained as barren morass what might otherwise have become rich meadow or luxuriant woodland. The terms, therefore, are used in a strictly geological sense, to denote the removal and re-deposition of material, and its agency in preserving what lies beneath it.

(A) _The Air._

As a geological agent, the air brings about changes partly by its
component gases and partly by its movements. Its destructive action is
both chemical and mechanical. The chemical changes are probably
mainly, if not entirely, due to the moisture of the air, and
particularly to the gases, vapours and organic matter which the
moisture contains. Dry air seems to have little or no appreciable
influence in promoting these reactions. As the changes in question are
similar to those much more abundantly brought about by rain they are
described in the following section under the division on rain.

Among the more recognizable mechanical changes effected in the
atmosphere, one of considerable importance is to be seen in the result
of great and rapid changes of temperature. Heat expands rocks, while
cold contracts them. In countries with a great annual range of
temperature, considerable difficulty is sometimes experienced in
selecting building materials liable to be little affected by the
alternate expansion and contraction, which prevents the joints of
masonry from remaining close and tight. In dry tropical climates,
where the days are intensely hot and the nights extremely cold, the
rapid nocturnal contraction produces a strain so great as to rival
frost in its influence upon the surface of exposed rocks,
disintegrating them into sand, or causing them to crack or peel off in
skins or irregular pieces. Dr Livingstone found in Africa (12 deg. S.
lat., 34 deg. E. long.) that surfaces of rock which during the day
were heated up to 137 deg. Fahr., cooled so rapidly by radiation at
night that, unable to sustain the strain of contraction, they split
and threw off sharp angular fragments from a few ounces to 100 or 200
[lb] in weight. In temperate regions this action, though much less
pronounced, still makes itself felt. In these climates, however, and
still more in high latitudes, somewhat similar results are brought
about by frost.

By its motion in wind the air drives loose sand over rocks, and in
course of time abrades and smoothes them. "Desert polish" is the name
given to the characteristic lustrous surface thus imparted. Holes are
said to be drilled in window glass at Cape Cod by the same agency.
Cavities are now and then hollowed out of rocks by the gyration in
them of little fragments of stone or grains of sand kept in motion by
the wind. Hurricanes form important geological agents upon land in
uprooting trees, and thus sometimes impeding the drainage of a country
and giving rise to the formation of peat mosses.

The reproductive action of the air arises partly from the effect of
the chemical and mechanical disintegration involved in the process of
"weathering," and partly from the transporting power of wind and of
aerial currents. The layer of soil, which covers so much of the
surface of the land, is the result of the decay of the underlying
rocks, mingled with mineral matter blown over the ground by wind, or
washed thither by rain, and with the mouldering remains of plants and
animals. The extent to which fine dust may be transported over the
surface of the land can hardly be realized in countries clothed with a
covering of vegetation, though even there, in dry weather during
spring, clouds of dust may often be seen blown away by wind from bare
ploughed fields. Intercepted by the leaves of plants and washed down
to their roots by rain, this dust goes to increase the soil below. In
arid climates, where dust clouds are dense and frequent, enormous
quantities of fine mineral particles are thus borne along and
accumulated. The remarkable deposit of "Loess," which is sometimes
more than 1500 ft. thick and covers extensive areas in China and other
countries, is regarded as due to the drifting of dust by wind. Again
the dunes of sand so abundant along the inner side of sandy
sea-beaches in many different parts of the world are attributable to
the same action.

(B) _Water._

In treating of the epigene action of water in geological processes it
will be convenient to deal first with its operations in traversing the
land, and then with those which it performs in the sea. The
circulation of water from land to sea and again from sea to land
constitutes the fundamental cause of most of the daily changes by
which the surface of the land is affected.

1. _Rain._--Rain effects two kinds of changes upon the surface of the
land. It acts _chemically_ upon soils and stones, and sinking under
ground continues a great series of similar reactions there. It acts
_mechanically_, by washing away loose materials, and thus powerfully
affecting the contours of the land. Its chemical action depends mainly
upon the nature and proportion of the substances which, in descending
to the earth, it abstracts from the atmosphere. Rain always absorbs a
little air, which, in addition to its nitrogen and oxygen, contains
carbonic acid, and in minute proportions, sodium chloride, sulphuric
acid and other ingredients, especially inorganic dust, organic
particles and living germs. Probably the most generally efficient of
these constituents are oxygen, carbonic acid and organic matter. Armed
with these reagents, rain effects a chemical decomposition of the
rocks on which it falls, and through which it sinks underground. The
principal changes thus produced are as follows: (a) Oxidation.--Owing
to the prominence of oxygen in rain-water, and its readiness to unite
with any substance which can contain more of it, a thin oxidized
pellicle is formed on the surface of many rocks on which rain falls,
and this oxidized layer if not at once washed off, sinks deeper until
a crust is formed over the stone. A familiar illustration of this
action is afforded by the rust, or oxide, which forms on iron when
exposed to moisture, though this iron may be kept long bright if
allowed to remain screened from moist air and rain. (b)
Deoxidation.--Organic matter having an affinity for more oxygen
decomposes peroxides by depriving them of some part of their share of
that element and reducing them to protoxides. These changes are
especially noticeable among the iron oxides so abundantly diffused
among rocks. Hence rain-water, in sinking through soil and obtaining
such organic matter, becomes thereby a reducing agent. (c)
Solution.--This may take place either by the simple action of the
water, as in the solution of rock-salt, or by the influence of the
carbonic acid present in the rain. (d) Formation of Carbonates.--A
familiar example of the action of carbonic acid in rain is to be seen
in the corrosion of exposed marble slabs. The carbonic acid dissolves
some of the lime, which, as a bicarbonate, is held in solution in the
carbonated water, but is deposited again when the water loses its
carbonic acid or evaporates. It is not merely carbonates, however,
which are liable to this kind of destruction. Even silicates of lime,
potash and soda, combinations existing abundantly as constituents of
rocks, are attacked; their silica is liberated, and their alkalis or
alkaline earths, becoming carbonates, are removed in solution. (e)
Hydration.--Some minerals, containing little or no water, and
therefore called anhydrous, when exposed to the action of the
atmosphere, absorb water, or become hydrous, and are then usually more
prone to further change. Hence the rocks of which they form part
become disintegrated.

Besides the reactions here enumerated, a considerable amount of decay
may be observed as the result of the presence of sulphuric and nitric
acid in the air, especially in that of large towns and manufacturing
districts, where much coal is consumed. Metallic surfaces, as well as
various kinds of stone, are there corroded, while the mortar of walls
may often be observed to be slowly swelling out and dropping off,
owing to the conversion of the lime into sulphate. Great injury is
likewise done from a similar cause to marble monuments in exposed
graveyards.

The general result of the disintegrating action of the air and of
rain, including also that of plants and animals, to be noticed in the
sequel, is denoted by the term "weathering." The amount of decay
depends partly on conditions of climate, especially the range of
temperature, the abundance of moisture, height above the sea and
exposure to prevalent winds. Many rocks liable to be saturated with
rain and rapidly dried under a warm sun are apt to disintegrate at the
surface with comparative rapidity. The nature and progress of the
weathering are mainly governed by the composition and texture of the
rocks exposed to it. Rocks composed of particles liable to little
chemical change from the influence of moisture are best fitted to
resist weathering, provided they possess sufficient cohesion to
withstand the mechanical processes of disintegration. Siliceous
sandstones are excellent examples of this permanence. Consisting
wholly or mainly of the durable mineral quartz, they are sometimes
able so to withstand decay that buildings made of them still retain,
after the lapse of centuries, the chisel-marks of the builders. Some
rocks, which yield with comparative rapidity to the chemical attacks
of moisture, may show little or no mark of disintegration on their
surface. This is particularly the case with certain calcareous rocks.
Limestone when pure is wholly soluble in acidulated water. Rain
falling on such a rock removes some of it in solution, and will
continue to do so until the whole is dissolved away. But where a
limestone is full of impurities, a weathered crust of more or less
insoluble particles remains after the solution of the calcareous part
of the stone. Hence the relative purity of limestones may be roughly
determined by examining their weathered surfaces, where, if they
contain much sand, the grains will be seen projecting from the
calcareous matrix, and where, should the rock be very ferruginous, the
yellow hydrous peroxide, or ochre, will be found as a powdery crust.
In limestones containing abundant encrinites, shells, or other organic
remains, the weathered surface commonly presents the fossils standing
out in relief. The crystalline arrangement of the lime in the organic
structures enables them to resist disintegration better than the
general mechanically aggregated matrix of the rock. An experienced
fossil collector will always search well such weathered surfaces, for
he often finds there, delicately picked out by the weather, minute
and frail fossils which are wholly invisible on a freshly broken
surface of the stone. Many rocks weather with a thick crust, or even
decay inwards for many feet or yards. Basalt, for example, often shows
a yellowish-brown ferruginous layer on its surface, formed by the
conversion of its felspar into kaolin, and the removal of its calcium
silicate as carbonate, by the hydration of its olivine and augite and
their conversion into serpentine, or some other hydrous magnesian
silicate, and by the conversion of its magnetite into limonite.
Granite sometimes shows in a most remarkable way the distance to which
weathering can reach. It may occasionally be dug into for a depth of
20 or 30 ft., the quartz crystals and veins retaining their original
positions, while the felspar is completely kaolinized. It is to the
endlessly varied effects of weathering that the abundant fantastic
shapes assumed by crags and other rocky masses are due. Most varieties
of rock have their own characteristic modes of weathering, whereby
they may be recognized even from a distance. To some of these features
reference will be made in Part VIII.

The mechanical action of rain, which is intimately bound up with its
chemical action, consists in washing off the fine superficial
particles of rocks which have been corroded and loosened by the
process of weathering, and in thus laying open fresh portions to the
same influences of decay. The detritus so removed is partly carried
down into the soil which is thereby enriched, partly held in
suspension in the little runnels into which the rain-drops gather as
they begin to flow over the land, partly pushed downwards along the
surface of sloping ground. A good deal of it finds its way into the
nearest brooks and rivers, which are consequently made muddy by heavy
rain.

It is natural that a casual consideration of the subject should lead
to an impression that, though the general result of the fall of rain
upon a land-surface must lead to some amount of disintegration and
lowering of that surface, the process must be so slow and slight as
hardly to be considered of much importance among geological
operations. But further attention will show such an impression to be
singularly erroneous. It loses sight of the fact that a change which
may be hardly appreciable within a human lifetime, or even within the
comparatively brief span of geological time embraced in the compass of
human history, may nevertheless become gigantic in its results in the
course of immensely protracted periods. An instructive lesson in the
erosive action of rain may be found in the pitted and channelled
surface of ground lying under the drip of the eaves of a cottage. The
fragments of stone and pebbles of gravel that form part of the soil
can there be seen sticking out of the ground, because being hard they
resist the impetus of the falling drops, protecting for a time the
earth beneath them, while that which surrounded and covered them is
washed away. From this familiar illustration the observer may advance
through every stage in the disappearance of material which once
covered the surface, until he comes to examples where once continuous
and thick sheets of solid rock have been reduced to a few fragments or
have been entirely removed. Since the whole land surface over which
rain falls is exposed to this waste, the superficial covering of
decayed rock or soil, as Hutton insisted, is constantly, though
imperceptibly, travelling outward and downward to the sea. In this
process of transport rain is an important carrying agent, while at the
same time it serves to connect the work of the other disintegrating
forces, and to make it conducive to the general degradation of the
land. Though this decay is general and constant, it is obviously not
uniform. In some places where, from the nature of the rock, from the
flatness of the ground, or from other causes, rain works under great
difficulties, the rate of waste may be extremely slow. In other places
it may be rapid enough to be appreciable from year to year. A survey
of this department of geological activity shows how unequal wasting by
rain, combined with the operations of brooks and rivers, has produced
the details of the present relief of the land, those tracts where the
destruction has been greatest forming hollows and valleys, others,
where it has been less, rising into ridges and hills (Part VIII.).

Rain-action is not merely destructive, but is accompanied with
reproductive effects, chief of which is the formation of soil. In
favourable situations it has gathered together accumulations of loam
and earth from neighbouring higher ground, such as the "brick-earth,"
"head," and "rain-wash" of the south of England--earthy deposits,
sometimes full of angular stones, derived from the subaerial waste of
the rocks of the neighbourhood.

2. _Underground Water._--Of the rain which falls upon the land one
portion flows off into brooks and rivers by which the water is
conducted back to the ocean; the larger part, however, sinks into the
ground and disappears. It is this latter part which has now to be
considered. Over and above the proportion of the rainfall which is
absorbed by living vegetation and by the soil, there is a continual
filtering down of the water from the surface into the rocks that lie
below, where it partly lodges in pores and interstices, and partly
finds its way into subterranean joints and fissures, in which it
performs an underground circulation, and ultimately issues once more
at the surface in the form of springs (q.v.). In the course of this
circulation the water performs an important geological task. Not only
carrying down with it the substances which the rain has abstracted
from the air, but obtaining more acids and organic matter from the
soil, it is enabled to effect chemical changes in the rocks
underneath, and especially to dissolve limestone and other calcareous
formations. So considerable is the extent of this solution in some
places that the springs which come to the surface, and begin there to
evaporate and lose some of their carbonic acid, contain more dissolved
lime than they can hold. They consequently deposit it in the form of
calcareous tuff or sinter (q.v.). Other subterranean waters issue with
a large proportion of iron-salts in solution which form deposits of
ochre. The various mineral springs so largely made use of for the
mitigation or cure of diseases owe their properties to the various
salts which they have dissolved out of rocks underground. As the
result of prolonged subterranean solution in limestone districts,
passages and caves (q.v.), sometimes of great width and length, are
formed. When these lie near the surface their roofs sometimes fall in
and engulf brooks and rivers, which then flow for some way underground
until the tunnels conduct them back again to daylight on some lower
ground.