Chapter XV: Part VI: , where the grouping of the stratified rocks into formations

and systems is described.

(b) _Igneous Rocks._--As part of the earth's crust these rocks present characters by which they are strongly differentiated from the stratified series. While the broad petrographical distinctions of their several varieties remain persistent, they present sufficient local variations of type to point to the existence of what have been called petrographic provinces, in each of which the eruptive masses are connected by a general family relationship, differing more or less from that of a neighbouring province. In each region presenting a long chronological series of eruptive rocks a petrographical sequence can be traced, which is observed to be not absolutely the same everywhere, though its general features may be persistent. The earliest manifestations of eruptive material in any district appear to have been most frequently of an intermediate type between acid and basic, passing thence into a thoroughly acid series and concluding with an effusion of basic material.

Considered as part of the architecture of the crust of the earth, igneous rocks are conveniently divisible into two great series: (1) those bodies of material which have been injected into the crust and have solidified there, and (2) those which have reached the surface and have been ejected there, either in a molten state as lava or in a fragmental form as dust, ashes and scoriae. The first of these divisions represents the plutonic, intrusive or subsequent phase of eruptivity; the second marks the volcanic, interstratified or contemporaneous phase.

1. The plutonic or intrusive rocks, which have been forced into the
crust and have consolidated there, present a wide range of texture
from the most coarse-grained granites to the most perfect natural
glass. Seeing that they have usually cooled with extreme slowness
underground, they are as a general rule more largely crystalline than
the volcanic series. The form assumed by each individual body of
intrusive material has depended upon the shape of the space into which
it has been injected, and where it has cooled and become solid. This
shape has been determined by the local structure of the earth's crust
on the one hand and by the energy of the eruptive force on the other.
It offers a convenient basis for the classification of the intrusive
rocks, which, as part of the framework of the crust, may thus be
grouped according to the shape of the cavity which received them, as
bosses, sills, dikes and necks.

Bosses, or stocks, are the largest and most shapeless extravasations
of erupted material. They include the great bodies of granite which,
in most countries of the world, have risen for many miles through the
stratified formations and have altered the rocks around them by
contact-metamorphism. Sills, or intrusive sheets, are bed-like masses
which have been thrust between the planes of sedimentary or even of
igneous rocks. The term laccolite has been applied to sills which are
connected with bosses. Intrusive sheets are distinguishable from true
contemporaneously intercalated lavas by not keeping always to the same
platform, but breaking across and altering the contiguous strata, and
by the closeness of their texture where they come in contact with the
contiguous rocks, which, being cold, chilled the molten material and
caused it to consolidate on its outer margins more rapidly than in its
interior. Dikes or veins are vertical walls or ramifying branches of
intrusive material which has consolidated in fissures or irregular
clefts of the crust. Necks are volcanic chimneys which have been
filled up with erupted material, and have now been exposed at the
surface after prolonged denudation has removed not only the
superficial volcanic masses originally associated with them, but also
more or less of the upper part of the vents. Plutonic rocks do not
present evidence of their precise geological age. All that can be
certainly affirmed from them is that they must be younger than the
rocks into which they have been intruded. From their internal
structure, however, and from the evidence of the rocks associated with
them, some more or less definite conjectures may be made as to the
limits of time within which they were probably injected.

2. The interstratified or volcanic series is of special importance in
geology, inasmuch as it contains the records of volcanic action during
the past history of the globe. It was pointed out in Part I. that
while towards the end of the 18th and in the beginning of the 19th
century much attention was paid by Hutton and his followers to the
proofs of intrusion afforded by what they called the "unerupted lavas"
within the earth's crust, these observers lost sight of the
possibility that some of these rocks might have been erupted at the
surface, and might thus be chronicles of volcanic action in former
geological periods. It is not always possible to satisfactorily
discriminate between the two types of contemporaneously intercalated
and subsequently injected material. But rocks of the former type have
not broken into or involved the overlying strata, and they are usually
marked by the characteristic structures of superficial lavas and by
their association with volcanic tuffs. By means of the evidence which
they supply, it has been ascertained that volcanic action has been
manifested in the globe since the earliest geological periods. In the
British Isles, for example, the volcanic record is remarkably full for
the long series of ages from Cambrian to Permian time, and again for
the older Tertiary period.

2. SUBSEQUENTLY INDUCED STRUCTURES

After their accumulation, whether as stratified or eruptive masses, all kinds of rocks have been subject to various changes, and have acquired in consequence a variety of superinduced structures. It has been pointed out in the part of this article dealing with dynamical geology that one of the most important forms of energy in the evolution of geological processes is to be found in the movements that take place within the crust of the earth. Some of these movements are so slight as to be only recognizable by means of delicate instruments; but from this inferior limit they range up to gigantic convulsions by which mountain-chains are upheaved. The crust must be regarded as in a perpetual state of strain, and its component materials are therefore subject to all the effects which flow from that condition. It is the one great object of the geotectonic division of geology to study the structures which have been developed in consequence of earth-movements, and to discover from this investigation the nature of the processes whereby the rocks of the crust have been brought into the condition and the positions in which we now find them. The details of this subject will be found in separate articles descriptive of each of the technical terms applied to the several kinds of superinduced structures. All that need be offered here is a general outline connecting the several portions of the subject together.

One of the most universal of these later structures is to be seen in
the divisional planes, usually vertical or highly inclined, by which
rocks are split into quadrangular or irregularly shaped blocks. To
these planes the name of joints has been given. They are of prime
importance from an industrial point of view, seeing that the art of
quarrying consists mainly in detecting and making proper use of them.
Their abundance in all kinds of rocks, from those of recent date up to
those of the highest antiquity, affords a remarkable testimony to the
strains which the terrestrial crust has suffered. They have arisen
sometimes from tension, such as that caused by contraction from the
drying and consolidation of an aqueous sediment or from the cooling of
a molten mass; sometimes from torsion during movements of the crust.

Although the stratified rocks were originally deposited in a more or
less nearly horizontal position on the floor of the sea, where now
visible on the dry land they are seldom found to have retained their
flatness. On the contrary, they are seen to have been generally tilted
up at various angles, sometimes even placed on end (crop, dip,
strike). When a sufficiently large area of ground is examined, the
inclination into which the strata have been thrown may be observed not
to continue far in the same direction, but to turn over to the
opposite or another quarter. It can then be seen that in reality the
rocks have been thrown into undulations. From the lowest and flattest
arches where the departure from horizontality may be only trifling,
every step may be followed up to intense curvature, where the strata
have been compressed and plicated as if they had been piles of soft
carpets (anticline, syncline, monocline, geo-anticline, geo-syncline,
isoclinal, plication, curvature, quaquaversal). It has further
happened abundantly all over the surface of the globe that relief from
internal strain in the crust has been obtained by fracture, and the
consequent subsidence or elevation of one or both sides of the
fissure. The differential movement between the two sides may be
scarcely perceptible in the feeblest dislocation, but in the extreme
cases it may amount to many thousand feet (fault, fissure,
dislocation, hade, slickensides). The great faults in a country are
among its most important structural features, and as they not
infrequently continue to be lines of weakness in the crust along which
sudden slipping may from time to time take place, they become the
lines of origin of earthquakes. The San Francisco earthquake of 1906,
already cited, affords a memorable illustration of this connexion.

It is in a great mountain-chain that the extraordinary complication of
plicated and faulted structures in the crust of the earth can be most
impressively beheld. The combination of overturned folds with rupture
has been already referred to as a characteristic feature in the Alps
(Part IV.). The gigantic folds have in many places been pushed over
each other so as to lie almost flat, while the upper limb has not
infrequently been driven for many miles beyond the lower by a rupture
along the axis. In this way successive slices of a thick series of
formations have been carried northwards on the northern slope of the
Alps, and have been piled so abnormally above each other that some of
their oldest members recur several times on different thrust-planes,
the whole being underlain by Tertiary strata (see ALPS). Further
proof of the colossal compression to which the rocks have been
subjected is afforded by their intense crumpling and corrugation, and
by the abundantly faulted and crushed condition to which they have
been reduced. Similar evidence as to stresses in the terrestrial crust
and the important changes which they produce among the rocks may also
be obtained on a smaller scale in many non-mountainous countries.

Another marked result of the compression of the terrestrial crust has
been induced in some rocks by the production of the fissile structure
which is typically shown in roofing-slate (cleavage). Closely
connected with this internal rearrangement has been the development of
microscopic microlites or crystals (rutile, mica, &c.) in argillaceous
slates which were undoubtedly originally fine marine mud and silt.
From this incipient form of metamorphism successive stages may be
traced through the various kinds of argillite and phyllite into
mica-schist, and thence into more crystalline gneissoid varieties
(foliation, slate, mica-schist, gneiss). The Alps afford excellent
illustrations of these transformations.

The fissures produced in the crust are sometimes clean, sharply
defined divisional planes, like cracks across a pane of glass. Much
more usually, however, the rocks on either side have been broken up by
the friction of movement, and the fault is marked by a variable
breadth of this broken material. Sometimes the walls have separated
and molten rock has risen from below and solidified between them as a
dike. Occasionally the fissures have opened to the surface, and have
been filled in from above with detritus, as in the sandstone-dikes of
Colorado and California. In mineral districts the fissures have been
filled with various spars and ores, forming what are known as mineral
veins.

Where one series of rocks is covered by another without any break or
discordance in the stratification they are said to be conformable. But
where the older series has been tilted up or visibly denuded before
being overlain by the younger, the latter is termed unconformable.
This relation is one of the greatest value in structural geology, for
it marks a gap in the geological record, which may represent a vast
lapse of time not there recorded by strata.