34. The student can hardly learn much about the mineralogical
composition of rocks, without at the same time acquiring some knowledge
of the manner of their occurrence in nature. We have already briefly
described certain sedimentary rocks, such as conglomerate, sandstone,
and shale, and have in some measure touched upon their structure as
rock-masses. These rocks, as we have seen, are arranged in more or less
thick layers or _beds_, which are piled one on the top of the other.
Rocks which are so arranged are said to be _stratified_, and are termed
_strata_. We may also use the word _stratum_ as an occasional substitute
for _bed_. The planes of _bedding_ or _stratification_ are sometimes
very close together, in other cases they are wide apart. When the
separate beds are very thin, as in the case of shale, it is most usual
to term them _laminae_, and to speak of the _lamination_ of a shale, as
distinguished from the _bedding_ of a sandstone. Planes of bedding are
generally more strongly marked than planes of lamination. The laminae
frequently cohere, while beds seldom do. In the above figure, which
represents a vertical cutting or _section_ through horizontal strata,
the planes of lamination are shewn at _l, l, l_, and those of
stratification at _s, s, s_. There are hardly any limits to the
thickness of a bed--it may range from an inch up to many feet or yards,
while _laminae_ vary in thickness from an inch downwards.
[Illustration: Fig. 1.--_st_, sandstone, and _sh_, shale: _s_,
lines or planes of bedding; _l_, lines or planes of
lamination.]
35. Hitherto we have been considering the _laminae_ and _strata_ as lying
in an approximately horizontal plane. Sometimes, however, the layers of
deposition in a single stratum are inclined at various angles to
themselves, as in the following figure. This structure is called _false
bedding_; the layers or laminae not coinciding with the planes of
stratification. It owes its origin to shifting currents, such as the ebb
and flow of the tide, and very often characterises deposits which have
been formed in shallow water. (Hillocks of drifting sand frequently shew
a similar structure, but their false bedding is, as a rule, much more
pronounced.)
[Illustration: Fig. 2.--False Bedding.]
36. _Mud-cracks and Rain-prints._--The surfaces of some beds
occasionally exhibit markings closely resembling those seen upon a flat
sandy beach after the retreat of the tide--hence they are called
_ripple-marks_ or _current-marks_. They are, of course, due to the
gentle current action which pushes along the grains of sand, and hence,
such marks may be formed wherever a current sweeps over the bottom of
the sea with energy just sufficient for the purpose. But since the
necessary conditions for the formation of _ripple-mark_ occur most
abundantly in shallow water, its frequent appearance in a series of
strata may often be taken as evidence, so far, for the shallow-water
origin of the beds. Besides ripple-marks we may also detect occasionally
on the surfaces of certain strata _mud-cracks_ and _rain-prints_. These
occur most commonly in fine-grained beds, as in flagstones, argillaceous
sandstones, shales, &c. The _mud-cracks_ resemble those upon a mud-flat
which are caused by the desiccation and consequent shrinkage of the mud
when exposed to the sun. The old cracks have been subsequently filled
up again by a deposition of mud or sand, usually of harder consistency
than the rock traversed by the cracks. Hence, when the bed that overlies
the mud-cracks is removed, we find a cast of these projecting from its
under surface, or frequently the cast remains in its mould, and forms a
series of curious ridges ramifying over the whole surface of the old
mud-flat. _Rain-prints_ are the small pits caused by the impact of large
drops. They are usually deeper at one side than the other, from which we
can infer the direction of the wind at the time the rain-drops fell.
Like the mud-cracks, they are most commonly met with in fine-grained
beds, and have been preserved in a similar manner. Some geologists have
also been able to detect _wave-marks_, 'faint outlinings of curved form
on a sandstone layer, like the outline left by a wave along the limit
where it dies out upon a beach.'
37. _Succession of Strata._--The succession of strata is often very
diversified. Thus, we may observe in one and the same section numberless
alternating beds of sandstone and shale from an inch or so up to several
feet each in thickness, with seams of coal, fireclay, ironstone, and
limestone interstratified among them. In other cases, again, the
succession is simpler, and some deep quarries shew only one bed, as is
the case with certain limestones, fine-grained sandstones (liver-rock),
and many volcanic rocks. Some limestones, indeed, shew small trace of
bedding throughout a vertical thickness of hundreds of feet.
38. _Beds, their Extent, &c._--Beds of rock are not only of very
different thicknesses, but they are also of very variable extent. Some
may thin gradually away, or 'die out' suddenly, in a few feet or
yards, while others may extend over many square miles. Beds of
limestone, for example, can often be traced for leagues in several
directions; and if this be the case with certain single beds, it is
still more true of groups of strata. Thus the coal-bearing strata
belonging to what is called the Carboniferous period cover large areas
in Wales, England, Scotland, and Ireland, not less, probably, than
6000 square miles; and strata belonging to the same great period
spread over considerable tracts on the Continent, and a very extensive
area in North America. It holds generally true that beds of
fine-grained materials are not only of more equal thickness
throughout, but have also a wider extension than coarser-grained
rocks. Fine sandstones, for example, extend over a wider area, and
preserve a more equable thickness throughout than conglomerates, while
limestones and coals are more continuous than either.
39. When a bed is followed for any distance it is frequently found to
thin away, and give place to another occupying the same plane or
_horizon_. Thus a shale will be replaced by a sandstone, a sandstone by
a conglomerate, and _vice versa_. Sometimes also we may find a shale, as
we trace it in some particular direction, gradually becoming charged
with calcareous matter, so as by and by to pass, as it were, into
limestone. Every bed must, of course, end somewhere, either by thus
gradually passing into another, or by thinning out so as to allow beds
which immediately overlie and underlie it to come together. Not
unfrequently, however, a bed will stop abruptly, as in fig. 3.
[Illustration: Fig. 3.--Sudden ending of Bed at x.]
40. _Sequence of Beds._--It requires little reflection to see that the
division plane between two beds may represent a very long period of
time. Let the following diagram represent a section of strata, _s_ being
beds of grit, and _a_, _b_, _c_, beds of sandstone and shale. It is
evident that the beds s must have been formed before the strata _b_ were
deposited above them. At x, the beds _a_ and _b_ come together, and were
attention to be confined to that part of the section, the observer might
be led to infer that no great space of time elapsed between the
deposition of these two beds. Yet we see that an interval sufficient to
allow of the formation of the beds _s_ must really have intervened. It
is now well known that in many cases planes of bedding represent 'breaks
in the succession' of strata--'breaks' which are often the equivalents
of considerable thicknesses of strata. In one place, for example, we may
have an apparently complete sequence of beds, as _a_, _b_, _c_, which a
more extended knowledge of the same beds, as these are developed in some
other locality, enables us to supplement, as _a_, _s_, _b_, _c_.
[Illustration: Fig. 4.--Sequence of Beds.]
41. _Joints._--Besides _planes_ or _lines of bedding_, there are certain
other division planes or _joints_ by which rocks are intersected. The
former, as we have seen, are congenital; the latter are subsequent.
Joints cut right across the bedding, and are often variously combined,
one set of joint planes traversing the rock in one direction, and
another set or sets intersecting these at various angles. Thus, in many
cases the rocks are so divided as easily to separate into more or less
irregular fragments of various sizes. Besides these confused joints
there are usually other more regular division planes, which intersect
the strata in some definite directions, and run parallel to each other,
often over a wide area: these are called _master-joints_. Two sets of
master-joints may intersect the same strata, and when such is the case,
the rock may be quarried in cuboidal blocks, the size of which will
vary, of course, according as the two sets of joints are near or wide
apart. Joints may either gape or be quite close; so close, indeed, as in
many cases to be invisible to the naked eye. Certain igneous rocks
frequently shew division planes which meet each other in such a way as
to form a series of polygonal prisms. The basalt of Staffa and Giants'
Causeway are familiar examples of this structure. Jointing is due to the
gradual consolidation of the strata, and hence, in a series of strata,
we may find the separate beds, according to their composition, very
variously affected, some being much more abundantly jointed than
others. Master-joints which traverse a wide district in some definite
direction probably owe their origin to tension, the strata having been
subjected to some strain by the underground forces.
[Illustration: Fig. 5.--Beds of Limestone (_a_), Sandstone
(_b_), and Shale (_c_), divided into cuboidal masses by
master-joints.]
[Illustration: Fig. 6.--Columnar Structure.]
[Illustration: Fig. 7.--Bedding, Joints, and Cleavage (after
Murchison).]
42. _Cleavage._--Fine-grained rocks, more especially those which are
argillaceous, occasionally shew another kind of structure, which is
called _cleavage_. Common clay-slate is a type of the structure. This
rock splits up into innumerable thin laminae or plates, the surface of
which may either be somewhat rough, or as smooth nearly as glass. The
cleavage planes, however, need not be parallel with the planes of
bedding; in most cases, indeed, they cut right across these, and
continue parallel to each other often over a very wide region. The
original bedding is sometimes entirely obliterated, and in most cases of
well-defined cleavage is always more or less obscure.
In the preceding diagram, the general phenomena of _bedding_,
_jointing_, and _cleavage_ are represented. The lines of bedding are
shewn at S, S; another set of division-planes (joints) is observed at J,
J, intersecting the former at right angles--A, B, C being the exposed
faces of joints. The lines of cleavage are seen at D, D, cutting across
the planes of bedding and jointing.
43. _Foliation_ is another kind of superinduced structure. In a
foliated rock the mineral ingredients have been crystallised and
arranged in layers along either the planes of original bedding or those
of cleavage. Mica-schist and gneiss are typical examples.
44. _Concretions._--In many rocks a concretionary structure may be
observed. Some sandstones and shales appear as if made up of spheroidal
masses, the mineral composition of the spheroids not differing
apparently from that of the unchanged rock. So in some kinds of
limestone, as in _dolomite_, the concretionary structure is often highly
developed, the rock resembling now irregular heaps of turnips with
finger-and-toe disease, again, piles of cannon-balls, or bunches of
grapes, and agglomerations of musket-shot. A spheroidal structure is
occasionally met with amongst some igneous rocks. This is well seen in
the case of rocks having the basaltic structure, in which the pillars,
being jointed transversely, decompose along their division planes, so as
to form irregular globular masses. In many cases, certain mineral matter
which was originally diffused through a rock has segregated so as to
form nodules and irregular layers. Examples of this are _chert_ nodules
in limestone; _flint_ nodules in chalk; _clay-ironstone_ balls in shale,
&c.
[Illustration: Fig. 8.--Dip and Strike of Strata.]
45. _Inclination of Strata._--Beds of aqueous strata must have been
deposited in horizontal or approximately horizontal planes; but we now
find them most frequently inclined at various angles to the horizon, and
often even standing on end. They sometimes, however, retain a horizontal
position over a large tract of country. The angle which the inclined
strata make with the horizon is called the _dip_, the degree of
inclination being the _amount_ of the dip; and a line drawn at right
angles to the dip is called the _strike_ of the beds. Thus, a bed
dipping south-west will have a north-west and south-east strike. The
_crop_ or _outcrop_ (sometimes also, but rarely, called the _basset
edge_) of a bed is the place where the edge of the stratum comes to view
at the surface. We may look upon inclined beds as being merely parts of
more or less extensive undulations of strata, the tops of the
undulations having been removed so as to expose the truncated edges of
the beds. In the following diagram, for example, the outcrops of
limestone seen at _l_, _l_, are evidently portions of one and the same
stratum, the dotted lines indicating its former extent. The
trough-shaped arrangement of the beds at _s_ is called a _synclinal
curve_, or simply a syncline; the arched strata at _a_ forming, on the
contrary, an _anticlinal curve_ or _anticline_.
[Illustration: Fig. 9.--Anticlines and Synclines.]
[Illustration: Fig. 10.--Contorted Strata.]
When strata shew many and rapid curves, they are said to be contorted.
The diagram section (fig. 10) will best explain what is meant by this
kind of structure.
46. In certain regions, the strata often dip in one and the same
direction for many miles, at an angle approaching verticality, as in the
following section. It might be inferred, therefore, that from A to B we
had a gradually ascending series--that as we paced over the outcrop we
were stepping constantly from a lower to a higher geological horizon.
But, in such cases, the dip is deceptive, the same beds being repeated
again and again in a series of great foldings of the strata. Such is
the case over wide areas in the upland districts of the south of
Scotland. The section (fig. 11) shews that the beds are actually
inverted, the strata at x x being bent back upon strata which really
overlie them.
[Illustration: Fig. 11.--Inversion of Strata.]
[Illustration: Fig. 12.--Contemporaneous Erosion.]
47. _Contemporaneous Erosion._--Occasionally a group of strata gives
proof that pauses in the deposition of sediment took place, during which
running water scooped out of the sediment channels of greater or less
width, which subsequently became filled up with similar or dissimilar
materials. The diagram (fig. 12) will render this plain. At _a_ we have
beds of sandstone, which it is evident were at one time throughout as
thick as they still are at x x. Having been worn away to the extent
indicated, a deposition of clay (_b_) succeeded; and this, in turn,
became eroded at _c_, _c_, the hollows being filled up again with coarse
sand and gravel. In former paragraphs, we found reason to believe that
lines of bedding indicated certain pauses in the deposition of strata.
Here, in the present case, we have more ample proof in the same
direction.
[Illustration: Fig. 13.--Unconformability.]
48. _Unconformability._--But the most striking evidence of such pauses
in the deposition of strata is afforded by the phenomenon called
_unconformability_. When one set of rocks is found resting on the
upturned edges of a lower set, the former are said to be _unconformable_
to the latter. In the above section (fig. 13), _a_, _a_, are beds of
sandstone resting on the upturned edges of beds of limestone, shale, and
sandstone, _l_, _s_. Figs. 14 and 15 give other examples of the same
appearance. It is evident that, in the case of fig. 14, the discordant
bedding chronicles the lapse of a very long period. We have to conceive
first of the deposition of the underlying strata in horizontal or
approximately horizontal layers; then we have to think of the time when
they were crumpled up into great convolutions, and the tops of the
convolutions (the anticlines) were planed away: all these changes
intervened, of course, after the lower set was deposited, and before the
upper series was laid down. In the case represented in fig. 15, we have
a double unconformability, implying a still more elaborate series of
changes, and probably, therefore, a still longer lapse of time.
[Illustration: Fig. 14.--Violent Unconformability.]
[Illustration: Fig. 15.--Double Unconformability.]
49. _Overlap._--When the upper beds of a conformable group of strata
spread over a wider area than the lower members of the same series, they
are said to _overlap_. The accompanying diagram shews this appearance.
An overlap proves that a gradual submergence of the land was going on at
the time the strata were being accumulated. As the land disappeared
below the water, the sediment gradually spread over a wider area, the
more recently deposited sediment being laid down in places which existed
as dry land at the time when the earliest accumulations were formed.
Thus, in the accompanying illustration (fig. 16), the stratum marked 1,
resting unconformably upon older strata, is overlapped by 2, as that is
by 3, and so on--all the beds in succession coming to repose upon the
older strata at higher and higher levels, as the old land subsided.
[Illustration: Fig. 16.--Overlap.]
[Illustration: Fig. 17.--Fault.]
50. _Dislocations or Faults._--When strata, once continuous, have been
broken across, and displaced or shifted along the line of breakage, they
are said to be _faulted_, the fissure along which the displacement
occurs being termed a _fault_ or _dislocation_. The simplest form of a
fault is that shewn in the following diagram, where strata of sandstone
and shale, with a coal-seam, S, have been shifted along the line _f_.
The direction in which the _fault_ is inclined[D] is its _hade_, and the
degree of _vertical displacement_ of the beds is the _amount_ of the
dislocation. Generally, the beds seem to be pulled _down_ in the
direction of the _downthrow_, and _drawn up_ on the opposite side of the
fault, as shewn in the diagram. Sometimes the rocks on each side of a
fault are smoothed and polished, and covered with long scratches, as if
the two sides of the fissure had been rubbed together. This is the
appearance called _slickensides_. Slickensides, however, may occur on
the walls of a fissure which is not a displacement, but a mere joint or
crack. A dislocation is spoken of as a downthrow or an upcast, according
to the direction in which it is approached. Thus, a miner working along
the coal-seam S, from _a_ to _b_, would describe the fault, _f_, as an
_upcast_, since he would have to mine to a _higher_ level to catch his
coal again. But, had he approached the fault from _c_ to _d_, he would
then have termed it a _downthrow_, because he would see from the hade of
the fault that his coal-seam must be sought for at a _lower_ level.
Faults are of all sizes, from a foot or two up to vertical displacements
of thousands of feet. Powerful dislocations can often be followed for
many miles across a country, running in a more or less linear direction.
Thus, one large fault has been traced across the breadth of Scotland,
from near St Abb's Head, in the east, to the coast of Wigtown, in the
west. Every large throw is accompanied by a number of smaller
ones--some of which run parallel to the main fault, while many others
seem to run out from this at various angles. Faults are of all
geological ages. Some date back to a most remote antiquity, others are
of quite recent origin; and no doubt faults are occurring even now. In
the following diagram, the strata, _a, a_, have been faulted and planed
away before the strata, _b_, were deposited. Hence, in this case, it is
evident that if we know the geological age of the beds, _a_ and _b_, we
can have an approximation to the age of the fault. If the beds, _a_, be
Carboniferous, and those at _b_ Permian, then we should say the fault
was _post-Carboniferous_ or _pre-Permian_.
[D] The degree of inclination is very variable. It may occur at
almost any angle up to vertical. But, as a rule, the hade of the
more powerful faults is steeper than that of minor displacements.
[Illustration: Fig. 18.--Ground-plan of Large Main Fault and
Minor Displacement Fissures.]
[Illustration: Fig. 19.--Faulted Strata covered by undisturbed
Strata.]
51. _Metamorphic and Igneous Rocks--mode of their occurrence._--In the
foregoing remarks on the structure and arrangement of rocks we have had
reference chiefly to the aqueous strata--that is to say, the
_mechanically_, _chemically_, and _organically_ formed rocks. We were
necessarily compelled, however, to make some reference to, and to give
some description of, certain structures and arrangements which are not
peculiar to aqueous strata, but characterise many metamorphic and
igneous rocks as well. To avoid repetition it was also necessary, while
treating of _joints_, &c., to give some account of certain structures
which are the result of metamorphic action. But, for sake of clearness,
we have reserved special account of the structure and mode of occurrence
of metamorphic and igneous rocks to this place. After what has been said
as to the structure and arrangement of aqueous strata, it is hardly
needful to say much about the crystalline schists. These the student
will understand to be merely highly altered aqueous rocks,[E] in which
the marks of their origin are still more or less distinctly traceable.
As a rule, metamorphic strata are contorted, twisted, and crumpled,
although here and there comparatively horizontal stretches of altered
rocks may be observed. The regions in which they occur are often hilly
and mountainous, but this is by no means invariably the case. The
greater part of the mountainous regions of the British Islands is
occupied by rocks which are more or less altered; the more crystalline
rocks, such as mica-schist, gneiss, &c., being abundantly developed in
the Scottish Highlands, and in the north and west of Ireland; while
those which are less altered cover large areas in the south of Scotland,
and in Wales and the north-west of England. Throughout these wide areas
the rocks generally dip at high angles, and contortion and crumpling are
of common occurrence. The finer-grained clay-rocks also exhibit fine
cleavage planes, and are in some places quarried for roofing-slates--the
Welsh quarries being the most famous. Here and there, bedding is
entirely effaced, and the resulting rock is quite amorphous, and,
becoming gradually more and more crystalline, passes at last into a rock
which in many cases is true granite. The original strata have
disappeared, and granite occupies their place, in such a way as to lead
to the inference that the granite is merely the aqueous strata which
have been fused up, as it were, _in situ_. At other times the granite
would appear to have been erupted amongst the aqueous strata, for these
are highly confused, and baked, as it were, at their junction with the
granite, from which, also, long veins are seen protruding into the
surrounding beds. Metamorphic granite, then, graduates, as a rule,
almost imperceptibly into rocks which are clearly of aqueous origin;
while on the contrary the junction-line between igneous granite and the
surrounding rocks is always well marked. The origin of granite, however,
is a difficult question, and one which has given rise to much
discussion. Some further remarks upon the subject will be found in the
sequel under the heading of _Metamorphism_.
[E] Igneous rocks have also in some cases undergone considerable
alteration; fine-grained tuffs, for example, occasionally
assume a crystalline texture.
52. True _igneous rocks_ occur either in beds or as irregular amorphous
masses. When they occur as beds interstratified with aqueous strata,
they are said to be _contemporaneous_, because they have evidently been
erupted at the time the series of strata among which they appear was
being amassed. When, on the other hand, they cut across the bedding,
they are said to be _subsequent_ or _intrusive_, because in this case
they have been formed at a period _subsequent_ to the strata among which
they have been _intruded_. The bed upon which a contemporaneous igneous
rock reclines, often affords marks of having been subjected to the
action of heat; sandstones being hardened, and frequently much jointed
and cracked, owing to the shrinking induced by the heat of the once
molten rock above, and clay-rocks often assuming a baked appearance.
There is generally, also, some discoloration both in the pavement of
rock upon which the igneous mass lies, and in the under portions of the
latter itself. The beds overlying a contemporaneous igneous rock,
however, do not exhibit any marks of the action of heat; the old
lava-stream having cooled before the sediment, now forming the overlying
strata, was accumulated over its surface. One may often notice how the
sand and mud have quietly settled down into the irregular hollows and
crevices of the old lava, as in the following section, where _i_
represents the igneous rock; _a_ being the baked pavement of sandstone,
&c.; and _b_ the overlying sedimentary deposits. When the igneous rock
itself is examined, its upper portions are often observed to be
scoriaceous or cinder-like, and the under portions likewise frequently
exhibit a similar appearance. It is generally most solid towards the
centre of the bed. The vesicles, or pores, in the upper and lower
portions are often flattened, and are frequently filled with mineral
matter. Sometimes these cavities may have been filled at the time the
rock was being erupted, but in most cases the mineral matter would
appear to have been introduced subsequently by the action of water
percolating through the rock. Occasionally we meet with igneous rocks
which are more or less vesicular and amygdaloidal throughout their
entire mass. Others, again, often shew no vesicular structure, but are
homogeneous from top to bottom. The texture is also very variable, and
this even in the same rock-mass; some portions being compact or
fine-grained, and others coarsely crystalline. As a rule the rock is
most crystalline towards the centre, and gets finer-grained as the top
and bottom of the bed are approached. Not unfrequently, however, an
igneous rock will preserve the same texture throughout. The jointing is
also highly irregular as a rule. But in many cases, especially when the
rock is fine-grained, the jointing is very regular. The basaltic columns
of the Giants' Causeway and the Isle of Staffa are well-known examples
of such regularly jointed masses. Igneous rocks frequently decompose
into a loose earthy mass (_wacke_), and this is most markedly the case
with those belonging to the basic group.
[Illustration: Fig. 20.--Contemporaneous Igneous Rock.]
53. Contemporaneous igneous rocks are frequently associated with more or
less regular beds of _breccia_, _conglomerate_, _ash_, _tuff_, &c. These
are evidently the loose volcanic ejectamenta which accompanied former
eruptions of lava, and have been arranged by the action of water. Beds
of such materials, however, frequently occur without any accompanying
lava-form rocks. Nor are they always arranged in bedded masses. They
sometimes appear filling vertical pipes which seem to have been the
funnels of old volcanoes. The following section exhibits the general
appearance of one of these volcanic _necks_. They are very common in
some parts of Scotland, as in Ayrshire, and are frequently ranged along
the line of a fault in the strata. Fig. 21 shews such a neck of
ejectamenta, made up of fragments of various kinds of rock, such as
sandstone, shale, limestone, coal, &c., sometimes without any admixture
of igneous rocks. The strata through which the pipe has been pierced
usually dip in towards the latter, and at their junction with the coarse
agglomerate often shew marks of the action of heat, coal-seams having
sometimes been 'burned' useless for a number of yards away from the
'neck.'
[Illustration: Fig. 21.--Neck filled with Volcanic Agglomerate.]
54. Intrusive igneous rocks occur as _sheets_, _dykes_, and _necks_. The
sheets frequently conform for long distances to the bedding of the
strata among which they occur, and are thus liable to be mistaken for
contemporaneous rocks. But when they are closely examined, it will be
seen that they not only bake or alter the beds above and below them, but
seldom keep precisely to one horizon or level--occasionally rising to a
higher, or sinking to a lower position in the strata, as shewn in the
following diagram-section. Dykes are wall-like masses of igneous strata
which cut across the strata, generally at a high angle (see _d, d_, fig.
22). In the neighbourhood of a recent volcanic orifice, numerous dykes
are seen ramifying in all directions. In the British Islands some dykes
have been followed in a linear direction for very long distances.
Sometimes these occupy the sites of large dislocations, at other times
they have cut through the strata without displacing them. Occasionally
they appear to have been the feeders of the great sheets of igneous rock
which here and there occur in their vicinity. The phenomena presented by
the _necks_ of intrusive rock do not differ from those characteristic of
_agglomerate_ or _tuff necks_. The strata are bent down towards the
central plug of igneous rock, and are generally more or less altered at
the line of junction.
[Illustration: Fig. 22.--Intrusive Sheet and Dykes: _i_, igneous
intrusive sheet; _d_, _d_, dykes; _s_, _s_, sedimentary strata.]
55. Intrusive rocks offer, as a rule, some contrasts in texture to
contemporaneous masses. They are seldom amygdaloidal, but when they are
so it is generally towards the centre of the mass. The kernels are
usually minute and more or less spherical.
[Illustration: Fig. 23.--Contemporaneous and Intrusive Igneous
Rocks: _c_, _c_, contemporaneous trap-rocks[1]; _t_, _t_,
contemporaneous fragmental igneous rocks; _i_, _p_, _n_, _d_,
intrusive igneous rocks.]
The diagram (fig. 23) shews the general mode of occurrence of igneous
rocks on the large scale. The stratified aqueous deposits are indicated
at _a_, _a_. These are overlaid by a series of alternating beds of
crystalline (_c_) and fragmental (_t_) igneous rocks. An irregular
intrusive sheet at _i_ cuts across the beds _a_, _a_. At _p_, another
intrusive mass is seen rising in a pipe, as it were, and overflowing the
beds _a_, _a_, so as to form a cap. A volcanic neck filled with angular
stones intersects the strata at _n_, and two dykes, approaching the
vertical, traverse the bedded rocks at _d_, _d_. It will be noticed that
the contemporaneous igneous rocks form a series of escarpments rising
one above the other.
The alteration effected by igneous rocks is generally greatest in the
case of intrusive masses. This is well seen in some of our coal-fields,
where the coal has frequently been destroyed over large areas by the
proximity of masses of what was once melted rock. It is curious to
notice how the intrusive sheets in a great series of strata have forced
their way along the lines of least resistance. Thus, in the Scottish
coal-fields, we find again and again that intrusive sheets have been
squirted along the planes occupied by coal-seams, these having been more
easily attacked than beds of sandstone or shale. The coal in such cases
is either entirely 'eaten up,' as it were, or converted into a black
soot. At other times, however, it is changed into a kind of coke, while
other seams at a greater distance from the intrusive mass have been
altered into a kind of 'blind coal' or _anthracite_.
These remarks on the mode of occurrence of igneous rocks are meant to
refer chiefly to those masses which occur in regions where volcanic
action has long been extinct, as, for instance, in the British Islands.
In the sequel, some account will be given of the appearances presented
by modern volcanoes and volcanic rocks.
[1] It has been usual to apply the term _trap_ or _trappean_ rock
to all the old igneous rocks which could neither be classed
with the granites and syenites, nor yet with the recent lavas,
&c., which are connected with a more or less well-marked
volcanic vent. The term _trap_ (Swedish _trappa_, a flight of
steps) was suggested by the terraced or step-like appearance
presented by hills which are built up of successive beds of
igneous rock. But the passage from the granitic into the
so-called trap rocks, and from these into the distinctly
volcanic, is so very gradual, that it is impossible to say
where the one class ends and the other begins. The term _trap_,
therefore, has no scientific precision, although it is
sometimes very convenient as a kind of broad generic term to
include a large number of rocks.