by geologists, piled in more or less irregular beds, one on top of
another, in a somewhat confused manner. The colour of the freshly
fractured rocks is very variable, being usually some shade of blue or
purple; but pink, red, brown, greenish, and dark grey or almost black
varieties also occur. Some of the rocks are finely crystalline; others,
again, are much coarser, while many are compact, or nearly so, a lens
being required to detect a crystalline texture. The mineral called
felspar is usually scattered more or less abundantly through the matrix
or base, which itself is composed principally of felspathic materials.
Besides distinct scattered crystals of felspar, other minerals often
occur in a similar manner; mica and hornblende being the commonest.
Occasionally the rocks contain numerous circular, oval, or flattened
cavities, which are sometimes so abundant as to give the appearance
of a kind of coarse slag to the porphyrite. These little cavities,
however, are usually filled up with mineral matter--such as calcspar,
calcedony, jasper, quartz, etc. Sometimes also cracks, crannies, and
crevices of some size have been sealed up with similar minerals. Now
nearly all these appearances are specially characteristic of rocks
which have at one time been in a state of igneous fusion; nor can
there be any doubt that the Cheviot porphyrites are merely solidified
lava-beds, which have been poured out from the bowels of the earth.
In modern lavas we may notice not only a crystalline texture, but
frequently also we observe those in our porphyrites. Such cavities are
due to the expansive force of the vapours imprisoned in the molten mass
at the time of eruption. They form chiefly towards the upper surface of
a lava stream, and are often drawn out or flattened in the direction
in which the lava flows. Thus a stream of lava, as it creeps on its
way, becomes slaggy and scoriaceous or cindery above and in front, and
as the molten mass within continues to flow, the slags and cinders
that cover its face tumble down before it, and form the pavement
upon which the stream advances. In this way slags and cinders become
incorporated with the bottom of the lava, and hence it is that so many
volcanic rocks are scoriaceous, as well below as above. The vapours
which produce the cavities usually contain minerals in solution, and
these, as the lava cools, are frequently deposited, partially filling
up the vesicles, so as to form what are called geodes. But many of
the cavities have been filled in another way--by the subsequent
infiltration of water carrying mineral matter in solution. And since
we know that all rocks are so permeated by water, it is clear that
the cavities may have received their contents during many successive
periods, after the solidification of the rock in which they occur. It
is in this manner that the jaspers, calcedony, and beautiful agates of
commerce have been formed. Rocks abundantly charged with cavities are
said to be _vesicular_, and when the vesicles are filled with mineral
matter, then the mass becomes, in geological language, _amygdaloidal_,
from the almond-like shape assumed by the flattened vesicles.
Now all the appearances described above, and many others hardly
less characteristic of true lavas, are to be met with amongst those
porphyrites which, as I have said, form the major portion of the
Cheviot Hills. From the valley of the Oxnam, east by Cessford,
Morebattle, and Hoselaw, and south by Edgerston, Letham, Browndeanlaws,
and Hindhope, the porphyrites extend over the whole area, sweeping
north-east across the border on to the heights above the Rivers Glen
and Till. In the hills at Hindhope we notice a good display of the
oldest beds of the series. At the base occurs a very peculiar rock
resting upon the Silurian, and thus forming the foundation of the
porphyrites. It varies in colour, being pink, grey, green, red, brown,
or variously mottled. Sometimes it is fine-grained and gritty, like
a soft, coarse-grained sandstone; at other times it is not unlike a
granular porphyrite; but when most typically developed it consists of a
kind of coarse angular gravel embedded in a gritty matrix. The stones
sometimes show distinct traces of arrangement into layers; but they are
often heaped rudely together with little or no stratification at all.
They consist chiefly of fragments of porphyrites; but bits of Silurian
rocks also occur amongst them. This peculiar deposit unquestionably
answers to the heaps of dust, sand, stones, and bombs which are shot
out of modern volcanoes; it is a true tuff--that is, a collection of
loose volcanic ejectamenta.
Upon what kind of surface did it fall? Long before the eruptions began,
the Silurian rocks had been sculptured into hills and valleys by the
action chiefly of the sub-aërial forces, and it was upon these hills
and in these valleys that the igneous materials accumulated. It is
difficult to say, however, whether at this period the Cheviot district
was above or under water. The traces of bedding in the tuff would seem
to indicate the assorting power of water; but the evidence is too
slight to found upon, because we know that in modern eruptions, loose
ejectamenta frequently assume a kind of irregular bedded arrangement.
For aught we can say to the contrary, therefore, dry land may have
extended across what is now southern Scotland and northern England
when the first rumblings of volcanic disturbance shook the Cheviot
area. Be that as it may, we know that the volcanic outbursts began in
those old times, as they almost invariably commence now, by a discharge
of sand, small stones, blocks, and cinders. These, we may infer,
covered a wide area round the centre of dispersion--the chief focus
of eruption being probably in the vicinity of the big Cheviot, where
a mass of granite seems to occupy the core or deep-seated portion of
the old volcanic centre. The locality where the tuff occurs is some
nine miles or so distant from this point, and the intervening ground
could hardly have escaped being more or less thickly sprinkled with
the same materials. The whole of that intervening ground, however, now
lies deeply buried under the massive streams of once-molten rock that
followed in succession after the first dispersion of stones and débris.
Although, as I have said, it may be doubted whether at the beginning of
their activity the Cheviot volcanoes were sub-aqueous, yet there are
not a few facts that lead to the inference that the eruption of the
porphyrites took place for the most part, if not exclusively, under
water. The beds are occasionally separated by layers of sandstone,
grit, and conglomerate; but such beds are rare, and true tuffs are
rarer still. If the outbursts had been sub-aërial, we ought surely to
have met with these latter in greater abundance, while we should hardly
have expected to find such evidently water-arranged strata as do occur
here and there. The porphyrites themselves present certain appearances
which lead to the same conclusion. Thus we may observe how the bottoms
of the beds frequently contain baked or hardened sand and mud, showing
that the molten rock had been poured out over some muddy or sandy
bottom, and had caught up and enclosed the soft, sedimentary materials,
which now bear all the marks of having been subjected to the action
of intense heat. Sometimes, indeed, the old lava-streams seem to have
licked up beds of unconsolidated gravel, the water-worn stones being
now scattered through their under portions. As no fossils occur in any
of the beds associated with the porphyrites, one cannot say whether
the latter flowed into the sea or into great freshwater lakes. Neither
can we be certain that towards their close the eruptions were not
sub-aërial. They may quite well have been so. The porphyrites attain
a thickness of probably not less than fifteen hundred or two thousand
feet, and the beds which we now see are only the basal, and therefore
the older portions of the old volcanoes. The upper parts have long
since disappeared, the waste of the igneous masses having been so great
that only the very oldest portions now remain, and these, again, are
hewn and carved into hill and valley. Any loose accumulation of stones
and débris, therefore, which may have been thrown out in the later
stages of the eruptions, must long ere this have utterly disappeared.
We can point to the beds which mark the beginning of volcanic activity
in the Cheviots; we can prove that volcanoes continued in action there
for long ages, great streams of lava being poured out--the eruptions of
which were preceded and sometimes succeeded by showers of stones and
débris; we can show, also, that periods of quiescence, more or less
prolonged, occasionally intervened, at which times water assorted the
sand and mud, and rounded the stones, spreading them out in layers.
But whether this water action took place in the sea or in a lake we
cannot tell. Indeed, for aught one can say, some of the masses of
rounded stones I refer to may point to the action of mountain torrents,
and thus be part evidence that the volcanoes were sub-aërial. If we
are thus in doubt as to some of the physical conditions that obtained
in the Cheviot district during the accumulation of the porphyrites
and their associated beds, we are left entirely to conjecture when
we seek to inquire into the conditions that prevailed towards the
close of the volcanic period. For just as we have proof that before
this period began the Silurian strata had been subjected to the most
intense denudation--had, in short, been worn into hill and valley--so
do we learn from abundant evidence that the rocks representing the old
volcanoes of the Cheviots are merely the wrecks of formerly extensive
masses. Not only have the upper portions of these volcanoes been swept
away, but their lower portions, likewise, have been deeply incised, and
thousands of feet of solid rock have been carried off by the denuding
forces. And by much the greater part of all this waste took place
before the accumulation of those sandstones which now rest upon the
worn outskirts of the old volcanic region.
III.
Some reference has already been made (see p. 64) to the general
appearance presented by the valleys of the Cheviots. In their upper
reaches they are often rough and craggy; narrow dells, in fact, flanked
with steep shingle-covered slopes, and occasionally overlooked by
beetling cliffs, or fringed with lofty scaurs of decomposing rocks. As
we follow down the valleys they gradually widen out; the hill-slopes
becoming less steep, and retiring from the stream so as to leave a
narrow strip of meadow-land through which the clear waters canter gaily
on to the low grounds of the Teviot. In their middle reaches these
upland dales are not infrequently well cultivated to a considerable
height, as in the districts between Hownam and Morebattle, and between
Belford and Yetholm--the former in the valley of the Kale, and the
latter in that of the Bowmont. It is noticeable that all the narrower
and steeper reaches lie among Silurian strata and Old Red Sandstone
porphyrites. No sooner do we leave the regions occupied by these tough
and hard rock-masses than the whole aspect of the scenery changes.
The surrounding hills immediately lose in height and fall away into a
softly undulating country, through which the streams and rivers have
dug for themselves deep romantic channels. Nevertheless, it is a fact,
as we shall see by-and-by, that south-west of the region occupied by
the igneous rocks of the Cheviot Hills, all the higher portions of
the range (Hungry Law, Carter Fell, Peel Fell, etc.) are built up of
sandstones. For the present, however, I confine attention to those
valleys whose upper reaches lie either wholly or in part among igneous
rocks or Silurian strata. A typical and certainly the most beautiful
example is furnished us by the vale of the River Jed. This stream
rises among the sandstone heights which have just been mentioned as
composing the south-west portion of the Cheviot range. The first seven
or eight miles of its course lead us through a broad open valley, which
has been hollowed out almost exclusively in sandstones and shales;
by-and-by, however, we are led into a Silurian tract, and thereupon
the valley contracts and the hill-slopes descend more steeply to the
stream. But we soon leave the grassy glades of this Silurian tract and
enter all at once upon what may be termed the lower reaches of the
Jed. No longer cooped up in the rocky gully, painfully worn for itself
in the hard greywacké and shales, the stream now winds through a much
deeper and broader channel which has evidently been excavated with
greater ease. Precipitous banks and scaurs here overlook the river at
every bend, the banks becoming higher and higher and retiring further
and further from each other, as the water glides on its way, until at
last they fairly open upon the broad vale of the Teviot. Sometimes the
river flows along one side of its valley for a considerable distance,
and whenever this is the case, it gives us a line of bold cliffs which
are usually flanked on the opposite side by sloping ground. This is
the general character of all valleys of erosion, and especially of the
lower reaches of the Jed.
A glance at the cliffs and scaurs of the Jed shows that they consist of
horizontal or gently undulating strata of soft earthy, friable, shaly
sandstone, arranged in thin beds and bands, which alternate rapidly
with crumbling, sandy, and earthy shales; the whole forming a loose
and unconsolidated mass that readily becomes a prey to the action of
the weather, rain, frost, and running water. The prevailing colour is
a dull red, but pale yellow, white, green, and purple discolorations
are visible when the strata are closely scanned. The finest sections
occur between Glen Douglas and Inchbonnie, and at Mossburnford, but
the cliffs throughout present the same general appearance, and are
picturesque in the highest degree. Everywhere the banks are thickly
wooded, and even the steep red scaurs are dashed and flecked with
greenery, which droops and springs from every ledge and crevice in
which a root can fix itself. How vivid and striking is the contrast
between the fresh delicate green of early summer and the rich warm
tint of these rocks, which when lit up by the setting sun seem almost
to glow and burn! Well may the good folk of Jedburgh be proud of the
lovely valley in which their lot is cast. In no similar district in
Scotland will the artist meet with a greater number of such "delicious
bits," in which all the charms of wood and water, of meadow and rock
are so harmoniously combined. It is not with the scenic beauties of
the Jed, however, that we have at present to do. I wish the reader
to examine with me certain appearances visible at the base of the
red beds, where these rest upon those older rocks which have formed
the subject of the preceding papers. In the bed of the river at
Jedburgh, we see the junction between the red beds and the Silurian
strata, and may observe how the bottom portions of the former, which
repose immediately upon the greywackés, are abundantly charged with
well-rounded and water-worn stones. Many of these stones consist of
greywacké, hardened grit, and other kinds of rock, and most of them
undoubtedly have been derived from Silurian strata. In other districts
where the old igneous rocks of the Cheviots form the pavement upon
which the red beds repose, the stones in the lower portions of the
latter are made up chiefly of rounded fragments of the underlying
porphyrites. All which clearly shows that the red beds have been
built out of the ruins of the older strata of the district. This is
unquestionably the origin not only of the conglomerates, but of all
the red beds through which the River Jed cuts its way from the base
of the hills to the Teviot. When we trace out the boundary of these
beds, we find that this leads us along the base of the hills, close to
the hill-foot; and not only so, but it frequently takes us into the
hill-valleys also. And this shows that the Cheviots had already been
deeply excavated by streams before any portion of the red beds was
deposited.
I have said that the red beds are approximately horizontal; sometimes,
however, they have a decided _dip_ or inclination, and when this is
continuous, it is invariably in a direction away from the hills. Thus
as we traverse the ground from the hill-foot to the Teviot, we pass
over the outcrops of the red beds and slowly rise from a lower to a
higher geological position. The strata, however, are generally so
flat that their dip is often not greater than the average slope or
inclination of the ground. Hence when we ascend the valley-slopes
from the stream, we soon reach the higher beds of the series, as,
for example, in the undulating heights that overlook the Jed in the
neighbourhood of Jedburgh. In that district a number of quarries
have been opened, in which the upper beds of the red series are well
exposed, as at Ferniehirst, Tudhope, etc. These consist of thick beds
of greyish white, yellowish, and reddish sandstones, which, unlike
the crumbling earthy deposits below, are quite suitable for building
purposes. Scales of fish and plant remains are often met with in the
thick sandstones, but the underlying earthy, friable red beds appear to
be quite destitute of any organic remains.
Let us now briefly recapitulate the main facts we have just
ascertained. They are these:--1. All the low grounds that abut upon the
hills are composed of horizontal or nearly horizontal strata, which
consist chiefly of red earthy beds, passing down into conglomerates,
and up into whitish and reddish sandstones. 2. The conglomeratic
portion forms the boundary of the series, fringing the outskirts of the
hills, and resting sometimes upon Silurian strata and sometimes upon
Old Red Sandstone igneous rocks. 3. Fossils occur in the white and red
sandstones, but seem to be wanting in the underlying red earthy beds.
[Illustration: Fig. 7.--S, Silurian strata; _i_, Old Red Sandstone
Igneous Rocks; _a^1_, Conglomerate; _a^2_, Red earthy beds; _a^3_,
White and Red Sandstones.]
The accompanying diagram (Fig. 7) gives a generalised view of the
relation borne by the red beds to the older rocks of the Cheviots.
It will be seen that the former rest _unconformably_ upon the Old
Red Sandstone igneous rocks, and also, of course, upon the Silurian
strata. The section shows that the red beds lie upon a worn and
denuded surface. Now this speaks to the lapse of a long period of
time. It may be remembered that we had some grounds for believing that
the latest eruptions of the Cheviot volcanoes were sub-aërial. The
evidence now enables us to advance further, and to state that after
the close of the volcanic period, the whole Cheviot district existed
as an elevated tract of dry land, from which streams flowed north and
south. And for so long a time did these conditions endure, that the
rivulets and streams were enabled to scoop out many channels and broad
valleys before any of the outlying red beds had come into existence.
Before the conglomerate beds were laid down, the ancient volcanic
bank of the Cheviots had thus suffered great erosion. This is what
"unconformability" means. It points to the prolonged continuance of
a land-surface, subject as that must always be to the wearing action
of the sub-aërial forces. Rain and frost disintegrate the rocks, and
running water rolls the débris from higher to lower levels, and piles
it up in the form of gravel, sand, and mud in lakes and the sea. While
the old volcanic country of the Cheviots was being thus denuded,
it would appear that a wide extent of land existed in the Northern
Highlands and Southern Uplands of Scotland, and also in what are now
the lake districts of England and the hilly tracts of Wales. And in all
these regions valleys were formed, which at a subsequent time were more
or less filled up with newer deposits.
The presence of the red beds that sweep round the base of the Cheviot
Hills shows unmistakably that a period of submergence followed these
land conditions. All the low grounds of Southern Scotland disappeared
beneath a wide sheet of water, which stretched from the foot of the
Lammermuirs up to the base of the Cheviots, and here and there entered
the valleys, and so extended into the hills. This water, however, does
not seem to have been that of an open sea; rather was it portion of a
great freshwater lake, brackish lagoon, or inland sea. The lowest beds
of the red series are merely hardened layers and masses of gravel and
rolled shingle, which would seem at first sight to indicate the former
action of waves along a sea-beach. There are certain appearances,
however, which lead one to suspect that these ancient shingle beds
may have had quite another origin. In some places the stones exactly
resemble those which are found so abundantly in glacial deposits. They
are sub-angular and blunted, and, like glaciated stones, occasionally
show striæ or scratches. This, however, is very rarely the case. Most
of the stones appear subsequently to have been rolled about in water,
and in this process they must have lost any ice-markings they may have
had, and become smoothed and rounded like ordinary gravel stones.
The same appearances may be noted in the glacier valleys of Norway
and Switzerland, where at the present day the glaciated stones which
are pushed out at the lower ends of the glaciers are rolled about in
the streams, and soon lose all trace of ice-work. It is impossible,
however, to enter here into all the details of the evidence which lead
one to suspect that glaciers may have existed at this early period
among the Cheviot and Lammermuir Hills. In the latter district, the
conglomerates occur in such masses and so exactly resemble the morainic
débris and ice-rubbish of modern glacial regions, that the late Sir A.
C. Ramsay long ago suggested their ice-origin.
Let us conceive, then, that when the ancient lake or inland sea of
which I have spoken reached the base of the Cheviots, glaciers may
have nestled in the valleys. Streams issuing from the lower ends of
these would sweep great quantities of gravel down the valleys to the
margin of the lake, and it is quite possible that there might be enough
wave-action to spread the gravel out along the shores. It is evident,
however, that the main heaps of shingle would gather opposite what were
at that time the mouths of glacier valleys; and it is just in such
positions that we now meet with the thickest masses of conglomerate.
Ere long, however, the supposed glaciers would seem to have melted
away, and only fine sand and mud, with here and there small rounded
stones and grit, accumulated round the shores of the ancient lake. Of
course, during all this time fine-grained sediment gathered over the
deeper parts of the lake-bottom.
We have no evidence to show what kind of creatures, if any, inhabited
the land at this time; nor do any fossils occur in the red earthy beds
to throw light upon the conditions of life that may have obtained in
the lake. If glaciers really existed and sent down ice-cold water,
the conditions would hardly be favourable to life of any kind; for
glacial lakes are generally barren. But the absence of fossils may
be due to other causes than this. It is a remarkable fact, that red
strata are, as a rule, unfossiliferous, and the few fossils which
they do sometimes yield are generally indicative rather of lacustrine
and brackish-water, than marine conditions. The paucity or absence of
organic remains seems to have been often due to the presence in the
water of a superabundance of salts. Now this excessive salinity may
have arisen in either of two ways. First, we may suppose some wide
reach of the sea to have been cut off from communication with the
open ocean by an elevation of a portion of its bed; and in this case
we should have a lagoon of saltwater, which evaporation would tend to
concentrate to such a degree, that by-and-by nothing would be able to
live in its waters. Or, again, we may have a lake so poisoned by the
influx of springs and streams, carrying various salts in solution,
as to render it uninhabitable by life of any kind, either animal or
vegetable. Many red sandstone deposits, as Sir A. C. Ramsay has pointed
out, are evidently lagoon-formations, which is proved by the presence
of associated beds of rock-salt, gypsum, and magnesian limestone.
They have slowly accumulated in great inland seas or lakes having no
outlet, whose waters were subject to evaporation and concentration,
although now and then they seem to have communicated more or less
freely with the ocean. The red earthy beds of the Jed, however, though
unfossiliferous, yet contain no trace of rock-salt or magnesian
limestone. The only character they have in common with the salt-bearing
strata of the New Red Sandstone of England is their colour, due to
the presence of peroxide of iron, which we can hardly conceive could
have been deposited in the mud of a sea communicating freely with the
ocean. But a quiet lake, fed by rivulets and streams that drained an
old volcanic district, is precisely the kind of water-basin in which
highly ferruginous mud and sand might be expected to accumulate. Such
a lake, tainted with the various salts, etc., carried into it by
streams and springs (some of which may have been thermal; for, as we
shall see presently, the volcanic forces, although quiescent, were yet
not extinct), might well be unfitted for either animal or plant, and
probably this is one reason why the red earthy beds of the Jed are so
unfossiliferous.