The Lines of the Geographer rarely right Lines.--These last,
however, always worth looking at when they occur.--Striking Instance
in the Line of the Great Caledonian Valley.--Indicative of the
Direction in which the Volcanic Agencies have operated.--Sections
of the Old Red Sandstone furnished by the Granitic Eminences of
the Line.--Illustration.--Lias of the Moray Frith.--Surmisings
regarding its Original Extent.--These lead to an Exploratory
Ramble.--Narrative.--Phenomena exhibited in the course of half an
hour's Walk.--The little Bay.--Its Strata and their Organisms.
The natural boundaries of the geographer are rarely described by
right lines. Whenever these occur, however, the geologist may
look for something remarkable. There is one very striking example
furnished by the north of Scotland. The reader, in consulting a map
of the kingdom, will find that the edge of a ruler, laid athwart the
country in a direction from south-west to north-east, touches the
whole northern side of the great Caledonian Valley, with its long,
straight line of lakes; and onwards, beyond the valley's termination
at both ends, the whole northern side of Loch Eil and Loch Linnhe,
and the whole of the abrupt and precipitous northern shores of the
Moray Frith, to the extreme point of Tarbat Ness--a right line of
considerably more than a hundred miles. Nor does the geography of the
globe furnish a line better defined by natural marks. There is both
rampart and fosse. On the one hand we have the rectilinear lochs and
lakes, with an average profundity of depth more than equal to that
of the German Ocean, and, added to these, the rectilinear lines of
frith; on the other hand, with but few interruptions, there is an
inclined wall of rock, which rises at a steep angle in the interior
to nearly two thousand feet over the level of the Great Canal, and
overhangs the sea towards its northern termination, in precipices of
more than a hundred yards.[AF]
[Footnote AF: The valley of the Jordan, from the village of Laish
to the southern extremity of the Dead Sea, furnishes another very
remarkable instance of a geographical right line.]
The direction of this rampart and fosse--this Roman wall of Scottish
geological history--seems to have been that in which the volcanic
agencies chiefly operated in upheaving the entire island from the
abyss. The line survives as a sort of foot-track, hollowed by the
frequent tread of earthquakes, to mark the course in which they
journeyed. Like one of the great lines in a trigonometrical survey,
it enables us, too, to describe the lesser lines, and to determine
their average bearing. _The volcanic agencies must have extended
athwart the country from south-west to north-east._ Mark in a map
of the island--all the better if it be a geological one--the line
in which most of our mountain ranges stretch across from the German
Ocean to the Atlantic,--the line, too, in which our friths, lochs,
and bays, on both the eastern and western coasts, and especially
those of the latter, run into the interior. Mark, also, the line
of the geological formations, where least broken by insulated
groups of hills--the line, for instance, of the Old Red Sandstone
belt, which flanks the southern base of the Grampians--the nearly
parallel line of our Scottish Coal-field, in its course from sea
to sea--the line of the Grauwacke, which forms so large a portion
of the south of Scotland--the line of the English Coal-field, of
the Lias, of the Oolite, of the Chalk--and how in this process of
diagonal lining, if I may so speak, the south-eastern portion of
England comes to be cut off from the secondary formations altogether,
and, but for the denudation of the valley of the Weald, would have
exhibited only tertiary depositions. In all these lines, whether of
mountains, lakes, friths, or formations, there is an approximation to
parallelism with the line of the great Caledonian valley--proofs that
the upheaving agency from beneath must have acted in this direction
from some unknown cause, during all the immensely extended term of
its operations, and along the entire length of the island. It is a
fact not unworthy of remark, that the profound depths of Loch Ness
undulated in strange sympathy with the reeling towers and crashing
walls of Lisbon, during the great earthquake of 1755; and that the
impulse, true to its ancient direction, sent the waves in huge
furrows to the north-east and the south-west.
The north-eastern portion of this rectilinear wall or chain runs,
for about thirty miles, through an Old Red Sandstone district. The
materials which compose it are as unlike those of the plain out of
which it arises, as the materials of a stone dike, running half-way
into a field, are unlike the vegetable mould which forms the field's
surface. The ridge itself is of a granitic texture--a true gneiss.
At its base we find only conglomerates, sandstones, shales, and
stratified clays, and these lying against it in very high angles.
Hence the geological interest of this lower portion of the wall.
As has been shrewdly remarked by Mr. Murchison,[AG] in one of his
earlier papers, the gneiss seems to have been forced through the
sandstone from beneath, in a solid, not a fluid form; and as the
ridge a-top is a narrow one, and the sides remarkably abrupt--an
excellent wedge, both in consistency and form--instead of having
acted on the surrounding depositions, as most of the south country
traps have done that have merely issued from a vent, and overlaid
the upper strata, it has torn up the entire formation from the very
bottom. Imagine a large wedge forced from below through a sheet of
thick ice on a river or pond. First the ice rises in an angle, that
becomes sharper and higher as the wedge rises; then it cracks and
opens, presenting its upturned edges on both sides, and through comes
the wedge. And this is a very different process, be it observed,
from what takes place when the ice merely cracks, and the water
issues through the crack. In the one case there is a rent, and water
diffused over the surface; in the other, there is the projecting
wedge, flanked by the upturned edges of the ice; and these edges, of
course, serve as indices to decide regarding the ice's thickness,
and the various layers of which it is composed. Now, such are the
phenomena exhibited by the wedge-like granitic ridge. The Lower
Old Red Sandstone, tilted up against it on both sides, at an angle
of about eighty, exhibits in some parts a section of well nigh two
thousand feet, stretching from the lower conglomerate to the soft,
unfossiliferous sandstone, which forms in Ross and Cromarty the upper
beds of the formation. There is a mighty advantage to the geologist
in this arrangement. When books are packed up in a deep box or chest,
we have to raise the upper tier ere we can see the tier below, and
this second tier ere we can arrive at a third, and so on to the
bottom. But when well arranged on the shelves of a library, we have
merely to run the eye along their lettered backs, and we can thus
form an acquaintance with them at a glance, which in the other case
would have cost us a good deal of trouble. Now, in the neighborhood
of this granitic wedge, or wall, the strata are arranged, not like
books in a box,--such was their original position,--but like books
on the shelves of a library. They have been unpacked and arranged by
the uptilting agent; and the knowledge of them, which could only have
been attained in their first circumstances by perforating them with
a shaft of immense depth, may now be acquired simply by passing over
their edges. A morning's saunter gives us what would have cost, but
for the upheaving granite, the labor of a hundred miners for five
years.
[Footnote AG: See _Transactions of the London Geological Society_ for
1828, p. 354.]
By far the greater portion of the life of the writer was spent within
less than half an hour's walk of one of these upturned edges. I have
described the granitic rock, with reference to the disturbance it has
occasioned as a wedge forced from below, and with reference to its
rectilinear position in the sandstone district which it traverses,
as a stone wall running half-way into a field. It may communicate a
still corrector and livelier idea to think of it as a row of wedges,
such as one sometimes sees in a quarry when the workmen are engaged
in cutting out from the mass some immense block, intended to form a
stately column or huge architrave. The eminences, like the wedges,
are separated; in some places the sandstone lies between--in others
there occur huge chasms filled by the sea. The Friths of Cromarty
and Beauly, for instance, and the Bay of Munlochy, open into the
interior between these wedge-like eminences;--the well-known Sutors
of Cromarty represent two of the wedges; and it was the section
furnished by the Southern Sutor that lay so immediately in the
writer's neighborhood. The line of the Cromarty Frith forms an
angle of about thirty-five degrees with that of the granitic line
of wedge-like hills which it bisects; and hence the peculiar shape
of that tongue of land which forms the lower portion of the Black
Isle, and which, washed by the Moray Frith on the one side, and by
the Frith of Cromarty on the other, has its apex occupied by the
Southern Sutor. Imagine a lofty promontory somewhat resembling a
huge spear thrust horizontally into the sea--a ponderous mass of
granitic gneiss, of about a mile in length, forming the head, and
a rectilinear line of the Old Red Sandstone, more than ten miles
in length, forming the shaft; and such is the appearance which
this tongue of land presents, when viewed from its north-western
boundary, the Cromarty Frith. When viewed from the Moray Frith,--its
south-western boundary,--we see the same granitic spear-head, but
find the line of the shaft knobbed by the other granitic eminences of
the chain.
Now on this tongue of land I first broke ground as a geologist.
The quarry described in my introductory chapter, as that in which
my notice was first attracted by the ripple markings, opens on the
Cromarty Frith side of this huge spear-shaft; the quarry to which
I removed immediately after, and beside which I found the fossils
of the Lias, opens on its Moray Frith side. The uptilted section of
sandstone occurs on both sides, where the shaft joins to the granitic
spear-head, but the Lias I found on the Moray Frith side alone. It
studs the coast in detached patches, sorely worn by the incessant
lashings of the Frith; and each patch bears an evident relation,
in the place it occupies, to a corresponding knob or wedge in the
granitic line. The Northern Sutor, as has been just said, is one of
these knobs or wedges. It has its accompanying patch of Lias upheaved
at its base, and lying unconformably, not only to its granitic
strata, but also to its subordinate sandstones. The Southern Sutor,
another of these knobs, has also its accompanying patch of Lias,
which, though lying beyond the fall of the tide, strews the beach,
after every storm from the east, with its shales and its fossils.
The hill of Eathie is yet another knob of the series, and it, too,
has its Lias patch. The granitic wedges have not only uptilted the
sandstone, but they have also upheaved the superincumbent Lias,
which, but for their agency, would have remained buried under the
waters of the Frith, and its ever accumulating banks of sand and
gravel. I had remarked at an early period the correspondence of the
granitic knobs with the Lias patches, and striven to realize the
original place and position of the latter ere the disturbing agent
had upcast them to the light. What, I have asked, was the extent of
this comparatively modern formation in this part of the world, ere
the line of wedges were forced through from below? A wedge struck
through the ice of a pond towards the centre breaks its continuity,
and we find the ice on both sides the wedge; whereas, when struck
through at the pond edge, it merely raises the ice from the bank, and
we find it, in consequence, on but one side the wedge. Whether, have
I often inquired, w T ere the granitic wedges of this line forced
through the Lias at one of its edges, or at a comparatively central
point? and about ten years ago I set myself to ascertain whether I
could not solve the question. The Southern Sutor is a wedge open to
examination on both its sides;--the Moray Frith washes it upon one
side, the Cromarty Frith on the other. Was the Lias to be found on
both its sides? If so, the wedge must have been forced _through_ the
formation, not merely _beside_ it. It occurs, as I have said, on the
Moray Frith side of the wedge; and I resolved, on carefully exploring
the Frith of Cromarty, to try whether it did not occur on that side
too.
With this object I set out on an exploratory excursion, on a
delightful morning of August, 1830. The tide was falling; it had
already reached the line of half ebb; and from the Southern Sutor
to the low, long promontory on which the town of Cromarty is built,
there extended a broad belt of mingled sand-banks and pools,
accumulations of boulders, and shingle, and large tracts darkened
with algæ. I passed direct by a grassy pathway to the Sutor, the
granitic spear-head of a late illustration,--and turned, when I
reached the curved and contorted gneiss, to trace through the broad
belt left by the retiring waters, and in a line parallel to what I
have described as the shaft of the huge spear, the beds and strata
of the Old Red Sandstone in their ascending succession. I first
crossed the conglomerate base of the system, here little more than
a hundred feet in thickness. The ceaseless dash of the waves, which
smooth most other rocks, has a contrary effect on this bed, except
in a few localities, where its arenaceous cement of base is much
indurated. Under both the Northern and Southern Sutors the softer
cement yields to the incessant action, while the harder pebbles
stand out in bold relief; so that, wherever it presents a mural
front to the breakers, we are reminded, by its appearance, of the
artificial rock work of the architect. It roughens as the rocks
around it polish. Quitting the conglomerate, I next passed over a
thick bed of coarse reel and yellow sandstone, with here and there a
few pebbles sticking from its surface, and here and there a stratum
of finer-grained fissile sandstone inserted between the rougher
strata: I then crossed over a strata of an impure grayish limestone,
and a slaty clay, abounding, as I long afterwards ascertained, in
ichthyolites and vegetable remains. There are minute veins in the
limestone (apparently cracks filled up) of a jet black bituminous
substance, resembling anthracite; the stratified clay is mottled
by layers of semi-aluminous, semi-calcareous nodules, arranged like
layers of flint in the upper Chalk. These nodules, when cut up and
polished, present very agreeable combinations of color; there is
generally an outer ring of reddish brown, an inner ring of pale
yellow, and a central patch of red, and the whole is prettily veined
with dark-colored carbonate of lime.[AH] Passing onwards and upwards
in the line of the strata, I next crossed over a series of alternate
beds of coarse sandstone and stratified clay, and then lost sight of
the rock altogether, in a wide waste of shingle and boulder-stones,
resting on a dark blue argillaceous diluvium, sometimes employed
in that part of the country, from its tenacious and impermeable
character, for lining ponds and dams, and as mortar for the
foundations of low-lying houses, exposed in wet weather to the
sudden rise of water. The numerous boulders of this tract have their
story to tell, and it is a curious one. The Southern Sutor, with its
multitudinous fragments of gneiss, torn from its sides by the sea,
or loosened by the action of frosts and storms, and rolled down its
precipices, is only a few hundred yards away;--its base, where these
lie thickest, has been swept by tempests, chiefly from the east, for
thousands and thousands of years; and the direct effect of these
tempests, regarded as transporting agents, would have been to strew
this stony tract with those detached fragments. The same billow that
sends its long roll from the German Ocean to sweep the base of the
Sutor, and to leap up against its precipices to the height of eighty
and a hundred feet, breaks in foam, only a minute after, over this
stony tract; which has, in consequence, its sprinkling of fragments
of gneiss, transported by an agency so obvious. But for every one
such fragment which it bears, we find at least ten boulders that have
been borne for forty and fifty miles in the opposite direction from
the interior of the country--a direction in which no transporting
agency now exists. The tempests of thousands of years have conveyed
for but a few hundred yards not more than a tithe of the materials
of this tract; nine tenths of the whole have been conveyed by an
older agency over spaces of forty and fifty miles. How immensely more
powerful, then, or how immensely protracted in its operation, must
that older agency have been!
[Footnote AH: A concretionary limestone of the Old Red system in
England, variegated with purple and green, was at one time wrought as
a marble.--_Silurian System_, Part i. p. 176.]
I passed onwards, and reached a little bay, or, rather, angular
indentation of the coast, in the neighborhood of the town. It was
laid bare by the tide, this morning, far beyond its outer opening;
and the huge, table-like boulder, which occupies nearly its centre,
and to which, in a former chapter, I have had occasion to refer,
held but a middle place between the still darkened flood-line that
ran high along the beach, and the brown line of ebb that bristled
far below with forests of the rough-stemmed tangle. This little
bay, or inflection of the coast, serves as a sort of natural wear
in detaining floating drift-weed, and is often found piled, after
violent storms from the east, with accumulations, many yards in
extent, and several feet in depth, of kelp and tangle, mixed with
zoöphytes and mollusca, and the remains of fish killed among the
shallows by the tempest. Early in the last century, a large body
of herrings, pursued by whales and porpoises, were stranded in it,
to the amount of several hundred barrels; and it is said that salt
and cask failed the packers when but comparatively a small portion
of the shoal were cured, and that by much the greatest part of them
were carried away by the neighboring farmers for manure. Ever since
the formation of the present coast-line, this natural wear has been
arresting, tide after tide, its heaps of organic matter, but the
circumstances favorable to their preservation have been wanting:
they ferment and decay when driven high on the beach; and the next
spring-tide, accompanied by a gale from the west, sweeps every
vestige of them away; and so, after the lapse of many centuries,
we find no other organisms among the rounded pebbles that form the
beach of this little bay, than merely a few broken shells, and
occasionally a mouldering fish-bone. Thus very barren formations may
belong to periods singularly rich in organic existences. When what is
now the little bay was the bottom of a profound ocean, and far from
any shore, the circumstances for the preservation of its organisms
must have been much more favorable. In no locality in the Old Red
Sandstone with which I am acquainted have such beautifully preserved
fossils been found. But I anticipate.
In the middle of the little bay, and throughout the greater part of
its area, I found the rock exposed--a circumstance which I had marked
many years before, when a mere boy, without afterwards recurring
to it as one of interest. But I had now learned to look at rocks
with another eye; and the thought which first suggested itself to
me regarding the rock of the little bay was, that I had found the
especial object of my search--the Lias. The appearances are in
some respects not dissimilar. The Lias of the north of Scotland is
represented in some localities by dark-colored, unctuous clays, in
others by grayish black sandstones, that look like indurated mud,
and in others by beds of black fissile shale, alternating with bands
of coarse, impure limestone, and studded between the bands with
limestone nodules of richer quality and finer grain. The rock laid
bare in the little bay is a stratified clay, of a gray color tinged
with olive, and occurring in beds separated by indurated bands of
gray, micaceous sandstone. They also abound in calcareous nodules.
The dip of the strata, too, is very different from that of the beds
which lean against the gneiss of the Sutor. Instead of an angle of
eighty, it presents an angle of less than eight. The rocks of the
little bay must have lain beyond the disturbing, uptilting influence
of the granitic wedge. So thickly are the nodules spread over the
surface of some of the beds, that they reminded me of floats of
broken ice on the windward side of a lake after a few days' thaw,
when the edges of the fragments are smoothed and rounded, and they
press upon one another, so as to cover, except in the angular
interstices, the entire surface.
I set myself carefully to examine. The first nodule I laid open
contained a bituminous looking mass, in which I could trace a
few pointed bones and a few minute scales. The next abounded in
rhomboidal and finely enamelled scales, of much larger size and more
distinct character. I wrought on with the eagerness of a discoverer
entering for the first time in a _terra incognita_ of wonders. Almost
every fragment of clay, every splinter of sandstone, every limestone
nodule, contained its organism--scales, spines, plates, bones, entire
fish; but not one organism of the Lias could I find--no ammonites,
no belemnites, no gryphites, no shells of any kind: the vegetable
impressions were entirely different; and not a single scale,
plate, or ichthyodorulite could I identify with those of the newer
formation. I had got into a different world, and among the remains of
a different creation; but where was its proper place in the scale?
The beds of the little bay are encircled by thick accumulations
of diluvium and debris, nor could I trace their relation to a
single known rock. I was struck, as I well might, by the utter
strangeness of the forms--the oar-like arms of the _Pterichthys_ and
its tortoise-like plates--the strange, buckler-looking head of the
_Coccosteus_, which, I suppose, might possibly be the back of a small
tortoise, though the tubercles reminded me rather of the skin of the
shark--the polished scales and plates of the _Osteolepis_--the spined
and scaled fins of the _Cheiracanthus_--above all, the one-sided
tail of at least eight out of the ten or twelve varieties of fossil
which the deposit contained. All together excited and astonished
me. But some time elapsed ere I learned to distinguish the nicer
generic differences of the various organisms of the formation. I
found fragments of the _Pterichthys_ on this morning; but I date
its discovery, in relation to the mind of the discoverer, more than
a twelvemonth later.[AI] I confounded the _Cheiracanthus_, too,
with its single-spined, membranous dorsal, with _Diplacanthus_
ichthyolite, furnished with two such dorsals; and the _Diplopterus_
with the _Osteolepis_. Still, however, I saw enough to exhilarate
and interest: I wrought on till the advancing tide came splashing
over the nodules, and a powerful August sun had risen towards the
middle sky; and were I to sum up all my happier hours, the hour would
not be forgotten in which I sat down on a rounded boulder of granite,
by the edge of the sea, when the last bed was covered, and spread out
on the beach before me the spoils of the morning.
[Footnote AI: I find, by some notes, which had escaped my notice when
drawing up for the _Witness_ newspaper the sketches now expanded
into a volume, that in the year 1834 I furnished the collection of
a geological friend, the Rev. John Swanson, minister of the parish
of Small Isles, in the Outer Hebrides, with a well-marked specimen
of the _Pterichthys Milleri_. The circumstance pleasingly reminds me
of the first of all my early acquaintance, who learned to deem the
time not idly squandered that was spent in exploring the wonders of
bygone creations. Does the minister of Small Isles still remember
the boy who led him in quest of petrifactions--himself a little boy
at the time--to a deep, solitary cave on the Moray Frith, where
they lingered amidst stalactites and mosses till the wild sea had
surrounded them unmarked, barring all chance of retreat, and the dark
night came on?]