THE CARBONIFEROUS ROCKS—CONTINUED.
The geology of the northern division of Scotland is, almost in every
particular in the series of rocks that have been described, the
counterpart of the southern, which now falls to be noticed. The fossils
so richly imbedded in the former are here repeated, more sparingly in
some, more abundantly in other families, and, in several instances,
in the introduction of entirely new forms of organic life. Along the
west and south border counties, the granites, with their associated
crystalline group, are sparingly developed, stretching, at intervals,
from the island of Arran through Galloway and Kirkcudbright into
Dumfries-shire. The silurians follow in their order of superposition,
occupying an extensive area from sea to sea across the island.
The devonian system, chiefly in the upper and middle beds, wraps
round the base of the older formation, and rests unconformably on
its highly-inclined strata. The carboniferous deposits are widely
distributed, some in isolated basins, and enveloped by the old red, and
pregnant all of them with the fossils of the period. The ignigenous
rocks, the traps and porphyries, are also very abundant, some in the form
of detached cones, some in extensive ranges, and all demonstrative of
their character as the agents that have lifted up, disrupted, and twisted
the strata of the district.
In passing over this section of our course, it will not be necessary,
therefore, to dwell in any minute or lengthened descriptions.
I. The geological student, in commencing his researches at Edinburgh,
is immediately arrested by the more prominent objects that everywhere
rise into view—the Castle Rock—the Calton Hill—Salisbury Crag—Arthur’s
Seat—and the Pentlands. A wondrous, glorious scene, every one
involuntarily exclaims, upon reaching any of these heights, thrown, as
if by the hand of an enchanter, in and around this lovely city. Geology
here has its favorite residence—the birth-place and cradle of the
HUTTONIAN THEORY—Arthur’s seat there to attest its truth. What a history
of bygone times recorded in these two words! What a revolution produced
in the sentiments of mankind as to the science of world-making! And,
still more, how deeply were men’s minds agitated, and the foundations of
religious truth stirred, by the novel views which were then announced!
The assumption lying at the foundation of the rival, or Wernerian
theory, is, that the materials of which the various strata of the globe
consist were originally dissolved or suspended in water: they were thus
in a condition to assume any form which their physical qualities and
the laws of matter might determine; and, accordingly, in this fluid
menstruum they were consolidated into various combinations, partly by
means of crystallization, and partly by mechanical deposition. The
Huttonian theory, on the other hand, employs the force of subterraneous
fire as its principal machinery, which is placed at immense depths,
and the materials on which it operates are under a vast pressure; and,
consequently, while they are indurated into limestone, sandstone, and
coal, along with their included fossils, their essential qualities are
but very slightly affected, and the arrangement and disposition of their
particles but little disturbed. The hills around, by which this theory
was to be tested, and to whose singular structure it owed its origin,
consist of an alternating series of tabular masses of trap and the
sedimentary deposits, basalt forming generally the central nucleus, with
tufa, greenstone, and sandstone variously disposed and folded over. All
the conditions of upheaval, tortion, angularity, induration, fracture,
and dislocation, are amply furnished; the columnar, jointed structure is
well defined in Samson’s Ribs; the very momentum of pressure, forcing
the sandstone into the perpendicular, may be studied as a nice dynamical
problem on the Castle rock; and when Sir James Hall brought from his
crucible a reconstructed whinstone, regularly jointed and with no
trace of vitreous fusion, the demonstration of the theory was felt and
acknowledged, in its leading features, to be complete.
Palæontology has added its living wonders to the mere lithological
speculations which were then in vogue and engrossed all attention. And
geologists can now afford to smile at the misinterpretations, made by
both parties, of established facts and well-ascertained things—nay, at
the eagerness with which they irrelevantly pressed facts to bend to
their conflicting views—the vehemence with which the Wernerian declared
whinstone to contain organisms, and to be no exception to the law of
mechanical deposition; while the Huttonian as deliberately set himself
to prove that the nodules in chalk could only be the product of fire—the
formation itself, as now determined, being merely a concretion of shells
of the most perfect structure and undiminished luster. Truth, like
light, emerges slowly, feeble in its dawnings when objects are obscurely
seen or readily mistaken, a portion of the view half in shade, and
half in brightness. And thus it has happened with both systems, as in
the progress of the science errors have been detected and deficiencies
supplied, peculiar to each. The acrimony of the contest, too, has passed
away. Theology has been disentangled, and declared by the divine to
be in no way affected by the issue. And while the scurrilities of the
indiscreet abettors of both are utterly forgotten, the deductions of
Hutton and the masterly expositions of his illustrator are in the main
adopted as the basis of the only true system of geology.
II. In the general structure of the environs of the Scottish metropolis
this plutonic machinery is deeply impressed, as it has been most
vigorously exerted. There is everywhere the greatest derangement
existing among the sedimentary deposits, everything is tossed out of its
original place, and divided into small sections and detached groups. The
connections and relative positions are very difficult to trace. Still,
amidst all the disorder, the more general bearings of the different
formations may be ascertained. Mr. Charles Maclaren, indeed, has examined
everything with a pains-taking care, and described them with a minuteness
and fidelity of detail, which cannot be surpassed, as they need scarcely
be repeated. His “Geology of Fife and the Lothians,” will be in every
student’s hands who desires to be acquainted with the structure of the
district, conversant more especially as this learned geologist is with
the position, fragments, and medal-stamp of every rock—their relations to
each other, historical value, and bearings in the science—and illustrated
with such diversity of section and diagram, that we feel as we accompany
him,—
“Panditur interea domus Omnipotentis.”
The general contour of the city, so picturesque and remarkable in its
grouping of streets, may be taken as a pretty safe guide in determining
the nature of the geology. The town is built over two parallel ridges,
which completely expose the character of the inferior minerals. The
northern division rests upon a series of beds, which appear immediately
to underlie the true workable coal seams; the Old Town ridge and Castle
rock bear up the lower members of the carboniferous deposit, while along
the extended plateau on the south the yellow sandstone of the old red
has been brought to the surface. The whole would thus seem to occupy the
upraised floor of the great coal basin of Mid Lothian, dislocated and
separated by the igneous matter of Arthur’s Seat and the Calton, whence
the metals all plunge to the eastward.—The flat, extending from Restalrig
toward Granton and Craigleith, consists of the same series of beds as
those upon which the New Town stands, and which have been elevated by the
dykes and bosses of trap that so frequently intersect the strata.
The range of the yellow deposit, supposed to belong to the old red, is
well defined; it commences on the northern slope and face of Salisbury
Crags, and covers nearly the whole eastern side, depending to the
Hunter’s Bog. The same series of beds, readily distinguished by their
reddish hue, train round by Samson’s Ribs, thence proceeding by St.
John’s Hill, Heriot’s Hospital, Burntsfield Links, they bear toward the
New Cemetery on the estate of Grange. The beds here, exposed in several
quarries, consist of an alternating series of marls, concretionary
limestone, and sandstone, similar in all their lithological characters to
the deposits of Dura Den and Glenvale. Not a fragment, indeed, of scale
or organism has yet, so far as we know, been detected in the locality
now defined, so as unequivocally to determine the position of the group
in question. But is not the absence of the fossil test as fatal to its
connection with the carboniferous series? while, considering its remote
geographical distance from the undisputed domain of the new red, and its
proximity to a surrounding belt of the true silurian, flanked with the
old red, the presumption is that the deposit will yet be classed with
the upper or yellow sandstone division of the devonian family. Still we
merely indicate an unpresuming judgment, leaving it to so much gifted
local research to confirm or disprove the correctness of the proposed
classification.
III. The Mid Lothian coal-basin, so rich in minerals, forms part of the
great carboniferous valley of Scotland, and may be considered as simply
an extension of the coal-field of Fifeshire, the metals dipping on both
sides toward the middle of the Frith. The out-crop rises toward the
Lammermuirs and the Pentlands.—The area occupied by the coal-measures
includes a space of about eighteen miles in length, by twelve in
breadth. The series of beds composing the formation, are nearly five
thousand feet thick, or about a mile in depth from the upper to the
lower strata, and the whole fractured and dislocated in every part of
the field. There are fifty-two slips, indeed, enumerated by the miners,
which occasion a depression toward the north to the extent of 5,196
feet; the metals are again raised by a series of thirty-seven slips to
the height of 2,412 feet; thereby causing a change of relative level in
the strata, corresponding to the altitude of the highest points in the
Lammermuir range, namely, 2,757 feet. The disturbances above and below
thus approximate to each other. Have they been directed and modified by
the same agencies, the silurian group rising higher and higher as the
carboniferous subsided into the depressions occasioned by the evolution
of the igneous matter? The Bass, North Berwick Law, and Arthur’s Seat,
are the products of the change, though indeed these scattered points of
igneous rock on the surface can give no idea of its subterranean extent,
since basalt and greenstone are met with at unvarying depths in a great
portion of the coal district in question.
The whole field is prolific in organic remains. But BURDIEHOUSE LIMESTONE
claims a separate notice, not only from the abundance but the very
remarkable characters of the fossils contained in it, many of them met
with for the first time in the progress of our sketches. This rock
immediately underlies the encrinital limestone beds of Gilmerton, and
is about twenty-seven feet thick, of a dark dingy color, arising from
so much bituminous matter mixed up with the calcareous. The vegetable
remains are very numerous, and in a state of beautiful preservation.
Nowhere, indeed, in the best arranged herbarium, have we anything so
graceful, so minutely and skillfully delineated, as are the figures of
these plants upon the stone. There are several species of lycopodium;
also stigmaria, sigillaria, equisetum, calamus, and cyclopteris, in
great abundance. The fronds of the fossil sphenopteris furnish exquisite
tracings of nature’s penciling. Nor are the relics belonging to the
animal kingdom less remarkable for their freshness and variety. Here
are the extremes of organic life, microscopic shells innumerable, with
the claws, eyes, slender feelers of their occupants, all entire; and
the gigantic _Megalichthys_, with a body sixty feet long, teeth of four
to six inches still sparkling with luster, and scales of corresponding
magnitude brightly enameled. There are also the bones and plates of
another huge creature, the _Gyracanthus_, along with the jaws of sauroid
fishes, measuring from a foot to a foot and half in length, thickly
studded with teeth. And there, too, lovely trout-like animals, the
_Palæoniscus_—with all the fins and organs and body fresh and glistering,
as if ready to leap to their prey, strewed in countless myriads around.
Nor is the enumeration complete as to the kind and quality of the
fossils of this curious deposit: there _coprolites_ mark the habits of
the predaceous monsters of the period—fæcal excrements composed of the
remains of their victims—and in some places so numerous as to outweigh
the calcareous matrix in which they are imbedded.
M. Agassiz, in his synoptical table of British fossil fishes, 1843, gives
the following list belonging to the Burdiehouse limestone. In the Order
of Placoids, _Ichthyodorulites_, there is a Ptychacanthus sublævis,
Sphenacanthus serrulatus, and Gyracanthus formosus: of _Cestraciontes_,
Otenoptychius pectinatus and denticulatus, and Ctenodus Robertsoni:
of _Hybodontes_, Cladodus acutus, parrus, and Hibberti, and Diplodus
gibbosus, and minutus. In the Order of Ganoids the following occur:
of _Lepidoides_, Palæoniscus ornatissimus, Robisoni, and striolatus,
and Eurynotus crenatus, and fimbriatus; of _Sauroids_, Megalichthys
Hibberti, Diplopterus Robertsoni, Pygopterus Bucklandi and Jamesoni; of
_Cœlacanthes_, Holoptychius Hibberti, sauroides, and striatus, Uronemus
lobatus, and a Phyllolepis tenuissimus. Since this list was drawn up,
many additional fossils have been obtained from the same locality; some
of them exhibit characters which will establish, in all probability, new
genera as well as species. The collection in the Edinburgh College Museum
contains gigantic specimens in the highest condition of preservation,
exciting our wonder at the strange forms which peopled our ancient
seas, and admiration of those singular processes by which they have
been embalmed by the chemistry of nature, surviving so many changes and
disturbances in the history of our planet.
The comparative history of the fishes enumerated, in relation to the
systems of rocks through which they extend, is both interesting and
curious. For example, the genus ptychacanthus begins in the devonian and
ends in the carboniferous period, one species peculiar to each formation.
Palæoniscus begins in the carboniferous, and continues through the
permian age, in five new specific forms. The megalichthys begins in the
devonian and becomes extinct in the carboniferous types; diplopterus,
holoptychius, and phyllolepis have each the same terms of existence; and
again the pygopterus begins in the carboniferous, and survives, in two
new species, through the permian era. Thus five genera are common to the
devonian and carboniferous systems; two to the carboniferous and permian;
eight belong exclusively to and become extinct in the carboniferous.
These results clearly manifest an adaptation on the part of nature, as
well as some arbitrary principle in the order of her creations, and all
speak to the fact of progression in the course of events and of direct
interposition in the successive origin of organic existence. Look again
into these rocks. Consider the causes which so filled them with these
memorials of warfare and death. Two families only, the least predaceous
of their kind, survived the age which produced them—one wide revolution
covered with its spoils the surface of the earth—the wreck is closed over
and silted beneath the waves—and the carboniferous era, teeming with
animal and vegetable life, forever passed away.
The deposit, so fruitful in these organisms, has, with much probability,
been regarded as a fresh-water limestone, from the circumstance that it
contains no corallines or marine shells. The plants, too, are all of a
terrestrial or fluviatile kind, and so perfectly entire as to warrant
the inference that they have not been tossed and drifted about in an
ocean nor transported from a distance, but have perished _in situ_, and
dropped amid still waters. It may have been an estuary on the borders
of an ancient sea, whither the Megalichthys resembling the crocodile
family in bulk, and the Gyracanthi akin to the sharks in voracity, may
have roamed in quest of food, gamboled for pleasure amidst a luxuriance
of tropical vegetation, or indolently reposed by the umbrageous shades
of slimy lagoons. How different the scene over which they maintained
undisputed sway from all that is now in these parts subject to man’s
dominion. Transpose the zones of the earth, and then only could there be
an approximation to the more ancient condition of things.
[Illustration: Basin Form of Coal-fields. 1.1. Mountain Limestone.]
IV. The Mid Lothian coal-basin is bounded on the west and north-west by
the Pentlands, the Braid, and Blackford Hills. The Corstorphine Hills
stand out in bold relief above the plain, and are remarkable of their
kind; they consist of a sandstone basis, capped by an enormous mass of
greenstone, in which the groovings and polish of diluvial or glacial
action have long been familiar to the geologist. The carboniferous beds
occupy, at intervals, the district toward Falkirk and Stirling, much
broken and intersected by the igneous rocks. Stirling, like Edinburgh,
is greatly indebted to its physical features, the Abbey-Crag, the
dome-shaped and wooded rock of Cragforth, the Castle-Hill, and the
Gillies Hill, overlooking and sharing in the glories of the plain of
Bannockburn. These all consist of greenstone or dolerite trap, resting
on sandstone, or often alternating in nearly conformable beds with
sandstone, ironstone, and limestone. The Pentlands stretch about sixteen
miles in length by six in their extreme breadth, the axis of the chain
bearing almost due N. E. and S. W. The eastern division presents the
different varieties of feldspathic rocks—in the center or middle group
of hills the graywacke series are more developed—and on the west the
old red sandstone and carboniferous deposits prevail. The axis of the
chain in some of the higher points is capped with the sedimentary rocks,
and along the entire range the phenomena of upheaval, dislocation,
subsidence, and denudation all present themselves in turn, and in most
instructive forms. The Carlops and Kaim-Valley coal-basin exhibits some
remarkable appearances; within a trough-shaped, narrow space, beds of
feldspar, porphyry, greenstone, and conglomerate are mixed up with the
coal metals, all less or more denuded, separated by transverse openings,
and irregularly broken off at their outcrop. Fossils, though sparingly,
are found in the graywacke, as trilobites and orthoceratites. The Braid
and Blackford Hills are outliers of the Pentlands, and present the same
varieties of rock and general lithological structure.
Rapid and brief as the above sketch is, let the reader be assured there
is much, very much, in the district to interest and instruct. Make the
circuit of the Pentlands when he may, and he will not be satisfied until
he has penetrated every valley, scaled every height, and become familiar
as household words with every name and calling through the length and
breadth of their varied range. Habbie’s How, a very pastoral in the
sound, Carlops, Kaim-Valley, Mount-Maw, Deerhoperigg, Dalmahoy-Crags,
the Mendick Hills, how dear to every lover of nature in their sweet
retreats and cool shady banks! And Tintock, rich in prophetic lore, to
be understood must be ascended, the eye ranging over the whole central
valley of Scotland, embracing both oceans in its field of vision,
and numbering all over the lofty granite peaks of the Grampians.
Resting-spots like these impart a delicious charm to the geologist amidst
his wanderings. If pregnant with the materials of doubtful reasonings,
perplexing arrangements, and intricate soundings, the science has its
sunny sides and cheerful fields of recreation. And if compelled to
traverse regions of dangerous stepping, dark profound abysses, he is
speedily again by the side of sparkling rivers, among grassy holms and
pastoral dales, redolent with the bracing airs of crag and mountain.
Nor are the moral influences of such speculations of a less healthful
and refreshing kind. Geology, which deals with the cycles of time, is
yet the youngest of the sciences. One exclusively of observation, all
its objects lie scattered around the daily pathways of men. And still,
but as yesterday, has it been looked upon with a favorable eye, as a
means of investigating and establishing truth, and its truths themselves
recognized as of good character and tendency. Herein, until very
recently, the tree of knowledge was supposed to yield of its fruits of
good and evil, most abundantly of the latter, and men long pertinaciously
refused to partake of, or even to look at, the precious things that
dropped from its numerous well-laden branches. Hume had attempted to
demonstrate that there was no external world at all. Researches into
the structure of mind, metaphysics, the domain of “common sense” as
distinguished from the abstractions of the ideal philosophy, engaged and
confounded alike the learned and the unlearned. Beneath, in the strata
of the earth, lay the records and memorials, it was said, of vast untold
ages, and all shrunk from an abyss on whose brink it was perilous to
walk. The interior was literally regarded as unhallowed ground, from
whose Pandora recesses, open who might, nothing but evils could issue, at
utter variance with every fixed and established principle.
“Hic specus horrendum et sævi spiracula ditis.”
Religion and science thus stood in direct antagonism to each other,
divorced by general consent from an unnatural alliance; and men, in
those days, in the Scottish metropolis, were grouped into coteries who
eyed each other with a bitter jealousy. Some more liberal mind, indeed,
a Blair and a Robertson, would pass occasionally into the hostile camp,
but returned again to his own ranks, to be received with no very cordial
embrace or flattering approval.
But now, were one of the sages of scarcely half a century ago permitted
to rise from the dust, and to take his place among the intellectuals
of the present time, nothing would be more likely to excite his wonder
than the controversies, and their subjects, which figure in their
works. Theory there is scarcely any among those who now give law in
Modern Athens in letters and science.—Whether in the regions of mental
philosophy, the walks of physical science, or the sacred precincts of
religion, men’s minds are nearly at one as to the objects and distinctive
province of each. They do not fear or dread the pursuits in which
they are respectively engaged, assured that skepticism, or any desire
to maintain it, has now neither party nor standing; or come to what
conclusion they may, the WISDOM FROM ABOVE will in its own pure and
elevated region remain scathless against any or all the bolts with which
it may be assailed. The everlasting hills are still there on their old
foundations—the remarkable variety of structure, which, all around the
city they so marvelously present, still speaks in impressive language
of order and disruption, stability and change—and underneath, in the
imperishable forms of buried generations, are the records of a history
in which man has no part, and with which his destiny would seem in no
wise concerned. But the language written thereon, and the leaves on which
it is impressed, are divested of the awful sibylline mystery in which
they were then involved. The saint is scared not away by the frightful
characters and dark meanings which the sage pretends he can trace in
them. Nor is the sage himself startled at the alchemy of his own art, and
the singular forms he can summon to his presence from his subterranean
domains.
A delightful repose all this from the fierce personal controversies of a
few years ago. Healthful truths are brought to light. On one and the same
page, penetrate as deep as they may, all professions and their abettors
join alike in admiration of the ineffaceable impress of the order,
wisdom, and goodness everywhere to be traced in the structure of the
globe. There is no longer the metaphysician vainly attempting to resolve
the whole concrete mass into the ideal; or ridiculously striving to
raise a structure of materialism, on the assumption that all our ideas,
whatever we know and all we excogitate about, are derived through the
medium of our sensations. The regions of infinite space are explored,
and the devotional tendencies of the age have become the more decided and
intense in proportion as the mental vision has been enlarged. The mind
seizes with a firmer grasp, and advances with a steadier pace over the
fields of creation, because there _is_ a Creator whose invisible Godhead
is understood from the things which are made. And now, in search of
truth, one and the same through all things, religion and science go hand
in hand, sanctified and enlightened by the union, and imparting the most
salutary lessons from the physical and moral revelations of Him whose
path is in the deep places of the earth, and who for the display of his
own glory has become the Instructor and Redeemer of the world.