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.
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.