The general structure of the district, as now indicated, is determined in the main by the Lammermuirs, a high mountain-range of sedimentary rocks, which formed the northern barrier of an extensive inland basin or sea, and of which the Solway and Tweed occupy the central stretch or depression. The old red sandstone was herein deposited, the strata of which rest unconformably upon the older rocks. The carboniferous beds succeeded, but at a period when the floor of the basin was elevated, and the dimensions contracted; hence these beds, though reposing conformably for the most part on those of the old red and not separable on physical grounds, do not occupy the same extent of surface. Creeks and bays existed around the silurian shores, into which the materials of the sandstone were carried, and thus along the southern slope, from St. Abb’s Head to Portpatrick, the old red is traceable in every opening and indentation, running up in long narrow tongues, or detached stripes, among the mountains. The coal series appear at various intervals in small isolated basins, forming on the west the coal-field of Whitehaven, which dips into the Solway, and on the east occupying from Kelso to Berwick, the valley of the Tweed, where the metals lie in very thin bands, and underneath the mountain limestone. Here are porphyries, which have disrupted and broken through the old red sandstone, and therefore, corresponding in age to those of the Sidlaws, Ochils, and Pentlands; and augite traps and greenstone, scattered over the coal-measures, which are as clearly the product of the movements that issued in the elevation of Arthur’s Seat and the Lomonds.
The Lammermuirs have an extent of nearly one hundred and fifty miles in length, by an average breadth of twenty-five to thirty miles. The axis of the chain runs from E.N.E. to W.S.W., broken at intervals by rivers and their divergent valleys, and constituting the great frontier barrier of Scotland. The Lowthers, Corston-cone, Queensberry, and the high grounds along the upper right bank of the Nith, form outliers or extensions of the general mass. Long regarded as furnishing a true type of Graywacke rock, the Lammermuirs are now, by general consent, admitted into the family of Silurians, bearing affinities both to the Upper and Lower series, and partaking likewise in some of the characteristics of the Cambrian group. The eastern division of the chain consists of very thick beds of a coarse brecciated rock, covered on the southern side by a fine-grained clay slate. In Kirkcudbrightshire, the slate-band and conglomerate, seen on the main land, and at White Bay, in Little Ross Island, are very closely allied in their mineralogical characters. And in the center division, about Innerleithen, the intrusive traps constitute a marked and interesting feature, more particularly as they there assume a subcrystalline granite structure, and convert the sedimentary deposits into hard flinty slates, or Lydian stone. The organic remains are not abundant: they are scattered, too, at wide intervals, but still sufficiently characteristic of the formation. They consist of graptolites, encrinites, trilobites, and several genera of shells. The list of conchiferæ, in some beds, is almost entirely Lower Silurian, while the smooth Asaphi would seem to connect this range of hills with the lower Silurian rocks of Tyrone and Fermanagh, in Ireland, which have furnished the only other specimens yet detected in Britain. The fossil localities are the lime quarries of Wrae, near Broughton, Greiston slate quarry, near Traquair, St. Mary’s Isle, Kirkcudbright, Loch Ryan, and Little Ross Island; and in certain graywacke beds in Liddesdale, Mr. Nicol records the discovery of “fragments of plants, not unlike the broken reeds, and other imperfect vegetable remains, seen on some carboniferous sandstones.” The collection of Lord Selkirk, from the vicinity of his residence, consists according to Mr. Salter, of—Terebratula semisulcata, Leptæna sarcinulata, Atrypa reticularis, Bellerophon trilobatus, Natica, Turritellæ, Murchisonia, Avicula lineata, Orthonaia cingulata, Phacops caudatus, Beyrichia tuberculata, and Graptolites ludensis. These characteristic Upper Silurian fossils are accompanied by a Leptæna sericea, and Orthoceras tenuicinctum of Portlock, and appear to be of the date of Wenlock shale. Their latest historian, indeed, ascribes a vast indefinite antiquity to the whole range, and considers that the depository matter has been twice reduced to a muddy arenaceous state: that a chain of hills existed in these parts at an age long anterior to the Lammermuirs, and that another stratified formation has to be intercalated in this district, between the oldest existing strata and the parent rock, whence the sediment was derived. This opinion is founded chiefly on the circumstance, that in none of the beds have there ever been observed any fragments of granite, or the associated crystalline gneiss and schists, while fragments of clay-slate and graywacke are not uncommon amongst the conglomerate or coarser varieties. But, admitting the truth of the statement, does it warrant the inference deduced? The clay-slates and graywacke of the Highlands are equally destitute of the inclosed granitoid portions so abundant in the superimposed conglomerates of the old red; and, upon the supposition of an intensely but unequally heated sea-bottom, and partial outbursts of irruptive matter, these appearances in the Lammermuirs, where certain strata contain included fragments of similar consolidated rock, receive an intelligible and less extravagant explanation. We more willingly accede to the conclusion of Mr. Nicol, that we have still here much of the original shape and contour of this ancient land; that the rivers and valleys are all in their olden places, and that since the elevation of the group there has been no important change in their general character and physical outline. The Lammermuirs, too, connected as they are with the great silurian deposits of England, Wales, and Ireland, lend confirmation to the theory, repeatedly adverted to, that these mountains, as well as those of the preceding epoch, formed the land on which grew part at least of the exuberant vegetation entombed in the coal formations of Great Britain. It may have been a peninsula projecting into a sea, whose waves washed the Grampians on the north, covered all the midland and eastern districts of England on the south, and were bounded by the primary and silurian girdle of rocks on the west. Through these depths roamed the successive races of holoptychius, palæoniscus, gyracanthus, and megalichthys; the shallows and bottom teemed with swarms of molluscs, trilobites, cephalaspes, pamphracti; while the dense forests of ferns, palms, and pines, which clothed the shores and uplands, have been distributed among the various basins of the coal-measures.
Ascend the Eildons, or, as the route may be, Carterfell, Hartfell, or Criffel, and witness the changes, as the different systems of rocks were drifted into their places, and rose above that expanse of waters. Criffel, the loftiest mountain on the west, is composed of granite, and formed a solitary islet there, or one of a series of islands, of the primary crystalline formation. The silurian chains, in their respective positions, are next elevated to the surface. The old red sandstones collect and form along their bases, spreading over vast areas all around. These are lifted into day by the Eildons, and the numerous hills of claystone porphyry, which give such diversity of character through Liddesdale, Lauderdale, Cheviotdale, and the whole border landscape. Carterfell, which consists of a dark greenstone trap, resting on a white or light reddish sandstone, marks the upheaval of the carboniferous strata, and probable retirement of the sea from these districts, where we find no traces of any of the secondary or newer systems of rocks. Thus the line of the Scottish Border, from Annan to Roxburghshire, consists of the under series of the coal formation passing down the Tweed, by Kelso, Sprouston, Coldstream, to Berwick. The old red sandstone is largely developed by Chester, Hawick, Melrose, Greenlaw, Dunse; and again, after an interruption of trap and the coal-measures, it resumes its course by Chirnside, Foulden, and Mornington, to the sea. Scales of the Holoptychius and Dendrodus are found in the strata at Prestonhaugh, near Jedburgh, and likewise at the Knock Hill, in Berwickshire. On the higher grounds, from the Eildons to Hartfell and Peebles, the graywacke and slate beds everywhere prevail, presenting at St. Abb’s Head, Selkirk, and Ettrick Bridge, interesting specimens of crumpled and bent strata. Remarkable veins of trap and calc-spar are to be observed near St. Mary’s Loch; silver and other metallic ores are said to have been found in the neighboring hills: near Moffat, gypsum, pyritous graywacke, and alum slate, are very abundant—formations probably connected with the mineral waters of Moffat Well and Hartfell spa. At Glendinning, in the parish of Westerkirk, an antimony mine has been long wrought, which is about twenty inches wide, and once rich in the valuable mineral. A vein of galena or lead, lined with heavy-spar, crosses the Esk at Broomholm, below Langholm, and here the usual series of shales, limestone, sandstone, and thin bands of coal, are developed for miles, resting on the graywacke of Hermitage, Ernton, and Witterhope Burn hills.
The valley of the Nith, from the pass of Dalveen to Barjarg, incloses a space of nearly ten miles in length by four in breadth, filled with red sandstone and beds of limestone, and exhibits one of those original creeks or bays in the primary and silurian rocks which characterize this ancient belt. The lower basin, toward Dumfries and the Solway, presents the same series of extremely fine-grained strata. The small isolated basin of red sandstone near Lochmaben, and which contains the celebrated impressions of foot-marks in the beds at Corncockle Moor, as well as the very limited patch of sandstone in the vale of the Annan near Moffat, are probably referable to one and the same system with the above. And what is that system—the Devonian or Permian? The position of all these beds, and of others in Annandale, has long formed a fruitful subject of discussion with geologists and practical engineers, whether to regard them as an extension of the English new red, or to refer them to the predominant rock of the country. The latter view is borne out so far by the fact, that no borings, which have been both numerous and deep, have penetrated to the coal metals. On Greenough’s map, on the other hand, the former theory is adopted, where the coal-measures are represented as extending underneath from Canobie, through Annandale to Arkit Muir, and as again emerging at Arbigland near Criffel. It has likewise been argued, that the foot-impressions on the slabs near Dumfries and at Corncockle Moor, have no analogues anywhere in the true old red of Scotland, while they are abundantly represented by the foot-prints of the newer sandstones of England and America. But now indeed such proofs are not wanting in America, that wide field of all organic things; the discovery has been made and assented to by the most competent authorities, that, in the old red sandstone of Pennsylvania, and 8,500 feet below the upper part of the coal formation, reptilian foot-prints are numerously and distinctly impressed, allied in form to the tread of the existing alligator. Then the larger orthoceræ and other testaceæ, found so plentifully in the limestones of Closeburn and Barjarg, also at Linburn and Shielgreen, would seem to claim very clearly and decidedly for the deposit in the middle basin of Nithsdale a Devonian origin.
We must refrain from entering upon the details of the extensive geological fields which have just been glanced at. A volume would not suffice to exhaust the subject. But, rapid as our sketch has necessarily been, enough has been advanced to show how intimately connected with the great fundamental principles of the science, and with the original configuration of our planet more especially, are all the deeply-interesting phenomena of the region in question. If at times the reader, as well as explorer, is apt to complain of the dryness of particulars, that the nomenclature is harsh and scholastic, how delightful to close, even in imagination, the day’s excursion amid these lovely valleys, to be steeped in fairy lore by St. Mary’s Loch, to dream of legends and minstrelsy until morning dawn by Newark’s Tower. Nith, Gala, Ettrick, Yarrow, Teviot, silvery Tweed!—who, indeed, will ever associate with the minerals, sections, and technicalities of geology. Still how refreshing to lie down, traveled and weary-worn, by their green pastures and pure waters. How beautifully do these rivers, all of them with an origin so remote, hold on in their pebbly courses—winding and gathering from so many rills amidst the pastoral uplands—“making sweet music with th’ enameled stones”—and anon with all their affluents sweeping in placid majesty to the main. Hither will men of all professions and pursuits,—the sportsman, poet, philosopher,—eagerly and rejoicingly resort, each with his own object or his own care. Finely, in imperishable verse, has the truth been expressed of that many-colored tide of human life on which all are embarked—
“Which, though it change in ceaseless flow,
Retains each grief, retains each crime”—
—but now these streams, dales, and hills retain no impress of strife or blood; and hence will the wish, age after age, breathe from many a heart—
“By Yarrow’s stream still let me stray,
Though none should guide my feeble way: