CHAPTER XI.THE LAMMERMUIRS—THE BORDER LAND—GENERAL STRUCTURE OF SCOTLAND.
The interest which attaches to this division of our sketches of Scottish geology is in no degree impaired by the consideration that the rocks, all of them, belong to one or other of the systems which have already passed under review. A belt of undisputed Silurian deposit here meets us for the first time, flanked on all sides, and nearly throughout its length, by the old red sandstone. Porphyritic hills, greenstone bosses and dykes, and the various phenomena of trap intrusion and dislocation, are again presented in many and very striking illustrative details. “The border land,” physically as well as morally, could not well be without its points of contention; and, accordingly, geologists have made “raide across the marches,” and claimed as of Scottish origin an extensive domain of the Englishnew redsandstone, or Permian system. Corncockle Moor, too, and the quarries near Dumfries, unfold as curious a page in the history of the old world as does the Crigup Lynn, hewn out of the same family of rocks, of the stern warfare and fierce contendings which adorn, as they likewise disgrace, the annals of the seventeenth century.
Thegeneral structureof 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, thoughreposing 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, andunderneaththe 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 ofGraywackerock, the Lammermuirs are now, by general consent, admitted into the family ofSilurians, 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 sedimentarydeposits 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 thesmooth Asaphiwould 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, andGraptolites ludensis. These characteristic Upper Silurian fossils are accompanied by aLeptæna sericea, andOrthoceras tenuicinctumof 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 beentwicereduced 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 suppositionof 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 orlight 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 theunder seriesof 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 extendingunderneathfrom 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 infairy 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”—
“Which, though it change in ceaseless flow,Retains each grief, retains each crime”—
“Which, though it change in ceaseless flow,Retains each grief, retains each crime”—
“Which, though it change in ceaseless flow,
Retains each grief, retains each crime”—
—butnowthese 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:Still feel the breeze down Ettrick break,Although it chill my withered cheek;Still lay my head by Teviot stone,Though there forgotten and alone.”
“By Yarrow’s stream still let me stray,Though none should guide my feeble way:Still feel the breeze down Ettrick break,Although it chill my withered cheek;Still lay my head by Teviot stone,Though there forgotten and alone.”
“By Yarrow’s stream still let me stray,Though none should guide my feeble way:Still feel the breeze down Ettrick break,Although it chill my withered cheek;Still lay my head by Teviot stone,Though there forgotten and alone.”
“By Yarrow’s stream still let me stray,
Though none should guide my feeble way:
Still feel the breeze down Ettrick break,
Although it chill my withered cheek;
Still lay my head by Teviot stone,
Though there forgotten and alone.”
And so human life will glide away, a new epoch will come, and the development of man’s immortal being will be accomplished in the new and brighter earth that is to arise.
We here close our review, over Scottish ground, of the earliest lessons to which we have access of the mineral structure of our globe. All the primary, palæozoic, and older class of the secondary rocks, are largely developed, leading us, indeed, a very little way into the inner chambers of the earth, but back through periods of time into the records of its history, for which the science itself furnishes no real standard of measurement. Vast, inconceivablecycles pass before the imagination, and fascinate the speculatist, while the sober inquirer pauses, doubts—nay, startles—at such remote undefined annals of creation. The legends and chronicles of Scotland are old indeed; but give geologists their own way, and what an antiquity would they assign to the mountains, valleys, and rivers of ancient Caledon! And yet, true it is, no rocks on the face of the earth can claim a deeper origin, an earlier arrangement, a more ancient ascent above the waters, than those whose nature and position we have so cursorily described.
Leaving for the present the question as to Time, geology has this advantage, in facilitating an acquaintance with its principles, that its lessons are as general as they are particular. Go where you will the record is before you, so that, generally speaking, what is observed of its subject in one district or country, or even continent, has its counterpart in some other place near or remote. The rocks of Scotland are all on the great scale, not solitary and individual specimens, but wide-spread formations along the face of the country. With scarcely a single exception, every class of rocks described in our course stretches from sea to sea over the island. The structure of Scotland is peculiar in this, that the bearing and the strike of the various strata, are correspondent and continuous. Hence the parallelism of the great straths and valleys. The principal rivers are observant of the same law. The several formations, from the primary crystalline to the coal and upper sandstones, have a common axis of elevation from nearly E. N. E. to W. S. W., partaking more of an equatorial than of a meridional direction. The porphyries and chains of the older traps maintain a similar direction. The greenstones and basalts, polygonal and jointed, or otherwise, are for the most part to be foundwithinthe area of the coal measures, or rising along the out-crop of the basins. Hence a description of any one locality will generally, in respect of the same series of rocks, be found applicable to another. The student may indifferently begin his researches as his convenience or sojourn for the time may direct. And whether it be the granite on the coasts of Aberdeen or of Arran—the schists of Glenisla or the Mull of Cantire—the silurian of St. Abb’s Head or Portpatrick—the devonian of Stonehaven or Girvan—the porphyries of Dundee or Largs—theshales and limestones of St. Andrews, Glasgow, or Ayr—the columnar basalts of Earlsferry, Orrock, Campsie, or Staffa—the lesson throughout will be one and the same, either as respects the mineral texture or the geological position of the rocks examined. A section, therefore, commencing at Ben Nevis and terminating at Kirkcudbright on the Solway, would present the very same series in all the main phenomena of superposition, structure, dislocation, and fossil remains, as the section adopted from Ben-Mac-Dhui to the Cheviots. Granite, gneiss, quartz rock, mica schist, and clay slate, underlie the old red sandstone which traverses the upper district of Stirlingshire. The traps of the Campsie Hills have thrown up, and form the boundary of the great coal-basin, of which Glasgow constitutes the center, and within whose area and suburbs are exhibited all the most striking features of the basalt and greenstone family—the elevation at the Necropolis beautifully showing the effects of their intrusion, and the induration of the sedimentary deposits. On the south, the coal metals are again succeeded by the old red sandstone and the porphyries, which in their turn are replaced by the silurian or graywacke rocks of the border counties. The section throughout is of the most varied and instructive character, diversified by the grandest mountain scenery, the loveliest of the Scottish lakes, and a development of the arts and sciences over inexhaustible coal and iron treasures which has rendered the name of the western metropolis illustrious among the cities of the world.
Should the geologist desire to extend his researches along the western coast and among the islands, he will experience an additional interest, arising chiefly from the numerous junctions of the different formations or sets of rocks which the constant erosion of the Atlantic has everywhere exposed to view. Gigantic isolated portions of granite or syenite, bared all around, are to be seen on every headland. The twistings and flexures of gneiss and the schists are frequent and remarkable. Columnar basalt, similar to Staffa, fringes the base of every islet and promontory; and from appearances like these, he will invariably infer the presence of the carboniferous deposits, which, in small detached patches are of common occurrence. Here, likewise, are to be found the lias and oolites, in marginal stripes on several of the islands, easily distinguishedby their characteristic fossils, and giving unequivocal indications of a far greater extension, and continuity with the main land, ere the inroads of the sea had broken up and parted so much of the aboriginal structure of the district. Thevi et sæpe cadendoof geological agency—the convulsions of subterranean forces, and the destroying powers of water—are exhibited in all their grandeur, where, in the face of cliffs exposed to their foundations, the hardest rocks may be observed yielding to every wave, and the whole inner machinery of granitic and basaltic dykes which upheaved them from their basis traced in their most varying forms and complicity. Out of the Ægean a finer group of islands is nowhere to be threaded—some scarcely raised above sea-level—some towering into the clouds, as in the lofty peaks of Mull, Jura, and Rum, with an altitude almost equal to their length—most of them glorying in names soft and euphonious as the choicest of classic Greece—and yet, all fragmentary and disrupted, as if but yesterday shivered by the thunder cloud. Skye exhibits an epitome not of the islands only, but nearly of our whole British geology, in which there is every variety of trap, combined with the primary series—coal, white sandstone, and limestone—the lias and oolites of secondary formation—and mountains 3,300 feet in height, composed of Labrador feldspar and hypersthene, whose crystals in the dark composite mass rival in structure, if not in beauty, the stalactitic concretions of the Spar Cave itself. Rocks, too, are here, of metamorphic texture, to which a Macculloch did not venture to assign a name or position in his list. And the serrated jagged pinnacles of the Coolin ridge, with the black Coruisk inclosed as in a crater, who will attempt to describe—unlike to everything else in bleak, naked, precipitous grandeur! The poet of the Isles has sketched the picture—
“Such are the scenes, where savage grandeur wakesAn awful thrill that softens into sighs;Such feelings rouse them by dim Rannoch’s lakes,In dark Glencoe such gloomy raptures rise:Or farther, where, beneath the northern skies,Chides wild Loch-Eribol, his caverns hoar—But, be the minstrel judge, they yield the prizeOf desert dignity to that dread shoreThat sees grim Coolin rise, and hears Coriskin roar.”
“Such are the scenes, where savage grandeur wakesAn awful thrill that softens into sighs;Such feelings rouse them by dim Rannoch’s lakes,In dark Glencoe such gloomy raptures rise:Or farther, where, beneath the northern skies,Chides wild Loch-Eribol, his caverns hoar—But, be the minstrel judge, they yield the prizeOf desert dignity to that dread shoreThat sees grim Coolin rise, and hears Coriskin roar.”
“Such are the scenes, where savage grandeur wakesAn awful thrill that softens into sighs;Such feelings rouse them by dim Rannoch’s lakes,In dark Glencoe such gloomy raptures rise:Or farther, where, beneath the northern skies,Chides wild Loch-Eribol, his caverns hoar—But, be the minstrel judge, they yield the prizeOf desert dignity to that dread shoreThat sees grim Coolin rise, and hears Coriskin roar.”
“Such are the scenes, where savage grandeur wakes
An awful thrill that softens into sighs;
Such feelings rouse them by dim Rannoch’s lakes,
In dark Glencoe such gloomy raptures rise:
Or farther, where, beneath the northern skies,
Chides wild Loch-Eribol, his caverns hoar—
But, be the minstrel judge, they yield the prize
Of desert dignity to that dread shore
That sees grim Coolin rise, and hears Coriskin roar.”