It is well to return on the record, and to read in its unequivocal characters the lessons which it was intended to teach. Infidelity has often misinterpreted its meaning, but not the less on that account has it been inscribed for purposes alike wise and benevolent. Is it nothing to be taught, with a demonstrative evidence which the metaphysician cannot supply, that races are not eternal—that every family had its beginning, and that whole creations have come to an end?
[CHAPTER VI.]
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]
[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.
[AG] See Transactions of the London Geological Society for 1828, p. 354.