Above the blended conglomerates and sandstones of this band a deposition of lime took place. Thermal springs, charged with calcareous matter slightly mixed with silex, seem to have abounded, during the period which it represents, over widely-extended areas; and hence, probably, its origin. An increase of heat from beneath, through some new activity imparted to the Plutonic agencies, would be of itself sufficient to account for the formation. I have resided in a district in which almost every spring was charged with calcareous earth; but in cisterns or draw-wells, or the utensils in which the housewife stored up for use the water which these supplied, no deposition took place. With boilers and tea-kettles, however, the case was different. The agency of heat was brought to operate upon these; and their sides and bottoms were covered, in consequence, with a thick crust of lime. Now, we have but to apply the simple principles on which such phenomena occur, to account for widely-spread precipitates of the same earth by either springs or seas, which at a lower temperature would have been active in the formation of mechanical deposits alone. The temperature sunk gradually to its former state; the purely chemical deposit ceased; the waters became populous as before with animals of the same character and appearance as those of the upper conglomerate; and layer after layer of yellow sandstone, to the depth of several hundred feet, were formed as the period passed. With this upper deposit the system terminated.
Though fish still remained the lords of creation, and fish of apparently no superior order to those with which the vertebrata began at least three formations earlier, they had mightily advanced in one striking particular. If their organization was in no degree more perfect than at first, their bulk at least had become immensely more great. The period had gone by in which a mediocrity of dimension characterized the existences of the ancient oceans, and fish armed offensively and defensively with scales and teeth scarcely inferior in size to the scales and teeth of the gavial or the alligator, sprung into existence. It must have been a large jaw and a large head that contained, doubtless among many others, a tooth an inch in diameter at the base. I may remark, in the passing, that most of the teeth found in the several formations of the system are not instruments of mastication, but, like those in most of the existing fish, mere hooks for penetrating slippery substances, and thus holding them fast. The rude angler who first fashioned a crooked bone, or a bit of native silver or copper, into a hook, might have found his invention anticipated in the jaws of the first fish he drew ashore by its means; and we find the hook structure as complete in the earlier ichthyolites of the Old Red Sandstone as in the fish that exist now. The evidence of the geologist is of necessity circumstantial evidence, and he need look for none other; but it is interesting to observe how directly the separate facts bear, in many examples, on one and the same point. The hooked and slender teeth tell exactly the same story with the undigested scales in the fœcal remains alluded to in an early chapter.
In what could this increase in bulk have originated? Is there a high but yet comparatively medium temperature in which animals attain their greatest size, and corresponding gradations of descent on both sides, whether we increase the heat until we reach the point at which life can no longer exist, or diminish it until we arrive at the same result from intensity of cold? The line of existence bisects on both sides the line of extinction. May it not probably form a curve, descending equally from an elevated centre to the points of bisection on the level of death? But whatever may have been the cause, the change furnishes another instance of analogy between the progress of individuals and of orders. The shark and the sword-fish begin to exist as little creatures of a span in length; they expand into monsters whose bodies equal in hugeness the trunks of ancient oaks; and thus has it been with the order to which they belong. The teeth, spines, and palatal bones of the fish of the Upper Ludlow Rocks are of almost microscopic minuteness; an invariable mediocrity of dimension characterizes the ichthyolites of the Lower Old Red Sandstone; a marked increase in size takes place among the existences of the middle formation; in the upper the bulky Holoptychius appears; the close of the system ushers in the still bulkier Megalichthys; and low in the Coal Measures we find the ponderous bones, buckler-like scales, and enormous teeth of another and immensely more gigantic Holoptychius—a creature pronounced by Agassiz the largest of all osseous fish.[BF] We begin with an age of dwarfs—we end with an age of giants. The march of Nature is an onward and an ascending march; the stages are slow, but the tread is stately; and to Him who has commanded, and who overlooks it, a thousand years are as but a single day, and a single day as a thousand years.[BG]
[BF] There have been fish scales found in Burdie House five inches in length, by rather more than four in breadth. Of the gigantic Holoptychius of this deposit we have still much to learn. The fragment of a jaw, in the possession of the Royal Society of Edinburgh, which belonged to an individual of the species, is 18-1/2 inches in length; and it is furnished with teeth, one of which, from base to point, measures five inches, and another four and a half.
[BG] See, on this subject, the introductory note to the present edition, and note [p. 154].
We have entered the Coal Measures. For seven formations together—from the Lower Silurian to the Upper Old Red Sandstone—our course has lain over oceans without a visible shore, though, like Columbus, in his voyage of discovery, we have now and then found a little floating weed, to indicate the approaching coast. The water is fast shallowing. Yonder passes a broken branch, with the leaves still unwithered; and there floats a tuft of fern. Land, from the mast-head! land! land!—a low shore, thickly covered with vegetation. Huge trees, of wonderful form, stand out far into the water. There seems no intervening beach. A thick hedge of reeds, tall as the masts of pinnaces, runs along the deeper bays, like water-flags at the edge of a lake. A river of vast volume comes rolling from the interior, darkening the water for leagues with its slime and mud, and bearing with it, to the open sea, reeds, and fern, and cones of the pine, and immense floats of leaves, and now and then some bulky tree, undermined and uprooted by the current. We near the coast, and now enter the opening of the stream. A scarce penetrable phalanx of reeds, that attain to the height and well nigh the bulk of forest trees, is ranged on either hand. The bright and glossy stems seem rodded like Gothic columns; the pointed leaves stand out green at every joint, tier above tier, each tier resembling a coronal wreath or an ancient crown, with the rays turned outwards; and we see a-top what may be either large spikes or catkins. What strange forms of vegetable life appear in the forest behind! Can that be a club-moss that raises its slender height for more than fifty feet from the soil? Or can these tall, palm-like trees be actually ferns, and these spreading branches mere fronds? And then these gigantic reeds!—are they not mere varieties of the common horse-tail of our bogs and morasses, magnified some sixty or a hundred times? Have we arrived at some such country as the continent visited by Gulliver, in which he found thickets of weeds and grass tall as woods of twenty years' growth, and lost himself amid a forest of corn, fifty feet in height? The lesser vegetation of our own country, reeds, mosses, and ferns, seems here as if viewed through a microscope: the dwarfs have sprung up into giants, and yet there appears to be no proportional increase in size among what are unequivocally its trees. Yonder is a group of what seem to be pines—tall and bulky, 'tis true, but neither taller nor bulkier than the pines of Norway and America; and the club-moss behind shoots up its green, hairy arms, loaded with what seems catkins above their topmost cones. But what monster of the vegetable world comes floating down the stream—now circling round in the eddies, now dancing on the ripple, now shooting down the rapid? It resembles a gigantic star-fish, or an immense coach-wheel, divested of the rim. There is a green, dome-like mass in the centre, that corresponds to the nave of the wheel, or the body of the star-fish; and the boughs shoot out horizontally on every side, like spokes from the nave, or rays from the central body. The diameter considerably exceeds forty feet; the branches, originally of a deep green, are assuming the golden tinge of decay; the cylindrical and hollow leaves stand out thick on every side, like prickles of the wild rose on the red, fleshy, lance-like shoots of a year's growth, that will be covered, two seasons hence, with flowers and fruit. That strangely formed organism presents no existing type among all the numerous families of the vegetable kingdom. There is an amazing luxuriance of growth all around us. Scarce can the current make way through the thickets of aquatic plants that rise thick from the muddy bottom; and though the sunshine falls bright on the upper boughs of the tangled forest beyond, not a ray penetrates the more than twilight gloom that broods over the marshy platform below. The rank steam of decaying vegetation forms a thick blue haze, that partially obscures the underwood; deadly lakes of carbonic acid gas have accumulated in the hollows; there is silence all around, uninterrupted save by the sudden splash of some reptile fish that has risen to the surface in pursuit of its prey, or when a sudden breeze stirs the hot air, and shakes the fronds of the giant ferns or the catkins of the reeds. The wide continent before us is a continent devoid of animal life, save that its pools and rivers abound in fish and mollusca, and that millions and tens of millions of the infusory tribes swarm in the bogs and marshes. Here and there, too, an insect of strange form flutters among the leaves. It is more than probable that no creature furnished with lungs of the more perfect construction could have breathed the atmosphere of this early period, and have lived.
Doubts have been entertained whether the limestone of Burdie House belongs to the Upper Old Red Sandstone or to the inferior Coal Measures. And the fact may yet come to be quoted as a very direct proof of the ignorance which obtained regarding the fossils of the older formation, at a time when the organisms of most of the other formations, both above and below it, had been carefully explored. The Limestone of Burdie House is unequivocally and most characteristically a Coal Measure limestone. It abounds in vegetable remains of terrestrial or lacustrine growth, and these, too, the vegetables common to the Coal Measures—ferns, reeds, and club-mosses. One can scarce detach a fragment from the mass, that has not its leaflet or seed-cone enclosed, and in a state of such perfect preservation, that there can be no possibility of mistaking its character. If in reality a marine deposit, it must have been formed in the immediate neighborhood of a land covered with vegetation. The dove set loose by Noah bore not back with it a less equivocal sign that the waters had abated. Now, in the Upper Old Red Sandstone none of these plants occur. The deposit is exclusively an ocean deposit, and the remains in Scotland, until we arrive at its inferior and middle formations, are exclusively animal remains. Its upper member, "the yellow sandstone," says Dr. Anderson, of Newburgh, "does not exhibit a single particle of carbonaceous matter—no trace or film of a branch having been detected in it, though, if such in reality existed, there are not wanting opportunities of obtaining specimens in some one of the twenty or thirty quarries which have been opened in the county of Fife in this deposit alone." No two bordering formations in the geological scale have their boundaries better defined by the character of their fossils than the Old Red Sandstone and the Coal Measures.
We pursue our history no further. Its after course is comparatively well known. The huge sauroid fish was succeeded by the equally huge reptile—the reptile by the bird—the bird by the marsupial quadruped; and at length, after races higher in the scale of instinct had taken precedence in succession, the one of the other, the sagacious elephant appeared, as the lord of that latest creation which immediately preceded our own. How natural does the thought seem which suggested itself to the profound mind of Cuvier, when indulging in a similar review! Has the last scene in the series arisen, or has Deity expended his infinitude of resource, and reached the ultimate stage of progression at which perfection can arrive? The philosopher hesitated, and then decided in the negative, for he was too intimately acquainted with the works of the Omnipotent Creator to think of limiting his power; and he could, therefore, anticipate a coming period in which man would have to resign his post of honor to some nobler and wiser creature—the monarch of a better and happier world. How well it is, to be permitted to indulge in the expansion of Cuvier's thought, without sharing in the melancholy of Cuvier's feeling—to be enabled to look forward to the coming of a new heaven and a new earth, not in terror, but in hope—to be encouraged to believe in the system of unending progression, but to entertain no fear of the degradation or deposition of man! The adorable Monarch of the future, with all its unsummed perfection, has already passed into the heavens, flesh of our flesh, and bone of our bone, and Enoch and Elias are there with him—fit representatives of that dominant race, which no other race shall ever supplant or succeed, and to whose onward and upward march the deep echoes of eternity shall never cease to respond.