[CHAPTER IX.]
Fossils of the Upper Old Red Sandstone much more imperfectly preserved than those of the Lower.—The Causes obvious.—Difference between the two Groups, which first strikes the Observer, a Difference in Size.—The Holoptychius a characteristic Ichthyolite of the Formation.—Description of its huge Scales.—Of its Occipital Bones, Fins, Teeth, and General Appearance.—Contemporaries of the Holoptychius.—Sponge-like Bodies.—Plates resembling those of the Sturgeon.—Teeth of various Forms, but all evidently the Teeth of Fishes.—Limestone Band, and its probable Origin.—Fossils of the Yellow Sandstone.—The Pterichthys of Dura Den.—Member of a Family peculiarly characteristic of the System.—No intervening Formation between the Old Red Sandstone and the Coal Measures.—The Holoptychius contemporary for a time with the Megalichthys.—The Columns of Tubal Cain.
The different degrees of entireness in which the geologist finds his organic remains, depend much less on their age than on the nature of the rock in which they occur; and as the arenaceous matrices of the Upper and Middle Old Red Sandstones have been less favorable to the preservation of their peculiar fossils than the calcareous and aluminous matrices of the Lower, we frequently find the older organisms of the system fresh and unbroken, and the more modern existing as mere fragments. A fish thrown into a heap of salt would be found entire after the lapse of many years; a fish thrown into a heap of sand would disappear in a mass of putrefaction in a few weeks; and only the less destructible parts, such as the teeth, the harder bones, and perhaps a few of the scales, would survive. Now, limestone, if I may so speak, is the preserving salt of the geological world; and the conservative qualities of the shales and stratified clays of the Lower Old Red Sandstone are not much inferior to those of lime itself; while, in the Upper Old Red, we have merely beds of consolidated sand, and these, in most instances, rendered less conservative of organic remains than even the common sand of our shores, by a mixture of the red oxide of iron. The older fossils, therefore, like the mummies of Egypt, can be described well nigh as minutely as the existences of the present creation; the newer, like the comparatively modern remains of our churchyards, exist, except in a few rare cases, as mere fragments, and demand powers such as those of a Cuvier or an Agassiz to restore them to their original combinations. But cases, though few and rare, do occur in which, through some favorable accident connected with the death or sepulture of some individual existence of the period, its remains have been preserved almost entire; and one such specimen serves to throw light on whole heaps of the broken remains of its contemporaries. The single elephant, preserved in an iceberg beside the Arctic Ocean, illustrated the peculiarities of the numerous extinct family to which it belonged, whose bones and huge tusks whiten the wastes of Siberia. The human body found in an Irish bog, with the ancient sandals of the country still attached to its feet by thongs, and clothed in a garment of coarse hair, gave evidence that bore generally on the degree of civilization attained by the inhabitants of an entire district in a remote age. In all such instances, the character and appearance of the individual bear on those of the tribe. In attempting to describe the organisms of the Lower Old Red Sandstone, where the fossils lie as thickly in some localities as herrings on our coasts in the fishing season, I felt as if I had whole tribes before me. In describing the fossils of the Upper Old Red Sandstone, I shall have to draw mostly from single specimens. But the evidence may be equally sound so far as it goes.
The difference between the superior and inferior groups of the system which first strikes an observer, is a difference in the size of the fossils of which these groups are composed. The characteristic organisms of the Upper Old Red Sandstone are of much greater bulk than those of the Lower, which seem to have been characterized by a mediocrity of size throughout the entire extent of the formation. The largest ichthyolites of the group do not seem to have much exceeded two feet or two feet and a half in length; its smaller average from an inch to three inches. A jaw in the possession of Dr. Traill—that of an Orkney species of Platygnathus, and by much the largest in his collection—does not exceed in bulk the jaw of a full-grown coal-fish or cod; his largest Coccosteus must have been a considerably smaller fish than an ordinary-sized turbot; the largest ichthyolite found by the writer was a Diplopterus, of, however, smaller dimensions than the ichthyolite to which the jaw in the possession of Dr. Traill must have belonged; the remains of another Diplopterus from Gamrie, the most massy yet discovered in that locality, seem to have composed the upper parts of an individual about two feet and a half in length. The fish, in short, of the lower ocean of the Old Red Sandstone—and I can speak of it throughout an area which comprises Orkney and Inverness, Cromarty, and Gamrie, and which must have included about ten thousand square miles—ranged in size between the stickleback and the cod; whereas some of the fish of its upper ocean were covered by scales as large as oyster-shells, and armed with teeth that rivalled in bulk those of the crocodile. They must have been fish on an immensely larger scale than those with which the system began. There have been scales of the Holoptychius found in Clashbennie which measure three inches in length by two and a half in breadth, and a full eighth part of an inch in thickness. There occur occipital plates of fishes in the same formation in Moray, a full foot in length by half a foot in breadth. The fragment of a tooth still attached to a piece of the jaw, found in the sandstone cliffs that overhang the Findhorn, measures an inch in diameter at the base. A second tooth of the same formation, of a still larger size, disinterred by Mr. Patrick Duff from out the conglomerates of the Scat-Craig, near Elgin, and now in his possession, measures two inches in length by rather more than an inch in diameter. (See [Plate X.], fig. 4.) There occasionally turn up in the sandstones of Perthshire ichthyodorulites that in bulk and appearance resemble the teeth of a harrow rounded at the edges by a few months' wear, and which must have been attached to fins not inferior in general bulk to the dorsal fin of an ordinary-sized porpoise. In short, the remains of a Patagonian burying-ground would scarcely contrast more strongly with the remains of that battle-field described by Addison, in which the pygmies were annihilated by the cranes, than the organisms of the upper formation of the Old Red Sandstone contrast with those of the lower.[AO]
[AO] I have permitted this paragraph to remain as originally written, though the comparatively recent discovery of a gigantic Holoptychius (?) in the Lower Old Red Sandstone of Thurso, by Mr. Robert Dick of that place, (see introductory note,) bears shrewdly against its general line of statement. But it will, at least, serve to show how large an amount of negative evidence may be dissipated by a single positive fact, and to inculcate on the geologist the necessity of cautious induction. An individual Holoptychius of Thurso must have been at least thrice the size of the Holoptychius of the Upper Old Red formation, as exhibited in the specimen of Mr. Noble, of St. Madoes.
PLATE X.
