As before, we must have recourse to the analogy of living nature. The existing ganoidal fishes chiefly inhabit lakes and rivers, especially near the confluence of the latter with the ocean. They feed on the decaying matter brought down from the land, or on the minute Crustacea that swarm upon the river-bottom. If, as seems probable, the ancient ganoids had habits similar to those of their present representatives, then the rocks wherein their remains occur abundantly may have originated on river-bottoms, and such may have been the case at Burdiehouse. So that here again we have corroborative evidence of the fluviatile origin of the limestone in question. But besides the remains of ganoidal fishes there occur the defensive spines of placoids. Now, the placoids are emphatically marine fishes, and the sole living representative of the most ancient genera of this order is the Port-Jackson shark, that haunts the seas round Australia. The ichthyodorulites of Burdiehouse, therefore, if we would apply analogy consistently, must be regarded as the relics of marine species. And this conclusion, too, will be found in entire harmony with those already obtained, for if we are right in assuming the Burdiehouse strata to have originated at a river-bottom, particularly near the sea, we may expect to find the remains of marine predaceous fishes imbedded in the sediment that gathered there, just as the teeth of the shark may be preserved among the mud forming in the upper reaches of many British estuaries, seeing that not a few instances are known where that fish has been stranded on such shores as those of the higher parts of the Firth of Forth. These Burdiehouse ichthyodorulites give positive proof that the limestone could not have originated in a lake, and the only explanation left is that of a river-bottom.
But it may perhaps be objected that, after all, these fish-remains are for the most part fragmentary, and may consequently be drifted specimens, so that no conclusion as to the source of the rock can be based on their occurrence there. The imbedded land-plants confessedly came from some distance, why may not the same have been the case with the bones and scales of the river-haunting ganoid fishes? And, indeed, did we regard these fish-bones and scales merely in themselves, the argument might not perhaps be very easily answered, although the great numbers and perfect outline of the bones, teeth, and scales, afford pretty strong evidence that the owners lived and died in the locality where their remains are found. But there is a curious kind of evidence to be gleaned from the rocks around them whereby this objection can be at once set aside. In the limestone itself, and especially in some of the shales above, there occur vast numbers of small oblong coprolitic concretions of a dirty yellow or brown colour, full of scales and fragments of bone. There can be no doubt that these are the excremental remains of predaceous animals, while their great number and perfect preservation assure us that they could not have been drifted from a distance, but must rather have been deposited on the spot where we now find them. And thus we conclude that the site of Burdiehouse must have been a favourite haunt of these bone-covered fishes; that the bulkier forms, armed with pointed teeth or barbed-spines, preyed upon their humbler congeners, while these in turn may have fed on the cyprides that swarmed by millions at the bottom of the estuary. I have often detected in these coprolites the peculiarly-sculptured scales of the palæoniscus. These graceful little animals must, therefore, have died that their lordlier brethren might dine.
On a survey, then, of the whole evidence from fossils, we are led to conclude that the Burdiehouse limestone was slowly elaborated at the bottom of an estuary, into which the remains of terrestrial plants were drifted from the land, while bone-covered fishes haunted the waters, and into these busy scenes huge sharks ascended from the sea to share in the decaying putrescent matter ever brought down from the interior.
The upper part of the limestone is shaly and argillaceous, and rests below a series of shales and thin sandstones. If the question were asked, what caused the change from limestone to shale, from the deposition of a calcareous to that of a muddy sediment, several answers might be given. The most probable seems to be the following. The limestone on weathered surfaces displays the mouldering casts of cypris-cases sometimes in such abundance as to show that the rock must be largely made up of them. The cyprides of the present day probably cast their shells annually; the integuments thus thrown off forming under favourable circumstances a thin mouldering calcareous marl at the bottom of the pond or marsh, along with the decaying shells of paludina, planorbis, limnea, or other fresh-water molluscs. We may conceive the Burdiehouse limestone to have had a similar origin. The cyprides, inhabiting water that contained little argillaceous matter, must have propagated by myriads, and during a long period of repose, in which the conditions of land and sea, and the directions of tidal currents and river-courses, appear not to have greatly varied in the neighbourhood of Burdiehouse, the calcareous exuviæ of these minute animals, along perhaps with the remains of other estuarine or fluviatile organisms,[60] would form each year a scarce appreciable stratum, until by slow aggregation a bed twenty-seven feet deep was elaborated. Each successive annual layer would hardly settle down more perceptibly or more rapidly than "the flickering dust that mottles the floor of some old haunted chamber."
[60] Though I have never observed molluscan remains in the limestone of Burdiehouse, they are abundant twelve miles to the west, in the equivalent strata around Mid-Calder, one little gastropod being especially plentiful near the base of the calcareous rock in a seam known to the quarrymen as the "Buckie fake." I have not met with specimens sufficiently perfect for identification, the hard splintery nature of the rock seldom allowing anything but a cross-section to be seen save on weathered specimens, where the general contour of the shells has sometimes reminded me of Paludina multiformis grouped together in a recent fresh-water marl. In the shales above the Burdiehouse limestone, Dr Hibbert states he found a unio (?), called by him U. nuciformis. Trans. Roy. Soc. Edin. vol. xiii. p. 245.
At last, however, this condition of things came to be modified. The direction of the river channel along some part of its course had varied, or some analogous change had taken place, so that muddy sediment transported from the land sank down amid the cyprides at the bottom. In circumstances so uncongenial these tiny denizens of the estuary diminished in numbers until the silt and sand came down so rapidly and in such abundance that they eventually died out. Alluvial matter still darkened the water and covered the river-bottom, enveloping now the fronds of a delicate fern that had waved along the margin of some sequestered lake far inland, anon a seed-cone that had been shaken by the breeze from the spiky branches of some tall club-moss. Among these muddy beds occur numerous coprolites and fish-scales, along with cypriscases and a few shells of unio (?), showing that though the cyprides were decreasing, the water still presented the old estuary conditions and still swarmed with life.
Eventually there came other changes in the direction or rapidity of river currents, and the accumulations of mud and silt were succeeded by a long protracted deposition of yellow sand, now forming the sandstone of Straiton. It enclosed many stems of stigmaria, lepidodendron, &c., and in certain limited areas these plants matted together in such quantities that their remains now form thin irregular seams of coal. It would appear, therefore, that notwithstanding these changes in the matter transported and deposited at the locality in question, the estuary character of the locality remained essentially the same. The sand was at length replaced by fresh accumulations of mud and sandy silt, which went to form the beds of shale and shaly sandstone now found above the Straiton rock.
When in the course of many long centuries a depth of strata amounting to fully 300 feet had been amassed, the area of Mid-Lothian underwent a total change. Owing to a depression of the earth's crust, that seems to have been general over the whole of central Scotland, the estuary in which the Burdiehouse limestone and superincumbent strata were deposited became open sea. As the evidence of this change rests solely on the character of the imbedded organic remains, we shall pursue our induction by examining the beds somewhat in detail.
Rather more than 300 feet above the limestone of Burdiehouse there occurs in the Mid-Lothian coal-field a series of shales and seams of limestone. The former are sometimes black and hard, sometimes bluish-grey, soft, and frequently imbedding the remains of several genera of mollusca and other organic remains. The limestones vary considerably in the thickness and general aspect of their several seams, some being highly crystallized and about two or three feet in depth, others dull, compact, and ranging up to twenty and thirty feet thick. The shales and limestones are intercalated with and sometimes pass into each other, through the gradations of shaly limestone and calcareous shales. The whole series may measure 150 to 200 feet, resting on the Straiton sandstone below, and passing upwards into the under part of the coal-bearing strata of Mid-Lothian known as the Edge series. These limestones form the northern marine equivalents of the mountain limestone of England, while the sandstones and shales on which they rest, including the Burdiehouse beds and all the Lower Carboniferous group, must probably be regarded as estuarine equivalents of the same formation. That is to say, while marine limestones were accumulating over the site of central England, sandstone, shale, and drifted plants, were slowly gathering in a wide estuary over what is now central Scotland, and only at the close of the period did marine limestones form simultaneously at both localities.
In examining these Mid-Lothian beds we are struck at once with the great dissimilarity that obtains between their organic remains and those of the underlying strata. All the land-plants disappear—ferns, lepidodendra, sigillariæ, and stigmariæ. The cyprides, too, no longer occur, though the shales seem, at a first glance, to differ in no respect from those underneath, in some of which the cypris-cases were seen to abound. Neither can we detect the glittering scales and teeth that stood out in such strong relief upon the rocks below. Yet the fossils are scarcely less numerous than they were in the lower beds. Nay, in some of the limestones they lie so crowded together that the rock seems entirely made up of them. Plainly such a total renovation of organic life points to some equally extensive change of a physical kind. Let us examine for a little some of the fossil remains occurring in the mountain limestone series of Mid-Lothian, and read off, if we can, the revolutions which they chronicle.