The Story of a Boulder; or, Gleanings from the Note-book of a Field Geologist

by Archibald Geikie

while the slow elevation of large tracts of country, or the sudden upheaval of others, shows us by how powerful an agency the balance of land and sea is preserved, and how sometimes the paroxysm of an hour may effect a mightier change than the wasting and decay of a thousand years. We choose to call these two principles antagonistic, because in their effects they are entirely opposite; yet there is no discordance, no caprice in their operation. Each works out its end, and the result is the harmony and stability of the face of nature.

In the world of life, too, there seems to have been a double principle of decline and renewal. The natural tendency of species and genera, like that of individuals, has been towards extinction. Why it should be so we know not, further than that they are for the most part influenced by every change in physical geography. But they probably obey a still higher law which governs their duration, as the laws of vitality govern the life of an individual If we are but slightly acquainted with the agency by which the degradation of land is counter-balanced, we are still more ignorant of the laws that preserve the balance of life. Creation is a mystery, and such it must for ever remain. So, too, are the principles on which it has been conducted. We can but mark their results. We see new species appear from time to time in the upward series of the geological formations, but they tell not whence they came. Of two genera created together at the beginning, one ere long died out, but the other still lives; yet here there is assuredly nought like discordance or caprice. Nay, these two principles—death and creation—have been in active operation all through the ages, and the result is that varied and exquisitely beautiful world wherein we dwell.

The Mollusca are so named from the soft nature of their bodies, and are familiar to us as exemplified in the garden-snail and the shells of the sea-shore. The general type upon which they are constructed is that of an external muscular bag, either entire or divided into two, called the mantle, in which the viscera are contained. In most of the orders, they have likewise an outer hard calcareous shell, consisting of one or more parts. It is of course this shell alone that can be detected in the rocks, but by attending to the relations between the living animals and their shells, we ascertain the nature and affinities of the fossil species.

Few who ramble by the sea-shore, gathering limpets, whelks, and cockles, are aware how complex an anatomy is concealed within one of those brown discoloured shells. There are elaborate nervous and muscular systems—sometimes several hearts with accompanying arteries and veins—often dozens of rudimentary eyes—capsules which perform the function of ears—jaws, teeth, a strongly armed tongue—gullet, gizzard, stomach, liver, intestine, and complete breathing apparatus. The structure and grouping of these parts vary in the different genera and orders, and upon such variations is founded the classification of the naturalist. Thus, the mollusca of the highest class are called the Cephalopoda, or head-footed, because their feet, or rather arms, are slung in a belt round the head. They contain, among their number, the cuttle-fish, with its curious internal bone that shadows forth, as it were, the coming of the vertebrate type; and the nautilus, with its many-decked vessel of pearl. The second class is termed the Gastropoda, or belly-footed, as the genera embraced under it creep on the under side of the body, which is expanded into a broad retractile foot. The common snail and whelk are familiar examples. The third class is formed by the Pteropoda, or wing-footed—delicate animals, found only in the open sea, and remarkable for a pair of wing-like expansions or fins on the sides of the mouth. The Lamellibranchiata form the fourth class, and receive their name from the laminated form of their branchia, or gills. They contain the two-valved shells, such as the oyster and scallop, and are one of the most abundant groups of animals on our coasts. The fifth class consists of the Brachiopoda, or arm-footed molluscs a name given to them from their long spiral arms, once thought to be the instruments of motion, but now ascertained only to assist in bringing the food to the mouth. The sixth, and humblest class, has received the designation of Tunicata, from the thick bladder-like tunic, or sac, which supplies the place of an outer shell.

The geologist finds the remains of all these classes in the different rock-formations of the crust of the earth. They flourished so abundantly in the earliest seas, that the first geological period has sometimes been called the Age of Molluscs; and, during all the subsequent eras, they held a prominent place among the inhabitants of the deep. Let us look for a little at their development in the times of the Carboniferous system.

As the Carboniferous group of rocks exhibits the remains of ocean-bed, lake-bottom, and land-surface, so we find in it shells of marine, fresh-water and (though rarely) terrestrial mollusca. The marine genera greatly predominate, just as the shells of the sea at the present day vastly outnumber those either of lakes or of the land. In England they occur chiefly in the lower part of the formation, giving a characteristic stamp to the deep series of beds known as the mountain limestone. There they are associated with the corals and stone-lilies already described—all productions of the sea. In Northumberland, however, and generally throughout Scotland, they occupy a somewhat different position. The great mountain limestone of central England gets split up into subdivisions as it proceeds northward, and beds of coal, full of land plants, become mingled with the ordinary marine strata. Sometimes we may find a group of brachiopods scattered over the macerated stem of a stigmaria; and the writer has himself collected a sigillaria in a limestone crowded with stone-lilies and producti. But this intermingling is still further carried on in the upper part of the series. The coal-beds, with their underclays and stigmaria rootlets, evidently representing ancient vegetation with the soils on which it grew, are succeeded by beds of limestone, full of marine mollusca; and these, again, are erelong replaced by sandstones, shales, and ironstones, charged with land-plants and fresh-water shells. To this curious blending of very different organic remains, I shall have occasion to refer more at large in a subsequent chapter. I mention it now as a sort of apology for the dryness of details which it is necessary to give, in order to complete our picture of the carboniferous fauna, and to understand the principles upon which the ancient history of the earth is deciphered.

Fig. 21.

Of the Pteropoda, we have, as yet, but one carboniferous genus, the conularia ([Fig. 21]). It was a slim delicate shell, in shape an oblong cone, having four sides, finely striated with a sort of zig-zag moulding like that of the Norman arch. Each of the four angles was traversed along its whole extent by a narrow gutter-like depression, and this style of fluting, combined with the markings on the sides, imparted no little elegance to the shell. The conularia is not a common fossil. It has been found among the coal-bearing strata of Coalbrook-Dale, and was noticed long ago by Dr. Ure in his History of Rutherglen.

The Brachiopoda are bivalve molluscs, but unlike most other molluscs they are rooted to one spot, and destitute of any power of locomotion. Their shells are unequal, the dorsal, or upper valve, being smaller and usually more bulged out than the under or ventral valve, which in most species is prolonged at its narrow end into a kind of beak. In the terebratula this beak has a little circular hole, from which there emerges a short peduncle or stalk, that fixes itself firmly to a rock or other substance at the sea-bottom, and serves the purpose of an anchor and cable to keep the little vessel safely moored. When the shells are detached, these perforated ventral valves have so exactly the form of the old Roman lamps, "that they were called Lampades, or lamp-shells, by the old naturalists."[36] Other species, as the lingulæ, have no beak, and the long peduncle passes out between the valves, which are of nearly equal size, and have been compared to the shape of a duck's bill. In yet another genus, the crania, there is no peduncle, but the animal adheres by its lower valve, much like the oyster, and may often be seen clustered in groups on decayed sea urchins or other organisms, particularly in the chalk formation.