The history of our globe during the deposition of the Chalk Formation, and the changes therewith connected, have now to be considered. By whatever causes effected—from whatever sources derived the materials—the line of demarkation is here complete. The sands of Africa, suddenly converted into or drifted over by the snows of the Alps, would not present a greater diversity of outline than is the transition to the geologist, when for the first time he steps into a chalk district, and marks the obvious contrast with all the surrounding scenery. From Gloucester into Wiltshire we pass, as it were, into a new zone of latitude.
The details contained in the two last chapters are in strict conformity with the laws of nature: the animals connected with the epoch possessed functions of life and an external adaptation of things suited to each other. Similar arrangements exist at present under nearly similar circumstances: the tropical animals bear a close affinity to the extinct races; and show that, however nature may contrive to display her exhaustless powers of invention in her forms of living creatures, she still conforms to a type, and has her limits of divergence. The past and the present so agree in all essential points as clearly to demonstrate a wise and controlling agency, a measure of enjoyment combined with an adjustment of figure, which, though approaching the marvelous, has resulted from design and the profoundest intelligence. Accordingly, the symptoms of the change we now witness are cotemporaneous. With the cretaceous system are introduced new mineral conditions, and along with these there are new forms of existence.
The animals of the chalk and oolite periods are essentially different—still generically shading, indeed, into each other—but so differing in species, and the appearance for the first time of new creations, as again to announce to us that we tread on sacred ground, and witness in its arrangement and contents the direct agency of Omnipotence. We can form no opinion, no notion whatever, in these changes, of the modus operandi. We remark simply the effects; and science, amidst all its otherwise barren and useless details, then achieves its loftiest purposes when it thus traces the footsteps and actings of the Great First Cause.
1. Range and Structure of the Chalk Formation. Considered mineralogically, this rock can never fail to arrest attention or inquiries, even among the least observant, as to its nature and origin. There is no trace of it in the northern portion of the island; and when one for the first time sees whole mountains of it, his sensations are not a little exciting. For our own part, we felt as if we entered a new world when we gazed upon hills, and their long-furrowed escarpments, of this calcareous snow-drift. Our acquaintance with the mineral had hitherto been limited to the fragments with which we were wont to trace the lines of our schoolboy pastimes. We got no deeper into its mysteries when, on a higher scale of action, we saw it delineate the diagram, or run over the fluxional problem on the black-board in academy and college-hall. But here! and half an island is covered with these stores of knowledge and of industry. Nine or ten counties on a stretch, from Dorset to Flamborough Head, and from Bridport to Deal, are covered over, and for hundreds of feet in depth, with the milk-white earth; and, whichever way you turn and bend toward the Capital, there are ample opportunities for the study of this curious page of geological history.
The chalk beds are not composed of one uniform compact mass of the useful mineral itself, which consists of nearly pure carbonate of lime, of soft earthy texture. Geologically considered, the cretaceous system comprises a series of green and ferruginous sands, clays, marls, gray and white limestones; and these again are arranged under three leading groups—chalk, gault, and green sand. The chalk—properly so called—is subdivided into the upper and lower,—containing in the upper numerous veins and nodules of flint, and varies in color through several intermediate hues, until, in its contact with trap, it assumes a deep red. The gault is an argillaceous deposit of stiff, dark blue clays, highly calcareous, and effervesces freely with acids. The green sand is a triple alternation of sands, cherty limestone, and friable sandstone, with beds, in some places of ocher and Fuller’s earth. The whole series may be estimated at nearly two thousand feet in thickness, formed in a deep sea basin, the materials floating in very still waters, and aggregated successively through the combined influence of mechanical, chemical, and organic agencies.
The mechanical influence is very apparent in the sands and marls, which are evidently the spoils of islands and continents, washed down by currents and floods of fresh water, and deposited over an ancient ocean bed. The chemical composition of the flints or concretionary nodules, which give such a remarkable character and appearance often to the chalk, is equally demonstrable; from fifty to a hundred beds of chalk, pure and beautifully white, will sometimes be seen alternating with as many bands of dark-colored flints, all regularly arranged as cannon-shot of all sizes on a floor, and presenting, for miles along the cliffs of the sea-shore, lines of beautifully defined fortifications. The organic agent is visible in the nucleus of these round masses, which consists of an animal or vegetable substance, as a coral, a shell, a piece of flustra, or sponge. The nodules assume various shapes, that seem to be molded according to the cavities of the matrix in which they are imbedded, but are actually the forms of the bodies or organic substances to which they are attached. The explanation given is, that a chemical attraction has taken place between the vegetable or animal remains, strewed abundantly through the waters, and the silicious matter held in solution. The silex in solution gradually incrusts, or incorporates with, the organized substance,—and thus were produced at once the flinty concretions and the wonderful petrifactions contained in them. Break any one of these nodular masses, and minute drops of moisture will, if immediately inspected, be seen to ooze out from its pores: thus clearly furnishing a proof of the state of solution in which it originally existed, and the watery menstruum in which it was produced.
The mineralogical history and arrangement of the chalk group of rocks are therefore in many respects very interesting. The chalk overlies the wealden, which was a mere delta at the river’s mouth. The bed of the river suddenly disappears, and now there rests upon it a deep sea formation. How stupendous and overwhelming the forces of nature through all her operations! How vast her affluence and prodigality, which could so thoroughly alter all her exterior and interior arrangements, and fill the seas with this new matter.
II. The Organic Remains display the boundless profusion of animal life which prevailed during the cretaceous period. The wealden furnishes no grounds of comparison, as that is simply a local fresh water deposit, and consequently can furnish no test of the general condition of life upon the surface of the globe. But when we go back to the oolitic period we obtain a standard by which to measure the doings of nature in the interval, what new creations started into being, and what provisions were made for their subsistence. The state of the temperature cannot be determined, as the products, with the rarest possible exceptions, are wholly marine, and therefore affected by atmospheric influences in a very small degree. Neither can much be conjectured concerning the state of the land, as scarcely a fragment of true terrestrial life has been detected in the deposit; and yet, from the stillness and comparatively small dimensions of sea-basins into which the earthy ingredients were floated, the probability is that the land was both lofty and widely extended. One mammalian, and the remains of a solitary bird, and a meager sprinkling of vegetables, constitute the whole, and even dubious, amount of contributions from this department of nature. To Neptune, therefore, the palæontologist turns his undivided attention; and, comparing one period with another, he finds the following results:
The cretaceous deposits all lie within the area of the oolites. They are conformable generally in position, and display, in proportion to their extent, a like superabundance of calcareous earth. Hence a return to polyp and shelly types of life, which we find so characteristic and diversified in both epochs.
Thus of the first order, Amorphozoa, the oolitic age produced only one genus; in the cretaceous we find thirteen genera, in the list Spongia, which is common to both. Of Zoophytes, there are twenty-three genera in the former, and seventeen in the latter—of which nine are common to both periods. The Echinodermata number eighteen in the oolite, and twenty-five in the chalk—five only common. The genus Foraminifera is entirely new in the latter formation, and consists of twelve ascertained genera, and nearly double the number of species. Of Annelida there are four genera in the oolite, and six in the chalk, in which the new order of Cirrhipeda occurs likewise. The Astacus is the only crustacean in the oolitic group: this and three new genera are found in the chalk. The Conchifera are very numerous in both deposits; forty-six in the older, and thirty-eight genera in the newer, of which eight are peculiar to the chalk. Monymaria are nearly in the same relative proportions. Rudistes occurs, as a new order, for the first time in the chalk, while again the Brachiopods, Gasteropods, and Cephalopods, are about equally abundant in both formations, with additions in the chalk to the generic models. And here too the new order Pteropoda, of a single genus and species, is introduced to our contemplation. Ammonites and Belemnites do not pass this age.