The World Before the Deluge

lively colour. It is the stone of which the portico of St. Paul's and many of the houses of London, built in Queen Anne's time, were constructed. The building-stone contains fossils exclusively marine. Upon this stratum rests a bed of limestone formed in lacustrine waters. Finally, upon this bed rests another deposit of a substance which consists of very well-preserved vegetable earth or humiis^ quite analogous to our vegetable soil, of the thickness, of from fifteen to eighteen inches, and of a blackish colour; it contains a strong proportion of carbonaceous earth; it abounds in the silicified remains of Conifers and other plants, analogous to the Zamia and Cycas — this soil is known as the " dirt-bed." The trunks of great numbers of silicified trees and tropical plants are found here erect, their roots fixed in the soil, and of species differing from any of our forest trees.

*' The ruins of a forest upon the ruins of a sea," says Esquiros, *•' the tnmks of these trees were petrified while still growing. The region now occupied by the narrow channel and its environs had been at first a sea, in whose bed the Oolitic deposits which now fomi the Portland stone accumulated: the bed of the sea gradually rose and emerged from the waves. Upon the land thus rescued from the deep, plants began to grow; they now constitute with their ruins the soil of the dirt-bed. This soil, with its forest of trees, was aftenvards plunged again into the waters — not the bitter waters of the ocean, but in the fresh waters of a lake fonned at the mouth of some great river."

Time passed on, however; a calcareous sediment brought from the interior by the waters, formed a layer of mud over the dirt-bed; finally, the whole region was covered by a succession of calcareous deposits, until the day when the Isle of Portland was again revealed to light. " From the facts observed," says Lyell, " we may infer:— * For details respecting these strata the reader may consult, with advantage, the useful handbook to the geology of Weymouth and Portland, by Robert Damon. t See Bristovv and Whitaker "On the Chesil Bank," Geol. Mag.^ vol. vi., p. 433.

I. That those beds of the Upper Oolite, called the Portland, which are full of marine shells, were overspread with fluviatile mud, which became dry land, and covered with a forest, throughout a portion of space now occupied by the south of England, the climate being such as to admit of the growth of the Zamia and Cycas. 2. This land at length sank down and was submerged with its forest beneath a body of fresh water from which sediment was thrown down enveloping fluviatile shells. 3. The regular and uniform preservation of this thin bed of black earth over a distance of many miles, shows that the change from dry land to the state of a fresh-water lake, or estuary, was not accompanied by any violent denudation or rush of water, since the loose black earth, together with the trees which lay prostrate on its surface, must inevitably have been swept away had any such violent catastrophe taken place. " *

The soil known as the dlrt-bcd is nearly horizontal in the Isle of Portland; but we discover it again not far from there in the sea-cliffs of the Isle of Purbeck, having an inclination of 45°, where a b the trunks continue perfectly parallel among themselves, affording a fine example of a change in the position of beds originally horizontal.

Fig. 124 represents this species of geological Jmmns. "Each dirt-bed^' says Sir Charles Lyell, "may, no doubt, be the memorial of

Fig. 124. — Geological humus, a. Fresh-water cal-many thousand years or cen- careous slate (Purbeck); b. Dirt-bed, with roots and* turies, because we find that steins of trees; c. Fresh-water beds; d, Portland Stone. tvvo or three feet of vegetable soil is the only monument which many a tropical forest has left of its existence ever since the ground on which it now stands was first covered with its shade. "t

This bed of vegetable soil is, then, near the summit of that long and complicated series of beds which constitute the Jurassic period; these ruins, still vegetable, remind us forcibly of the coal-beds, for they are nothing else than a less advanced state of that kind of vegetable fossilisation which was perfected on such an immense scale, and during an infinite length of time in the coal period.

t Ibid, p. 391. " Elements of Geology," p. 389.

The Purbeck beds, which are sometimes subdivided into Lower,; Middle, and Upper, are mostly fresh-water formations, intimately-connected with the Upper Portland beds. But there they begin and end, being scarcely recognisable except in Dorsetshire, in the sea-cliffs of which they were first studied. They are finely exposed in Durdlestone Bay, near Swanage, and at Lulworth Cove, on the same coast. The lower beds consist of a purely fresh-water marl, eighty feet thick, containing shells of Cypris^ LivincEa, and some Serptilce in a bed of marl of brackish -water origin, and some Q^m-bearing shales, strangely broken up at the west end of the Isle of Purbeck.

The Middle series consis ts of twelve feet of marine strata known as

the " cinder-beds," formed of a vast accumulation of Ostrea disforla, resting on fresh-water strata full of Cypris fascicnlata, Fla?wrlns, and Limncea, by which this strata has been identified as far inland as the vale of Wardour in Wiltshire. Above the cinder-beds are shales and limestones, partly of fresh-water and partly of brackish-water origin, in which are Fishes, many species of Lepidotus, and the crocodilian reptile, Macrorhyiichus. On this rests a purely marine deposit, with Pecten, Avicula, &c. Above, again, are brackish beds with Cy^'ena, overlying which is thirty feet of fresh-water limestone, with Fishes, Turtles, and Cyprides. The tipper beds are purely fresh-water strata, about fifty feet thick, containing Paludiiia^ Pkysa, Liumcea, all very abundant. In these beds the Purbeck marble, formerly much used in the ornamental architecture of the old English cathedrals, was formerly quarried. (See Note, page 274.)

A few words may be added, in explanation of the term oolite, applied to this sub-period of the Jurassic formation. In a great number of rocks of this series the elements are neither crystalline nor amorphous — they are, as we have already said, oolitic; that is to say, the mass has the form of the roe of certain fishes. The question naturally enough arises. Whence this singular oolitic structure assumed by the components of certain rocks? It is asserted that the grinding action of the sea acting upon the precipitated limestone produces rounded forms analogous to grains of sand. This hypothesis may be well founded in some cases. The marine sediments which are deposited in some of the warm bays of Teneriffe are found to take the spheroidal granulated form of the oolite. But these local facts cannot be made to apply to the whole extent of the oolitic formations.- We must, therefore, look further for an explanation of the phenomena.

It is admitted that if the cascades of Tivoli, for example, can give birth to the oolitic grains, the same thing happens in the quietest basins, that in stalactite-caverns oolitic grains develop themselves, which afterwards, becoming cemented together from the continued, but very slow, affluence of the calcareous waters, give rise to certain kinds of oolitic rocks. On the other hand, it is known that nodules, more or less large, develop themselves in marls in consequence of the concentration of the calcareous elements, without the possibility of any wearing action of water. Now, as there exists every gradation of size between the smallest oolitic grains and the largest concretions, it is reasonable to suppose that the oolites are equally the product of concentration.

Finally, from research to research, it is found that perfectly constituted oolites — that is to say, concentric layers, as in the Jurassic limestone — develop themselves in vegetable earth in places where the effects of water in motion is not more admissible than in the preceding instances. Thus we arrive at the conclusion, that if Nature sometimes forms crystals with perfect terminations in magmas in the course of solidification, she gives rise also to spheroidal forms surrounding various centres, which sometimes originate spontaneously, and in other cases are accumulated round the debris of fossils, or even mere grains of sand. Nevertheless, all mineral substances are not alike calculated to produce oolitic rocks; putting aside some particular cases, this property is confined to limestone and oxide of iron.

With regard to the distribution of the Jurassic formation on the terrestrial globe, it may be stated that the Cotteswold Hills in England, and in France the Jura mountains, are almost entirely composed of these rocks, the several series of beds being all represented in them — this circumstance, in fact, induced Von Humboldt to name the formation after this latter range. The Upper Lias also exists in the Pyrenees and in the Alps; in Spain; in many parts of Northern Italy; in Russia, especially in the government of Moscow, and in the Crimea; but it is in Germany where it occupies the most im-i portant place. A thin bed of oolitic limestone presents, at Solenhofen in Bavaria, a geological repository of great celebrity, containing fossil Plants, Fishes, Insects, Crustaceans, with some Pterodactyles, admirably preserved; it yielded also some of the earliest of the feathered race. The fine quarries of lithographic stone at Pappenheim, so celebrated all over Europe, belong to the Jurassic formation.

It has recently been announced that these rocks have been found in India; they contribute largely to the formation of the main mass of the Himalayas, and to the chain of the Andes in South America;

finally, from recent investigations, they seem "to be present in NewZealand.

In England the Lias constitutes a well-defined belt about thirty miles broad, extending from Dorsetshire, in the south, to Yorkshire, in the north, formed of alternate beds of clay, shales, and limestone (with layers of jet), on the coast near Whitby. It is rich, as we have seen, in ancient life, and that in the strongest forms imaginable. From the unequal hardness of the rocks it comprises, it stands out boldly in some of the minor ranges of hills, adding greatly to the picturesque beauty of the scenery in the centre of the country. In Scotland the formation occupies a very limited space.

A map of the country at the close of the Jurassic period would probably show double the extent of dry land in the British Islands, compared with what it displayed as an island in the primordial ocean j but Devon and Cornwall had long risen from the sea, and it is probable that the Jurassic beds of Dorsetshire and France were connected by a tongue of land running from Cherbourg to the Liassic beds of Dorsetshire, and that Boulogne, still an island, was similarly connected with the Weald.

125. — Crioceras Duvallii, Sowerby. A non-involuted Ammonite. (Neocomian.)

Note. — Sections of the Purbeck strata of Dorsetshire have been constructed by Mr. Bristow, from actual measurement, in the several localities in the Isle of Purbeck, where they are most clearly and instructively displayed.

These sections, published by the Geological Survey, show in detail the beds in their regular and natural order of succession, with the thickness, mineral character, and contents, as well as the fossils, of each separate bed.