Part 36 (1/2)

7. Marine Portland stone.

The annexed tabular view will enable the reader to take in at a glance the successive changes from sea to river, and from river to sea, or from these again to a state of land, which have occurred in this part of England between the Cretaceous and Oolitic periods. That there have been at least four changes in the species of testacea during the deposition of the Wealden, seems to follow from the observations recently made by Professor E. Forbes, so that, should we hereafter find the signs of many more alternate occupations of the same area by different elements, it is no more than we might expect. Even during a small part of a zoological period, not sufficient to allow time for many species to die out, we find that the same area has been laid dry, and then submerged, and then again laid dry, as in the deltas of the Po and Ganges, the history of which has been brought to light by Artesian borings.[235-A] We also know that similar revolutions have occurred within the present century (1819) in the delta of the Indus in Cutch[235-B], where land has been laid permanently under the waters both of the river and sea, without its soil or shrubs having been swept away.

Even, independently of any vertical movements of the ground, we see in the princ.i.p.al deltas, such as that of the Mississippi, that the sea extends its salt waters annually for many months over considerable s.p.a.ces, which, at other seasons, are occupied by the river during its inundations.

It will be observed that the division of the Purbecks into upper, middle, and lower, has been made by Professor E. Forbes, strictly on the principle of the entire distinctness of the species of organic remains which they include. The lines of demarcation are not lines of disturbance, nor indicated by any striking physical characters or mineral changes. The features which attract the eye in the Purbecks, such as the dirt-beds, the dislocated strata at Lulworth, and the Cinder-bed, do not indicate any breaks in the distribution of organized beings. ”The causes which led to a complete change of life three times during the deposition of the freshwater and brackish strata must,” says this naturalist, ”be sought for, not simply in either a rapid or a sudden change of their area into land or sea, but in the great lapse of time which intervened between the epochs of deposition at certain periods during their formation.”

Each dirt-bed may, no doubt, be the memorial of many thousand years or centuries, because we find that 2 or 3 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. Yet, even if we imagined the fossil soils of the Lower Purbeck to represent as many ages, we need not expect on that account to find them const.i.tuting the lines of separation between successive strata characterized by different zoological types. The preservation of a layer of vegetable soil, when in the act of being submerged, must be regarded as a rare exception to a general rule. It is of so perishable a nature, that it must usually be carried away by the denuding waves or currents of the sea or by a river; and many dirt-beds were probably formed in succession, and annihilated in the Wealden, besides those few which now remain.

[Ill.u.s.tration: Fig. 244. Cone from the Isle of Purbeck, resembling the _Dammara_ of the Moluccas. (Fitton.)]

The plants of the Wealden, so far as our knowledge extends at present, consist chiefly of Ferns, Coniferae (see fig. 244.), and Cycadeae, without any exogens; the whole more allied to the Oolitic than to the Cretaceous vegetation, although some of the species seem to be common to the chalk.

But the vertebrate and invertebrate animals indicate, in like manner, a relations.h.i.+p to both these periods, though a nearer affinity to the Oolitic. Mr. Brodie has found the remains of beetles and several insects of the h.o.m.opterous and trichopterous orders, some of which now live on plants, like those of the Wealden, while others hover over the surface of our present rivers. But no bones of mammalia have been met with among those of land-reptiles. Yet, as the reader will learn, in Chapter XX., that the relics of marsupial quadrupeds have been detected in still older beds, and, as it was so long before a single portion of the jaw of an iguanodon was met with in the Tilgate quarries (see p. 228.), we need by no means despair of discovering hereafter some evidence of the existence of warm-blooded quadrupeds at this era. It is, at least, too soon to infer, on mere negative evidence, that the mammalia were foreign to this fauna.

In regard to the geographical extent of the Wealden, it cannot be accurately laid down; because so much of it is concealed beneath the newer marine formations. It has been traced about 200 English miles from west to east, from Lulworth Cove to near Boulogne, in France; and about 220 miles from north-west to south-east, from Whitchurch, in Buckinghams.h.i.+re, to Beauvais, in France. If the formation be continuous throughout this s.p.a.ce, which is very doubtful, it does not follow that the whole was contemporaneous; because, in all likelihood, the physical geography of the region underwent frequent change throughout the whole period, and the estuary may have altered its form, and even s.h.i.+fted its place. Dr. Dunker, of Ca.s.sel, and H. Von Meyer, in an excellent monograph on the Wealdens of Hanover and Westphalia, have shown that they correspond so closely, not only in their fossils, but also in their mineral characters, with the English series, that we can scarcely hesitate to refer the whole to one great delta. Even then, the magnitude of the deposit may not exceed that of many modern rivers. Thus, the delta of the Quorra or Niger, in Africa, stretches into the interior for more than 170 miles, and occupies, it is supposed, a s.p.a.ce of more than 300 miles along the coast, thus forming a surface of more than 25,000 square miles, or equal to about one half of England.[237-A] Besides, we know not, in such cases, how far the fluviatile sediment and organic remains of the river and the land may be carried out from the coast, and spread over the bed of the sea. I have shown, when treating of the Mississippi, that a more ancient delta, including species of sh.e.l.ls, such as now inhabit Louisiana, has been upraised, and made to occupy a wide geographical area, while a newer delta is forming[237-B]; and the possibility of such movements, and their effects, must not be lost sight of when we speculate on the origin of the Wealden.

If it be asked where the continent was placed from the ruins of which the Wealden strata were derived, and by the drainage of which a great river was fed, we are half tempted to speculate on the former existence of the Atlantis of Plato. The story of the submergence of an ancient continent, however fabulous in history, must have been true again and again as a geological event.

The real difficulty consists in the persistence of a large hydrographical basin, from whence a great body of fresh water was poured into the sea, precisely at a period when the neighbouring area of the Wealden was gradually going downwards 1000 feet or more perpendicularly. If the adjoining land partic.i.p.ated in the movement, how could it escape being submerged, or how could it retain its size and alt.i.tude so as to continue to be the source of such an inexhaustible supply of fresh water and sediment? In answer to this question, we are fairly ent.i.tled to suggest that the neighbouring land may have been stationary, or may even have undergone a contemporaneous slow upheaval. There may have been an ascending movement in one region, and a descending one in a contiguous parallel zone of country; just as the northern part of Scandinavia is now rising, while the middle portion (that south of Stockholm) is unmoved, and the southern extremity in Scania is sinking, or at least has sunk within the historical period.[237-C] We must, nevertheless, conclude, if we adopt the above hypothesis, that the depression of the land became general throughout a large part of Europe at the close of the Wealden period, a subsidence which brought in the cretaceous ocean.

FOOTNOTES:

[227-A] Dr. Fitton, Geol. Trans. vol. iv. p. 320. Second Series.

[230-A] Mantell, Geol. of S. E. of England, p. 244.

[231-A] ”On the Dorsets.h.i.+re Purbecks,” by Prof. E. Forbes, Edinb. Brit.

a.s.soc., Aug. 1850.

[233-A] Mr. Webster first noticed the erect position of the trees and described the Dirt-bed.

[233-B] Fitton, Geol. Trans., Second Series, vol. iv. pp. 220, 221.

[233-C] See Flinders' Voyage.

[233-D] Fitton, ibid.

[233-E] Buckland and De la Beche, Geol. Trans., Second Series, vol. iv.

p. 16. Mr. Forbes has ascertained that the subjacent rock is a freshwater limestone, and not a portion of the Portland oolite, as was previously imagined.

[234-A] E. Forbes, ibid.

[235-A] See Principles of Geol., 8th ed. pp. 260-268.

[235-B] Ibid. p. 443.

[237-A] Fitton, Geol. of Hastings, p. 58.; who cites Lander's Travels.

[237-B] See above, p. 85.; and Second Visit to the U. S. vol. ii.