Geological facts; or, the crust of the earth, what it is, and what are its uses

by W. G. Barrett

2. SKELETON RESTORED.

3. THE PTERODACTYLE.

CHAPTER X.
SECONDARY ROCKS.
No. 4. The Wealden.

“The works of the Lord are great, sought out of all them that have pleasure therein.”—David.

Lying immediately between the oolite and the chalk, is a small formation of fresh water, and not of marine deposit, to which the term Wealden has been given. This name has been given to it because it has been found developed chiefly in the Wealds, or Wolds,[^[98]] of Sussex, Surrey, and Kent; and to Dr. Mantell belongs the honour of investigating this singularly interesting formation, and of giving us its history, after months of patient research and laborious toil conducted on the spot, just as Hugh Miller has become the historian and explorer of the Old Red Sandstone of Cromarty. But although the Wealden is small when compared with the vast extent of some other formations, previously or subsequently added to the material of this “great globe’s” crust, it possesses unusual interest on account of the strange organic remains that are found in it, and of the evidence which these remains supply of vast changes in the conditions and characters of the living beings, found at that period roaming at large in the once tropical swamps of the Wealden.

The Wealden is almost if not wholly of fresh water origin; and a word or two on the formation of the deltas of great rivers, now going on in various parts of the world, will help us rightly to appreciate the character of this formation. It is seldom, some one remarks, that we “can catch a mountain in process of making,” and hence we have much difficulty in arriving at definite ideas concerning the times that the sedimentary rocks occupied in their deposit. But we can catch deltas in the process of manufacture,—the deltas of the Nile, the Mississippi, the Amazon, and other huge rivers that pour down, not their “golden streams,” but their muddy accumulations, gathered from the mountain sides whose slopes they wash into the great ocean basin waiting to receive them. “All the rivers run into the sea, yet the sea is not full.”

These deltas,—for we wish to explain as we go along, and would rather fall into the error of explaining where there is no necessity, than of leaving one word that might prove a stumbling-block unexplained,—these deltas are all the mouths, (les embouchères,) of oceanic rivers, that is, of rivers that pursue their impetuous course towards their native ocean bed; and the character of these deltas necessarily varies with the character of the coast through which they pursue the uneven “tenor of their way.” As the level of the sea is approached, the rapidity of the mountain stream is necessarily checked. No longer dashing down steep mountain precipices, as it did in the heyday of its youth, it finds itself grown into a majestic breadth and depth; and as it nears its maternal home, leaving the high land of its origin far behind, it traverses with slow and measured pace the slightly descending planes by which it falls into the ocean. This gradual process enables it to deposit on each side the alluvial soil, and decayed vegetable matter, and so on, that have been held by it in suspension during its rapid progress; and as the current slackens still more as it approaches its final destiny, portions of land gradually rise into view, the results of these deposits, which from their triangular shape have received the name of deltas, from their likeness to the Greek letter Δ.

Thus the mouths of the Ganges form a vast breadth of waters, with intervening islands and strips of land, 200 miles in width. Into this delta the river Ganges runs laden with the rich spoils of clay, sand, vegetable matter, &c., gathered from the Himalaya Mountains, and thus those dismal-looking islands have been formed in the mouth of this river, that have become the home of tigers, crocodiles, &c.

In North America, that “father of waters,” the Mississippi, sometimes called the Missouri, runs through a glorious valley of unexhausted and inexhaustible wealth of 3,000 miles, and runs out into the sea at least fifty miles, while its currenmiles further. “Like all other great rivers, the Mississippi does not empty itself into the sea in one continuous channel, but in a great variety of arms or mouths, which intersect in sluggish streams the great alluvial delta, which is formed by the perpetual deposit of the immense volume of water which rolls into the ocean. Between these mouths of the river a vast surface, half land, half water, from 50 to 100 miles in width, and 300 in length, fringes the whole coast; and there the enormous mass of vegetable matter constantly brought down by the Mississippi is periodically deposited. A few feet are sufficient to bring it above the level of the water, except in great floods; and as soon as that is done, vegetation springs up with the utmost rapidity in that prolific slime. No spectacle can be conceived so dreary and yet so interesting, as the prospect of these immense alluvial swamps in the course of formation. As far as the eye can reach, over hundreds of square leagues, nothing is to be seen but marshes bristling with roots, trunks, and branches of trees. In winter and spring, when the floods come down, they bring with them an incalculable quantity of these broken fragments, technically called logs, which not only cover the whole of this immense semi-marine territory, but, floating over it, strew the sea for several miles off to such an extent, that ships have often no small difficulty in making their way through them. Thus the whole ground is formed of a vast network of masses of wood, closely packed and rammed together to the depth of several fathoms, which are gradually cemented by fresh deposits, till the whole acquires by degrees a firm consistency. Aquatic birds, innumerable cranes and storks, water serpents, and huge alligators, people this dreary solitude. In a short time a kind of rank cane or reed springs up, which, by retarding the flow of the river, collects the mud of the next season, and so lends its share in the formation of the delta. Fresh logs, fresh mud, and new crops of cane or reed, go on for a series of years, in the course of which the alligators, in enormous multitudes, fix in their new domain, and extensive animal remains come to mingle with the vegetable deposits. Gradually, as the soil accumulates and hardens, a dwarfish shrub begins to appear above the surface; larger and larger trees succeed with the decay of their more stunted predecessors; and at length, on the scene of former desolation, the magnificent riches of the Louisianian forest are reared.”[^[99]]