OF THE EFFECTS OF RAIN—OF MARSHES, SUBTERRANEOUS WOOD, AND WATER.
We have already observed that rains, and the currents of water they produce, continually detach from the heights of mountains sand, earth, gravel, &c. which they carry into plains, from whence the rivers convey a part of them into the sea. Plains therefore are successively filled, and by degrees raised higher, while mountains daily diminish. Joseph Blancanus relates various facts on this subject, which were of public notoriety in his time, and which prove that mountains have been considerably lowered. In the county of Derby, in England, the steeple of the village Craich was not visible in 1572 from a certain mountain, on account of the height of another which intervened; in eighty or an hundred years after, not only this steeple but every part of the church became visible from that very spot. Dr. Plot gives a similar example of a mountain between Sibbertoft and Ashby, in Northamptonshire. The rain waters not only carry with them the lightest parts of the mountains, as earth, gravel, and small stones, but even undermine and roll down large rocks, which considerably diminish the height of them. The mountains of Wales are very steep and high, and the fragments of these rocks are to be seen in large pieces at their feet, which as well as all fragments of rocks met with in vallies are the works of frosts and water. It is not mountains of sand and earth alone which the rain causes to sink, for they attack the hardest rocks, and carry with them large fragments into the vallies. In a valley in Nant-phrancon, in 1685, a part of a large rock, which rested on a narrow base, having been undermined by the waters, fell and broke in many pieces, the largest of which, in descending, tore up a considerable trench in the plain, and crossed a small river on the other side of which it stopped. It is to similar accidents we must attribute the origin of all the large stones found adjacent to the mountains. We must recollect, as before observed, that these large stones, scattered abroad, are more common in countries whose mountains are composed of sand and free-stone, than in those where their composition is marble and clay, because sand is a less solid foundation than clay.
To give an idea of the quantity of earth which the rain detaches from mountains and carries into the vallies, we shall quote a circumstance related by Dr. Plot; he says, in his Natural History of Staffordshire, that 18 feet deep in the earth a great number of pieces of money had been found, coined in the reign of Edward IV. two hundred years before his time, from which he concludes that the ground which is marshy has increased above a foot in eleven years, or an inch and a twelfth every year. A similar observation occurs with respect to some trees buried seventeen feet deep from the surface under which medals of Julius Cæsar were found; so the earth, brought from the top of mountains by the waters, considerably increases the elevation of the ground of plains.
This gravel, sand, and earth which the waters from mountains convey into plains form strata, which must not be confounded with the ancient and original strata of the globe. In the former class must be placed those of soft stone, gravel, and sand, the grains of which are washed and rounded; to these may be added, the strata of stones which are formed by a kind of incrustation; neither of which owe their origin to the motion or sediments of the sea. In these strata of soft and imperfect stones are found an infinity of vegetables, leaves, land and river shells, and small bones of terrestrial animals, but never sea shells, or other marine productions; which evidently proves, together with their want of solidity, that these strata are formed on the surface of the dry land; and that they are more modern than those of marble and other stones which contain shells, and were originally formed by the sea. All these modern stones appear to be hard and solid when they are first hewn out, but when, exposed to the weather, the air and rain presently dissolve them; their substance is so different from true stone, that when reduced into minute parts, to make sand of them, they are converted into a kind of earth or clay. Stalactites, and other stony concretions, which Tournefort took for vegetated marble, are not real stones, no more than those formed by the incrustations. We have already shewn that tufa is not of ancient formation, and must not be ranked in the class of stones. Tufa is an imperfect matter, differing from stone or earth, but which derives its origin from both by the means of rain water, as incrustations derive theirs from the deposit of the water of certain springs; therefore the strata of these matters are not ancient, nor been formed like the rest, by the sediments of the sea. The strata of turf are also modern, and have been produced by the successive assemblage of leaves and other perishable vegetables, and which are only preserved by a bitumous earth. Among these modern strata we never meet with any marine production; but, on the contrary, many vegetables, bones of land animals, and land and river shells, as may be seen in the meadows, near Ashby, in the county of Northampton, where a great number of snail shells, plants, herbs, and many river shells are found all in good preservation, some feet deep in the earth, but not a single marine shell among them.[AM]
[AM] See Philosophical Trans. Abridg. XI. page 271.
These new strata have been formed by the water which runs on the surface of the earth, often changing situation and dispersing on every side. Part of these waters penetrate internally and flow across the clefts of rocks and stones; and the reason we meet with no water in high lands, no more than on the tops of hills, is, because all elevations are generally composed of stone and rocks, therefore to find water we must dig through the rock till we come to clay, or firm earth, on which these rocks stand, and we shall not meet with any water until the stone is pierced to the bottom: therefore, when the thickness of the rock, which must be pierced, is very considerable, as in lofty mountains, where they are often upwards of 1000 feet in height, it is impossible to dig to their base, and of course to have any water. There are even large parts of land where there is not any water, as in Arabia Petrea, which is a desert where no rain ever falls, where the scorching sand covers the whole surface of the earth, where there is scarcely any vegetable soil, where the few plants found are sickly, and where springs and wells are so very scarce that only five are reckoned between Cairo and Mount Sinai, and the water of them is salt and bitter.
When the waters on the surface cannot find vent to flow they form marshes and fens. The most famous marshes in Europe are those of Muscovy, at the source of the Tanais; and those of Savolaxia and Enasak, in Finland; there are also some in Holland, Westphalia, and other low countries: in Asia are the marshes of the Euphrates, of Tartary, and of the Palus Meotis; nevertheless there are fewer of them in Asia and Africa than in Europe but America may be said to be but one continued marsh throughout all its plains, which is a greater proof of the modern date of the country, and of the small number of inhabitants than of their want of industry.
There are very great bogs in England, especially near the sea in Lincolnshire, which has lost much ground on one side and gained it on the other. In the ancient ground a great number of trees are found buried below the new ground, which has been deposited there by the water. The same also are met with in Scotland, particularly at the mouth of the river Ness. Near Bruges, in Flanders, in digging to the depth of 40 or 50 feet, a great quantity of trees were found, as close to each other as in a forest: the trunks, branches, and leaves, were so well preserved that the different kinds were easily distinguished. About 500 years since, the land where these trees were found was covered by the sea, and before that time there is no trace or tradition that it ever existed; nevertheless it must have been so, when these trees stood and vegetated; therefore this ground, which formerly was covered with wood, has been overwhelmed by the sea, the waters of which has, by degrees, deposited there between 40 and 50 feet of earth upon the former surface, and then retired. A number of subterraneous trees have been also found at Youle, in Yorkshire, 12 miles below the town, near the river Humber; there are some large enough for building; and it is said, perhaps improperly, that this wood is as durable as oak. The people cut them into long thin slips, and sell them in the neighbouring towns, where they are used for lighting of pipes. All these trees appear broken, and the trunks are separated from the roots as if they had suffered the violence of a hurricane, or an inundation. This wood greatly resembles willow, it has the same smell when burnt, and makes charcoal exactly like it.[AN] In the Isle of Man there is a marsh six miles long by three broad, it is called Curragh; subterraneous trees, like willows, are found there, and although they are 18 or 20 feet high, they are, nevertheless, firm on their roots.[AO] Trees are met with in almost every morass, bog, and marsh, in Somerset, Chester, Lancaster, and Stafford. There are some places where trees are found under the earth, which have been cut, sawed, squared, and worked by the labour of man; and even wedges and saws have likewise been found by them. Between Birmingham and Bromley, in the county of Lincoln, there are lofty hills of fine light sand, which the rain and wind sweep away, leaving uncovered the roots of large willow trees, on which the impression of the axe is exceedingly plain. These hills, without doubt, have been formed like downs, by the accumulation of sand, which the waters of the sea has brought there and deposited at different periods. A great number of these subterraneous trees are also found in the marshy lands of Holland, Friesland, and near Groningen, from whence the turfs which they burn are dug.