第58章

The amount of fine earth brought to the surface under the form of castings, and afterwards transported by the winds accompanied by rain, or that which flows and rolls down an inclined surface, no doubt is small in the course of a few scores of years; for otherwise all the inequalities in our pasture fields would be smoothed within a much shorter period than appears to be the case.But the amount which is thus transported in the course of thousands of years cannot fail to be considerable and deserves attention.E.de Beaumont looks at the vegetable mould which everywhere covers the land as a fixed line, from which the amount of denudation may be measured. He ignores the continued formation of fresh mould by the disintegration of the underlying rocks and fragments of rock; and it is curious to find how much more philosophical were the views maintained long ago, by Playfair, who, in 1802, wrote, "In the permanence of a coat of vegetable mould on the surface of the earth, we have a demonstrative proof of the continued destruction of the rocks."

Ancient encampments and tumuli.--E.de Beaumont adduces the present state of many ancient encampments and tumuli and of old ploughed fields, as evidence that the surface of the land undergoes hardly any degradation.But it does not appear that he ever examined the thickness of the mould over different parts of such old remains.He relies chiefly on indirect, but apparently trustworthy, evidence that the slopes of the old embankments are the same as they originally were; and it is obvious that he could know nothing about their original heights.In KnolePark a mound had been thrown up behind the rifle-targets, which appeared to have been formed of earth originally supported by square blocks of turf.The sides sloped, as nearly as I could estimate them, at an angle of 45 degrees or 50 degrees with the horizon, and they were covered, especially on the northern side, with long coarse grass, beneath which many worm- castings were found.These had flowed bodily downwards, and others had rolled down as pellets.Hence it is certain that as long as a mound of this kind is tenanted by worms, its height will be continually lowered.The fine earth which flows or rolls down the sides of such a mound accumulates at its base in the form of a talus.A bed, even a very thin bed, of fine earth is eminently favourable for worms; so that a greater number of castings would tend to be ejected on a talus thus formed than elsewhere; and these would be partially washed away by every heavy shower and be spread over the adjoining level ground.The final result would be the lowering of the whole mound, whilst the inclination of the sides would not be greatly lessened.The same result would assuredly follow with ancient embankments and tumuli; except where they had been formed of gravel or of nearly pure sand, as such matter is unfavourable for worms.Many old fortifications and tumuli are believed to be at least 2000 years old; and we should bear in mind that in many places about one inch of mould is brought to the surface in 5 years or two inches in 10 years.Therefore in so long a period as 2000 years, a large amount of earth will have been repeatedly brought to the surface on most old embankments and tumuli, especially on the talus round their bases, and much of this earth will have been washed completely away.We may therefore conclude that all ancient mounds, when not formed of materials unfavourable to worms, will have been somewhat lowered in the course of centuries, although their inclinations may not have been greatly changed.

Fields formerly ploughed.--From a very remote period and in many countries, land has been ploughed, so that convex beds, called crowns or ridges, usually about 8 feet across and separated by furrows, have been thrown up.The furrows are directed so as to carry off the surface water.In my attempts to ascertain how long a time these crowns and furrows last, when ploughed land has been converted into pasture, obstacles of manykinds were encountered.It is rarely known when a field was last ploughed; and some fields which were thought to have been in pasture from time immemorial were afterwards discovered to have been ploughed only 50 or 60 years before.During the early part of the present century, when the price of corn was very high, land of all kinds seems to have been ploughed in Britain.There is, however, no reason to doubt that in many cases the old crowns and furrows have been preserved from a very ancient period. That they should have been preserved for very unequal lengths of time would naturally follow from the crowns, when first thrown up, having differed much in height in different districts, as is now the case with recently ploughed land.

In old pasture fields, the mould, wherever measurements were made, was found to be from 0.5 to 2 inches thicker in the furrows than on the crowns; but this would naturally follow from the finer earth having been washed from the crowns into the furrows before the land was well clothed with turf; and it is impossible to tell what part worms may have played in the work.Nevertheless from what we have seen, castings would certainly tend to flow and to be washed during heavy rain from the crowns into the furrows.But as soon as a bed of fine earth had by any means been accumulated in the furrows, it would be more favourable for worms than the other parts, and a greater number of castings would be thrown up here than elsewhere; and as the furrows on sloping land are usually directed so as to carry off the surface water, some of the finest earth would be washed from the castings which had been here ejected and be carried completely away.The result would be that the furrows would be filled up very slowly, while the crowns would be lowered perhaps still more slowly by the flowing and rolling of the castings down their gentle inclinations into the furrows.