OCR Text |
Show 308 CONCLUSION. CHAP. VIT. of a year, a horizontal line oue yard iu length~ so that 240 cubic inches would cross a line 100 yards in leng·th. This latter amount in a damp state would weigh 11~ pounds. Thu · a considerable weight of earth is continually moving down each side of every valley, and will in time reach its bed. Finally this eartl1 will be transported by the streams flowing in the valleys into the ocean, the great receptacle for all n1atter denuded from the land. It is known from the amount of sediment annually delivered into the sea by the Mississippi, that its enormous drainage-area must on an average be lowered ·00263 of an inch each year ; and tlJi would suffice in four and half million years to lower the whole drainage-area to tho level of the sea-shore. So that, if a small fraction of the layer of fine earth, ·2 of an inch in thickness, which is annually brought to the surface by worms, is carried away, a great result cannot fail to be produced within a period which no geologist considers extremely long. ..Archreologists ought to be grateful to worms, as they protect and preserve for an CiiAP. vn. CONCLUSION. 309 indefinitely long periocl every object, not liable to decay, which is dropped on the surface of tlte land, by burying it beneath their castings. Thus, also, many elegant and curious tesselated pavements and other ancient remains have been preserved ; though no doubt the worms have in these cases been largely aided by earth washed and blown from the adjoining lan !, especially when cultivated. The old tesselated pavements have, however, often sufferecl by having subsided unequally from being unequally undermined by the worms. Even olcl massive walls may be undermined and subside ; and no building is in this respect safe, unless the foundations lie 6 or 7 feet beneath the surface, at a depth at which worms cannot work. It is probable that many monoliths and some old walls have fallen down from having been undermined by worms. vVorms prepare the ground in an excellent manner for the growth of fibrous-rooted plants and for seedlings of all kinds. They periodically expose the mould to the air, and sift it so that no stones larger than the par- |