The pretended exactness of statistical tables is too often little better than an imposture; and those founded not on direct estimation by competent observers, but on the report of persons who have no particular interest in knowing the truth, but often have a motive for distorting it, are commonly to be regarded as but vague guesses at the actual fact.] It is, indeed, not to be supposed that all the degradation of the mountains is due to the destruction of the forests--that the flanks of every Alpine valley in Central Europe below the snow-line were once covered with earth and green with woods, but there are not many particular cases in which we can, with certainty, or even with strong probability, affirm the contrary. Mountain Slides. Terrible as are the ravages of the torrent and the river-flood, the destruction of the woods exposes human life and industry to calamities even more appalling than those which I have yet described. The slide in the Notch of the White Mountains, by which the Willey family lost their lives, is an instance of the sort I refer to, though I am not able to say that in this particular case the slip of the earth and rock was produced by the denudation of the surface. It may have been occasioned by this cause, or by the construction of the road through the Notch, the excavations for which, perhaps, cut through the natural buttresses that supported the sloping strata above. Not to speak of the fall of earth when the roots which held it together, and the bed of leaves and mould which sheltered it both from disintegrating frost and from sudden drenching and dissolution by heavy showers, are gone, it is easy to see that, in a climate with severe winters, the removal of the forest, and, consequently, of the soil it had contributed to form, might cause the displacement and descent of great masses of rock. The woods, the vegetable mould, and the soil beneath, protect the rocks they cover from the direct action of heat and cold, and from the expansion and contraction which accompany them. Most rocks, while covered with earth, contain a considerable quantity of water. [Footnote: Rock is permeable by water to a greater extent than is generally supposed. Freshly quarried marble, and even granite, as well as most other stones, are sensibly heavier, as well as softer and more easily wrought, than after they are dried and hardened by air-seasoning. Many sandstones are porous enough to serve as filters for liquids, and much of that of Upper Egypt and Nubia hisses audibly when thrown into water, from the escape of the air forced out of it by hydrostatic pressure and the capillary attraction of the pores for water. Even the denser silicious stones are penetrable by fluids and the coloring matter they contain, to such an extent that agates and other forms of silex may be artificially stained through their substance. The colors of the stones cut at Oberstein are generally produced, or at least heightened, by art.