I have already referred to the rapid denudation which is going on in these islands, and to the extent to which they have been denuded within comparatively recent times. The floors of caves, from their being cemented into a nearly homogeneous mass by stalagmitic matter, are much harder than the ordinary porous blown limestone; and it seems that in many cases, after the rocks forming the walls and roof have been removed, disintegration has been at all events temporarily arrested by the floor. Where there is a flat surface of rock exposed anywhere on the island, it very generally bears traces of having been at one time the floor of a cave; and as the weather-wearing of the surface goes on, the old concretionary structures are gradually brought out again, the parts specially hardened by a localized slow infiltration of lime resist integration longest and project above the general surface. Often a surface of weathered rock is so studded with these symmetrical concretions, that it is hard to believe that one is not looking at the calcified stumps of a close-growing grove of palms. [Illustration] THE BIG TREES OF CALIFORNIA (FROM STUDIES SCIENTIFIC AND SOCIAL.) BY ALFRED RUSSEL WALLACE. [Illustration] In the popular accounts of these trees it is usual to dwell only on the dimensions of the very largest known specimens, and sometimes even to exaggerate these. Even the smaller full-grown trees, however, are of grand dimensions, varying from fourteen to eighteen feet in diameter, at six feet above the ground, and keeping nearly the same thickness for perhaps a hundred feet. In the south Calaveras grove, where there are more than a thousand trees, the exquisite beauty of the trunks is well displayed by the numerous specimens in perfect health and vigor. The bark of these trees, seen at a little distance, is of a bright orange brown tint, delicately mottled with darker shades, and with a curious silky or plush-like gloss, which gives them a richness of color far beyond that of any other conifer. The tree which was cut down soon after the first discovery of the species, the stump of which is now covered with a pavilion, is twenty-five feet in diameter at six feet above the ground, but this is without the thick bark, which would bring it to twenty-seven feet when alive. A considerable portion of this tree still lies where it fell, and at one hundred and thirty feet from the base I found it to be still twelve and a half feet in diameter (or fourteen feet with the bark), while at the extremity of the last piece remaining, two hundred and fifteen feet from its base, it is six feet in diameter, or at least seven feet with the bark. The height of this tree when it was cut down is not recorded, but as one of the living trees is more than three hundred and sixty feet high, it is probable that this giant was not much short of four hundred feet. [Illustration: THE "MOTHER OF THE FOREST."]