The action of vegetation is not confined merely to imbibing the water falling from the clouds; it often, also, assists the superabundant moisture in penetrating the interior of the ground. Trees, after they have received the water upon their foliage, let it trickle down drop by drop on the gradually softened earth, and thus facilitate the gentle permeation of the moisture into the substratum; another part of the rain-water, running down the trunk and along the roots, at once finds its way to lower strata. On mountain slopes, the mosses and the freshly growing carpet of Alpine plants swell like sponges when they are watered with rain or melted snow, and retain the moisture in the interstices of their leaves and stalks until the vegetable mass is thoroughly saturated and the liquid surplus flows away. Peat-mosses especially absorb a very considerable quantity of water, and form great feeding reservoirs for the springs which gush out at a lower level. The immense fields of peat which cover hundreds and thousands of acres on the mountain slopes of Ireland and Scotland may, notwithstanding their elevation and inclined position, be considered as actual lacustrine basins containing millions of tons of water dispersed among their innumerable leaflets. The superabundant water of these tracts of peat-mosses issues forth in springs in the plains below. Rocks, like vegetable earth, also absorb water in greater or less quantities, according to their fissures and the density of their particles. If the soil is formed of volcanic scoriæ, or porous beds of pebbles, gravel, or sand, the water rapidly descends toward the underlying strata. Some of the harder rocks, especially certain kinds of granite, absorb but a very small quantity of water, on account of the small number of their clefts; others, on the contrary, as most of the calcareous masses, imbibe every drop of water which falls on their surface. There are some rocks which have their layers broken and cracked to such an extent that they resemble enormous walls of rubble-work; the rain instantly disappears on them as if it had fallen into a sieve. But the greater part of the calcareous rocks belonging to various geological periods are formed of thick and regular strata, cleft at intervals by long vertical crevices. Below the surface-beds, perhaps, are layers of soft marl, which the water penetrates with difficulty, although it can soften and carry away its particles. Here are formed, rill by rill, the subterranean rivulets which ultimately spread all over the substratum of marl, following the general slope of the bed. After a more or less considerable lapse of time, the stratum of marl ultimately becomes saturated, and the water then flows out through caverns which are variously modified by subsidences--faults in the strata and the perpetual action of the streams. The springs which proceed from calcareous rocks of this nature are in general the most abundant, owing to the length of their