If all soils were absolutely impervious, there would be no springs, and the whole of the liquid mass furnished by rain and snow would flow away over the surface of the ground like the torrents and flood-waters of the mountains. The greater part, however, of the water which falls upon the ground sinks in the first place into the depths of the earth. There it becomes more or less perfectly purified from the foreign bodies with which it was charged, gradually rising to the temperature of the strata through which it passes, and becoming impregnated with the soluble salts which it meets with. Ultimately, when the water, in sinking down, encounters impervious beds, it can penetrate no further, and, flowing laterally to the outcrop of the beds, makes its escape in the form of springs. The absorption of the rain and melted snow takes place in various ways, according to the nature of the soil. Ordinary vegetable earth only allows the water to penetrate to a very slight depth, especially when the rain falls in showers and the slope of the ground is favorable for drainage. As mould is capable of absorbing a very large quantity--indeed, more than half its own weight, it prevents the strata beneath from receiving its due share of moisture, retaining almost the whole of it for the use of the vegetation which it nourishes. In fact, it requires an altogether exceptional rainfall to saturate any ordinary arable soil to the extent of a yard below the surface. Water passes with much more facility through sandy and gravelly beds; but compact loams and clay will not allow it to penetrate through them, retaining it in the form of pools or ponds on the surface of the ground.