ing eventually at the watershed, and cutting through it so as to form apparently but a single valley, though on either side of the highest portion of its bottom, the waters flowed in opposite directions.
The mention of springs recalls another denuding agent, which has been already discussed in connection with caverns, and seems to have assisted in moulding the surface of the country and in excavating the valleys. It is well known that the water flowing in the streams of a chalk-country contains, in solution, a considerable amount of chalk, or rather, of bi-carbonate of lime; the water on entering the ground deriving a certain amount of carbonic acid from the decaying vegetable matter contained in the soil, and when thus charged, becoming capable of dissolving a corresponding quantity of the chalk. The amount is usually 17 or 18 grains in the gallon; and even in the Thames at London, not a purely chalk - stream, there are about 14 grains. Taking the proportion of 17 grains to the gallon, it will be found by calculation that every inch of rain which falls over a square mile of chalk-country, and passes off by springs, carries with it, in solution, and without in the slightest degree interfering with its brightness, no less than from 15 to 16 tons of solid chalk. The quantity of rain which thus finds its way to the springs has, as already stated, been ascertained by experiment to be as much as 9 inches per annum in average seasons, giving an amount of about 140 tons of chalk thus annually carried away from each square mile of country at the present day; so that the loss is still going on at the rate of 140,000 tons of dry chalk to each square mile in every ten centuries.
The lowering of level from this cause is probably not uniform over the whole surface. For the acidulated water sinking into the chalk on the top of a hill, and descending one or two hundred feet before reaching the surface of "the subterranean reservoir,"[1] might, in its almost vertical passage, become saturated with carbonate of lime, and only render the chalk through which it passed somewhat more porous, without materially affecting the level of its surface. On the other hand, that absorbed in a valley would probably, to some extent, acquire the chalk which it eventually held in solution during its almost horizontal passage to the point of its delivery by springs; and as this would be at no great depth, the abstraction of solid matter would become more perceptible on the surface, so that the level of the valley would be lowered more
- ↑ See p. 664.
