either carried off again by evaporation and vegetation, or descends to a point at which the chalk is saturated with water, which is, however, constantly being drained off by springs along the valleys. This body of water has been termed "the subterranean reservoir" in the Chalk. The consequence of this absorbent power of the soil is that the streams and rivers in a Chalk country are not liable to floods, and moreover that their flow is but little affected at the time by rain; they being almost entirely dependent on perennial springs, which, during the driest of summers, still continue to deliver the water that in the course of the preceding winter, or even previously, has accumulated in the body of the Chalk.
The surface of the "subterranean reservoir" in the Chalk is by no means level, but always presents a gradient towards the point at which the springs are delivering its contents, so that within a chalk-hill forming a watershed between two streams there is what may be termed a hill of subterranean water, the summit of which need not, and often does not, correspond with the apparent watershed on the surface. The angle of the water-surface gradient depends principally on two factors, the degree of friction in passing through the chalk, and the amount of rain that finds its way down from the surface.
The height of saturation varies much in different seasons, as is evinced by the intermitting streams, often known as bournes,[1] which perhaps only flow for a few months once in every six or seven years. Near the Chalk escarpment in Hertfordshire, at a spot several miles distant from any stream, I have known this height of saturation, as shown by the level of water in a deep well, to vary as much as 70 feet in the course of a single year. But with a greater rainfall than at present, the Chalk might at all times be in a state of saturation up to within a few feet of the surface; and this would be materially assisted, were there no deep valleys in existence into which the subterranean water could be delivered; as, of course, if the outfall were raised, the level of permanent saturation would be raised also. Were the Chalk in a less porous condition than at present, of course also its absorbent powers would not be so great. Under the circumstances, therefore, which have been supposed, the river- and spring-water from a Chalk district would be delivered in a manner very materially differing from that which at present prevails. The delivery of water by springs would be but small in shallow valleys; and, indeed, the only
- ↑ See Trans. Watford Nat. Hist. Soc., vol. i., 1878, p. 137.
