Page:A Treatise on Geology, volume 2.djvu/37

 CHAP. VI. have exerted all their force in merely smoothing and filling the inequalities of their valleys, and this partial labour they have not accomplished. Will any one, after this, require to be told that rivers did not make their own valleys; and only yield to this truth when, on the chalk and limestone hills, hundreds of valleys are shown him, down which water never runs, and which, indeed, have no trace of a channel?

The upfillings of a valley by the operations of a river ever tend to be formed in horizontal laminæ; or at least their surface is generally level in the direction across the valley, whatever undulations exist beneath, and however rapid may be the longitudinal declivity of the valley. This is well seen in many valleys of the Swiss Jura, the Cotswold Hills, &c.



a. Irregular surface which is the original basis of the valley, b. The sediment left in it, with a plane surface as if deposited in a lake. c. The surface of the valley, uniformly declining among A, the bordering mountains.

When the materials are gravel and coarse sand, deposited by an impetuous stream, the general surface may be level, and yet the laminæ beneath are frequently much inclined, with slopes in various directions, as Mr. Lyell has noticed with regard to the detritus left by the stormy waters of the Arve. The same thing occurs in many of the stratified rocks which appear to have been accumulated under violent agitation near the sea-shore. (See Diag. No. 20. p. 61. Vol. I.)

Plane surfaces existing along the course of valleys,