This type of disposition of fluviatile deposits is of common occurrence. We will assume that the valley is carved out of clay (shown by horizontal lines and dots). On both sides of it, and at the same relative heights, are two masses (marked 1 and 2) of brick-earths and gravels running along so as to form two distinct broad terraces. These beds were laid down when the river, in flood, though occupying only a small portion of the valley, was approximately of the height shown by the dotted lines _a b_. Denudation has been hard at work, however, since then, and only vestiges of these beds clinging to the sides of the valley, as shown, remain. At a later period, and coming on towards modern times, the broad expanse of beds (comparable in disposition with those depicted in Fig. 2) some miles in width, marked 3, were laid down, and we notice the river channel, as it now is, cutting its way through them. Thus it comes to pass that brickyards may be situated in terraces one above the other; and what is much more important, the brick-earths may vary very widely in quality along these horizons, those in 1 differing from 2 and both from 3. The brickyards may be quite close to each other, and to the unscientific eye the earths are of similar appearance, but they do not yield the same class of brick, and no one seems to trouble to enquire the reason why. These differences have resulted primarily from the materials having been derived from other collecting grounds, other watersheds, than those comprised within the basin of the river as at present constituted. They are the inevitable accompaniment of the evolution of the river system, and throw light on successive phases of the past history of the stream and its tributaries. For us, as we have seen, they possess considerable practical value of the first importance in selecting the site for a brickyard. Apart from differences of the character just described, serious alterations sometimes take place on these brick-earths being traced higher up the valley, and indeed an excellent brickmaking material may become absolutely worthless in that respect, for the reasons about to be explained. The reader will agree that neither stones nor sediment can travel up a valley, and he will understand that no sediment can be found in the valley earths other than that derived from the destruction of rocks within the watershed of the river system, to which the valleys belong, or did belong, at the time the earths were formed. We desire to put the case in a very simple light, so as to be clearly comprehended. Let us contemplate Fig. 4. [Illustration: Fig. 4.--Map shewing river basin, with geological formations depicted.]