In general a permanent stream at one point in a valley means a continuous stream from that point to the sea or lake which the valley joins; but to this rule there are many exceptions. They are likely to arise where a stream heads in a region of abundant precipitation, and flows thence through an arid tract where the ground-water level is low, and evaporation great. In such cases, evaporation and absorption may dissipate the water gathered above, and the stream disappears (Fig. 2, Pl. III, near Paradise, Nev.). [Illustration: PLATE III. Fig. 1. KANSAS. U. S. Geol. Surv. Fig. 2. NEVADA. U. S. Geol. Surv.] [Illustration: PLATE IV. Fig. 1. ILLINOIS. U. S. Geol. Surv. Fig. 2. NORTH DAKOTA. U. S. Geol. Surv.] =Other modes of valley development.=—If as a new area of land emerges from the sea its surface has a depression without an outlet, and such an assumption is by no means improbable, the depression would be filled with sea-water. The inflowing water from the surrounding land might fill the basin to overflowing, and the outflow, finding exit at the lowest point in the rim of the basin, would flow thence toward the sea. Such a stream would develop a valley, the history of which would be somewhat different from that which has been sketched. Instead of developing headward from the sea, the valley would be in process of excavation all the way from the initial basin to the sea at the same time (Fig. 54). The upper end of the valley might ultimately be cut to the level of the bottom of the basin, when the lake would disappear. The head of the valley might then work back across the former site of the lake into the territory beyond. Valleys might have developed above the lake before it was drained, and after this event, such valleys would make connections with the valley below (Fig. 55). A valley developed in this manner is not simply a gully grown big by head erosion, and the valley would not precede the stream. [Illustration: +Fig.+ 54.—Diagram to show how a valley may be developing all the way from a water-filled basin (lake) to the sea at the same time. Small valleys leading to the lake are also developing. The black area = the sea.] [Illustration: +Fig.+ 55.—The stream leading out from the lake (Fig. 54) has drained the lake, and the valleys above and below the site of the former lake have united.]