If the layers of unequal hardness in a stream’s bed be vertical and the course of the stream at right angles to the strike, rapids, and perhaps falls, will develop (Fig. 115). The chances for falls are greater, the greater the difference in hardness. Falls developed under these conditions, as well as the rapids preceding and following, would remain constant in position until the resistant layer was brought to grade, but they would ultimately disappear as in the preceding cases. Falls are not likely to develop where the strata of the stream’s bed dip down-stream, though they may develop even under these conditions if the gradient of the stream is greater than the dip of the strata (Fig. 116). [Illustration: +Fig.+ 114.—Falls in Utica shale, Canajoharie, N. Y. (Darton, U. S. Geol. Surv.)] The inequality of resistance in the rock which occasions a fall may be original or secondary. In the case of Niagara Falls[50] (Fig. 113) relatively resistant limestone overlies relatively weak shale. At the Falls of St. Anthony (Minneapolis) limestone overlies friable sandstone. The falls of the Yellowstone and the Shoshone Falls of the Snake River (Idaho), are in igneous rock. In the former case the unequal resistance is occasioned by unequal decay of the rock, due perhaps to the rise of hot vapors which have decomposed the rock along the lines of their ascent; in the latter, a more resistant sort of igneous rock overlies a less resistant. Structural features, such as jointing, sometimes give rise to falls, or determine their distinctive features (Fig. 117), even where the formations involved are of uniform hardness. A joint plane has the effect of a weak vertical or highly inclined bed. If an open joint is discovered in a stream’s bed, the water enters it. If it finds an outlet below, a channel is worn along the new line of flow, with rapids or falls where the water descends. Rock originally homogeneous may be much fractured in some parts, while it remains unbroken in others. Where a stream passes from the solid to the broken portion rapids, or even falls, may develop. [Illustration: +Fig.+ 115.—Diagram illustrating the development of falls over a vertical hard layer.] [Illustration: +Fig.+ 116.—Diagram illustrating the possibility of falls where the beds dip down-stream.]