. Elementary physical geography . see in the plains bordering the coast of Texas,the latter in the Colorado canon (Fig. 142). In the hardrocks of the Colorado the form of the canon is preserved,and this is also favored by the dry climate; but the soft,clay banks of the _ Texas streams readily crumble under the action of weathering in a moist climate. The development of the latter to the state of maturity, will therefore be much more rapid than that of the former, — just as some animals or plants pass through life in a few weeks, while others live for a century. In a mountainous country th
. Elementary physical geography . see in the plains bordering the coast of Texas,the latter in the Colorado canon (Fig. 142). In the hardrocks of the Colorado the form of the canon is preserved,and this is also favored by the dry climate; but the soft,clay banks of the _ Texas streams readily crumble under the action of weathering in a moist climate. The development of the latter to the state of maturity, will therefore be much more rapid than that of the former, — just as some animals or plants pass through life in a few weeks, while others live for a century. In a mountainous country the elevation is so great, and the rock structure so complex, that gorges will remain for long periods of time ; and ages must elapse before the erosive action of the river becomes less rapid than weather-ng. Now and then a deep mountain lake may check thevork of the river, and serve as a temporary base level, belowfhich, for the time being, the stream cannot cut; and solere, for a short distance, the valley may become Fig. 143. A broad Alpine valley. 272 PHYSICAL GEOGRAPHY. While the prevailing type of mountain stream valley isthat of the gorge (Fig. 134), there are mountain valleys ofgreat breadth and depth. These are not true stream valleys,but great synclinal valleys of rock folding (Fig. 143) whichthe rivers have occupied because of their convenient passing through a deep defile (Fig. 144), a tiny streammay emerge into one of these great, park-like valleys (Figs. 143 and 221); andthen we see, sideby side, the valleyof stream forma-tion and that ofrock the aid ofweathering, eventhe mountain gorgewill in time broad-en out into a widevalley. Adjustment ofStreams. — Whena river begins tocut its valley upona new land, thereis no necessaryrelation betweenstream course and rock structure. The stream may flowacross hard and soft layers alike, the course being consequenton the topography, because the river was guided down theoriginal slopes
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