STUDY OP TORRENTS 269 



be borne in mind that Avherever a stream fails to work downward as 

 rapidly as that of the adjacent channels, its drainage is invaded by the 

 extension of the side slopes of its competitors, with the result that it still 

 further loses its capacity to hold its original down-cutting power, its area 

 being soon shared in something like equal measure by the more success- 

 ful streams. The further this process continues the more complete the 

 work of effacing the original irregularities of interval. 



APPLICATION OF EXPERIMENTS AND OBSERVATIONS TO STUDY OF NATURAL 



CONDITIONS 



If from these observations on the convenient miniature specimens, 

 showing the conditions under which drainage channels are established 

 and developed, we turn to the larger examples of such work, as are ex- 

 hibited in river systems, we note at once the advantage of approaching 

 our problem in the manner which has been pursued. In the first place, 

 we see that the equality of the spacings in the larger streams is most 

 perfect in those which in size and character nearly approach the small 

 temporary gullies of newly bared soil. The torrent gorges which are cut 

 in uniformly yielding rock are often as accurately spaced as in the smaller 

 samples, though they may have the depth of hundreds of feet and an 

 average width of a mile or more. As we pass to the true rivers the 

 symmetry becomes less and less perfect as they increase in size. It may 

 be said that in proportion as the grade of the side slopes leading to the 

 divide diminishes and becomes thereby less regular, the spacing becomes 

 less even. 



The reason for the less regular intervals between the valleys of gentle 

 declivity and moderate side slopes and those of rapid fall with steep 

 slopes are numerous. Among them we may note the following : As the 

 stream approaches its baselevel it is proportionately more and more 

 affected by various influences which tend to deflect it from the center of 

 its valley. Obstacles which should have been disregarded or easily over- 

 come while the descent was steep now control its movement. This causes 

 the lateral extension of the valley to be forced this way or that ; it no 

 longer, as before, induces the attack on the adjacent slopes to be tolerably 

 uniform. In the basins of gentle descent the nature of the underlying 

 rock is likely to play a more important part in controlling the topography 

 and the share of solutional action greater than is the case in the torrential 

 valleys. It is also to be noted that in districts which are underlaid by 

 limestones, except they be dolomites, where the position of streams may 

 be fixed by the formation of underground water-courses, which in the 

 course of time become open channels, the arrangement of the streams is 

 often extremely irregular. In such case their order is determined by the 



XL— Bull. Geol. Soc. Am., Vol. 10, 1898 



