GEOLOGY. 243 



students of engiiieering problems, as well as togeolopsts concerned with 

 the various and increasingly important problems of degradation. 



Unquestionably the most noteworthy contribution to objective geol- 

 ogy during the biennial period is the recognition by several students of 

 the intimate relation between topographic configuration and geologic 

 history. Although fully matured only within a few months, the concep- 

 tion found birth during the days of active geologic investigation in 

 western Territories, originating, as have so many other wide-reaching 

 inductions, in the fertile brain of Powell, but receiving notable impetus 

 through the early work of Button. The former geologist pointed out 

 that one of the leading determinants of degradation is found in the 

 declivity of streaujs, and that there is a certain minimum declivity be- 

 yond which there is neither corrasiou nor transportation. It is of 

 course evident that no stream can sink its channel below the level of 

 the sea ; and as Powell pointed out, it is equally evident that at a dis- 

 tance from the sea a stream can not sink its channel below a certain 

 altitude above tide which will give a slopejust sufficient to permit tiow- 

 age; and also that no tributary can corrade its channel below the level 

 of its primary. This lowest limit of corrasiou and transportation may 

 be called the base level of erosion.* The importance of this conce[)tion 

 was realized by several of the leading geologists of the world, but cir- 

 cumstances prevented further development of the subject, and the con- 

 ception lay dormant for years. 



Peceutly, different geologists at work in eastern United States have 

 noticed certain intermontane plains and ancient terraces which can be 

 explained only upon the hypothesis that during some past period the 

 laud stood lower than now, and remained stationary until not only the 

 rivers and their tributaries, but the minor streamlets — and even the 

 rain-born rills and so the entire surface, were reduced to a level below 

 Avhich degradation was impossible; and the term base level was ex- 

 tended not only to rivers and their tributaries, but to whole continents, 

 so that to-day the term is applied to a bi-dimensional surface rather than 

 a series of uni-dimensional lines. Moreover, it was found that the inter, 

 montane plains are in many cases bounded by steep slopes and inter- 

 sected by sharp-cut gorges and ravines which could only be exiilained 

 on the assumption that at the close cf the base-level period the land 

 was lifted so high that degradation was greatly accelerated, but for a 

 period too short to permit the general reduction of the area to a new 

 base level; and the geologists concluded that the plains tell of altera- 

 tions in level of the land as well as of long standing at a single level. 

 The conception expanded still further as study progressed until it came 

 to be perceived that every hill and valley is a record of geologic activ- 

 ity depending upon the relation between land and sea, and thus indi- 

 cating the geographic configuration of past periods. So a new geologic 



• Exj)loratiou of tho Co lorado River, 1875, p. 203. 



