BEARING OF PHYSIOGRAPHY ON UNIFORMITARIANISM. 9 



Hutton, Playfair and Lyell, especially safe if the very remote hypothetical past of 

 unrecorded time is not considered. 



Now, the conditions and processes postulated in the physiographic study of land 

 forms are among the cardinal principles of uniformitarianism. The success in the 

 interpretation of nature by arguments based on these postulates confirms tlieir cor- 

 rectness, and thus brings to the support of uniformitarianism a large class of facts, 

 whose bearing on these principles was not at all perceived when they were an- 

 nounced by their early advocates ; indeed, at that time and for a long interval 

 afterward the facts here referred to were not known. The facts are those concern- 

 ing the form and arrangement of rivers and river valleys, as dependent on the 

 denudation of the lands. 



In older geological and geographical writings, valleys have been explained as 

 fractures in the earth's crust or as the channels formed by ocean currents during a 

 time of submergence. The modern explanation, so apparent and so acceptable to 

 us now, that most valleys are the result of the wasting of the land under the 

 guidance of the local stream or river, slowly gained acceptance through the middle 

 of this century ; but the wide application of this explanation — that land areas may 

 be practically baseleveled by the wasting of slopes until the hills disappear and the 

 valley floors become essentially confluent — is still overlooked by many geologists. 

 Singularly enough, the British school of geologists is especially slow in making this 

 application of the principles of uniformitarianism ; even when furthest inland the 

 British geologist is still so little removed from the ocean shore that he prefers to 

 look on evenly denuded areas as surfaces of marine planation and not as subaerial 

 peneplains. 



In discussing the origin and arrangement of valleys as the result of every-day 

 processes there are two important groups of considerations to be borne in mind : 

 First, the incision of valleys along lines where the streams took their positions when 

 the land mass under investigation assumed essentially its present attitude with re- 

 spect to baselevel : antecedent, consequent and revived streams would come under 

 this heading; second, the spontaneous development of new streams and rearrange- 

 ment of old streams, resulting from the reaction of the streams on the structures : 

 subsequent and obsequent streams,* as well as all questions of migration of divides 

 and diversion of streams, would come under this heading. 



Those geologists who some fifty or sixty years ago turned attention to the pro- 

 duction of valleys by the work of the streams that occupy them considered only 

 the first of these groups of considerations, and that incompletely. They did not 

 give particular attention to the manner in which streams were located at the begin- 

 ning of their erosive work, and they did not carry the process of valley-widening 

 to its legitimate conclusion. They were chiefly occupied with the production of 

 young, adolescent and mature valleys. The result of their teaching is seen in the 

 selection of narrow valleys to illustrate the success of streams as valley-makers : the 

 stream cutting down its own channel and carrying away the waste that falls into it 

 from the side slopes. But if success is measured by the magnitude of work accom- 

 plished, wide-open valleys or, still better, peneplains of subaerial denudation, and 

 not gorges and canyons, should be cited in illustration of what streams can do in 

 the way of valley-making. This, however, is seldom the custom even today. The 

 deep and narrow valley is truly more immediately impressive than the peneplain. 



* For definition of terms see (London) Geographical Journal, February, 1895, p, 134. 

 II— BvLL, Gkol. Soc. Am., Vol. 7, 1895. 



