264 N. S. SHALER — SPACING OF RIVERS 



order ia their placetneut, except such as was determined by the jointing 

 of the limestone rocks in which they were excavated, the surface streams 

 of the neighboring cavernless countrj^ were grouped with rather definite 

 intervals which did not distinctly vary, whatever the character of the 

 subjacent rock might be. A debate which occurred in the geological 

 conference of Harvard University in 1897, on a paper presented by Pro- 

 fessor Penck, of Vienna, showed me that this distinguished geographer 

 had noted like facts in the Alps and in the Cordilleras, and indeed it 

 must have attracted the attention of many observers. This revival of 

 the inquiry has led me to the observations and inferences which are set 

 forth below. 



EVIDENCE FURNISHED BY TOPOGRAPHIC MAPS 



First, let us note in a more detailed way the certain important facts 

 concerning the placement of streams which has just been stated as a 

 very general proposition. Taking a series of maps which show in an 

 accurate manner the geological and topographical aspects of diversely 

 conditioned areas, we may readily observe that on those areas which 

 have been long and continuousl}^ exposed to the effective work of streams 

 the spacing of the channels is in the greatest measure uniform. On the 

 other hand, where, as in the coastal plain districts of the United States, 

 the surface has recentl}^ risen from the sea, the intervals between the 

 streams of all sizes is much more irregular. Again, in countries of in- 

 considerable reliefs which have had their former drainage system effaced 

 by a deep coating of glacial drift the order of position of the newly de- 

 termined channels has something like the irregularity which is exhibited 

 by newly elevated area. The fact that the likeness and the order of 

 streams is least evident where they have acted for the shortest time, and 

 most so where they have l:)een long in operation, suggests the hypothesis 

 that the distribution of their channels in an equable manner is in some 

 way brought about by the action of the streams themselves; that in 

 wearing downward they work in a manner which tends to equalize their 

 intervals. 



VALUE OF STUDYING BEGINNINGS OF STREAM WORK 



Apparently the best method of approaching the discussion of the view 

 just stated is by observing what takes place in those beginnings of stream- 

 work which we may observe wherever a sloping surface of earth has been 

 exposed to the action of the rain. On such surfaces, as is well known, 

 a drainage system in miniature is quickly developed, the channels at the 

 beginnings of the process being very small and much branched, and the 

 whole appearing, as has often been remarked, like a reduced model of a 



