Januaey 10, 1896.] 



SCIENCE. 



51 



(1847) found general acceptance, and in 

 England to this day hardly any serious con- 

 sideration is given to any other explana- 

 tion. The production of plains of abrasion 

 at the completion of a cycle of subaerial 

 denudation, advocated by Powell in con- 

 nection with the idea of the baselevel of 

 erosion (1875), has found wide acceptance 

 in this country, but it is less approved 

 abroad. The paper considered the criteria 

 by which plains of abrasion of one origin or 

 the other may be distinguished. When 

 such plains are uplifted and maturely dis- 

 sected in a second cycle of denudation the 

 difiSculty of determining their origin in- 

 creases. It is suggested that plains of 

 subaerial denudation may be recognized, 

 even when uplifted and dissected, by the 

 degree of adjustment of their streams to 

 their structures ; thorough adjustment re- 

 quires a longer time of stream action than 

 has passed since uplift ; much of the ad- 

 justment must be referred to a previous 

 cycle of denudation, which is thus shown to 

 have been a subaerial cycle. 



Considerable discussion followed by 

 Messrs. Willis, Reid, Hayes, Van Hise and 

 Gilbert, the speakers giving instances from 

 different parts of the continent, which il- 

 lustrated one or the other interpretation 

 cited, or which emphasized the large part 

 played by the character of the rocks con- 

 cerned or by isostatic adjustments. 



Fore-lands. F. P. Gullivee, Cam- 

 bridge, Mass. 



1. Action of waves, tides and currents. 

 Waves attack the whole coast, but erode 

 more rapidly on headlands than at bay 

 heads. Tides are less effective agents of 

 transportation along shore on exposed coasts 

 than currents, but they are the important 

 agents in sounds, channels and inlets. 



2. Current cusps. Tj'pe, Cape Hatteras. 

 The cusp is formed in the dead water be- 

 tween two eddy currents. 



3. Tidal cusps. Type, West Point, Puget 

 Sound, Washington. The cusp is formed be- 

 tween eddies of in- and out-flowing tides. 



4. Delta cusps. Type, Tiber delta, Italy. 

 The mouth of the river forms the point of 

 the cusp, on either side of which the along- 

 shore currents arrange the detritus. 



The paper was illustrated by pilot charts, 

 which somewhat unfortunately were not all 

 used, as space for display was limited. 

 Bailey Willis remarked on the applications 

 of the views advanced to localities in the 

 Puget sound region. 



Drainage Modifications and their Interpreta- 

 tion. M. E. Campbell, Washington, 

 D. C. 



This paper opened with a discussion of 

 the subject of stream modification under 

 the influence of slow elevation or depres- 

 sion of the earth's surface. From this was 

 derived the Law of the Migration of Divides 

 which control, to a greater or less extent, 

 the alignment of all drainage systems. 

 The Law of the Migration of Divides is in 

 brief that divides migrate toward a region 

 of uplift and away from a region of depres- 

 sion. The relations of divides may there- 

 fore be significant indicators of the lines 

 of upheaval or depression even when these 

 are comparatively slight. Criteria were 

 given by which these modifications may be 

 recognized and the character of the crustal 

 movement determined. 



A brief description followed of some of 

 the drainage systems of the Appalachian 

 province, south of the glaciated region, to 

 show that similar modifications of the 

 drainage are of common occurrence, not 

 only in the regions of horizontal rocks, but 

 also occur in the highly complicated geo- 

 logic structure of the Appalachian valley. It 

 was shown that some of these changes are 

 of recent occurrence, whereas some proba- 

 bly date back to the time of the Jura-Trias 

 depression. 



