522 T. C. CHAM BERLIN 



portation toward the continental interiors, with a possibiHty or 

 probability that more or less of the sediment will at length reach 

 the coast by a long and circuitous route but with a liability also of 

 lodgment in lake basins, on low plains, in estuaries, embayments, 

 gulfs, or other dependencies of the ocean, in consequence of which 

 the sediments are consumed in building up depressions in the 

 interiors of continents rather than building out their borders; and 

 (3) direct transportation to the ocean down the border slopes of the 

 continents; or in briefer terms: (i) internal drainage with no 

 oceanic connection, (2) internal drainage with indirect oceanic con- 

 nection, and (3) direct oceanic drainage. Murray places the area 

 of the first class at 11 . 5 million square miles out of a total of 55.7 

 million, or a little more than 20 per cent. Leaving out the circum- 

 polar region that cannot now be treated, there remain about 40 

 million square miles to be placed under classes two and three. 

 I know of no authoritative division of this area between these 

 classes, nor do I see how any but a somewhat arbitrary division 

 can be made, for the two classes grade into one another in a very 

 intricate way. If we put into class three only the drainage from 

 the coastward sides of the mountains and other elevated tracts 

 that so generally border the continents and add the drainage from 

 the coastal flats where elevations are absent, the resulting area will 

 be inferior to that of the great interior basins. So hkewise if, for 

 another line of approach, we turn to the geological record, as now 

 known, the amount of clastic sediments embraced in the coastal 

 slopes seems much inferior to that which is embraced in the great 

 interior terranes. But, as a portion of the sediment that goes 

 inward at first, later reaches the exterior of the continent, let the 

 40 million square miles be divided equally between the class which 

 contributes to building up the interior and the class that contrib- 

 utes to building the continents outward. We have already taken 

 the length of the circum-continental shelf belt as 100,000 miles. 

 The assigned 20 million square miles tributary to it gives a work- 

 ing mean of 200 square miles tributary to every mile of length of 

 the shelf. 



Much study has been given to the determination of the average 

 rate of denudation of land surfaces under present conditions in the 



