Appalachian Geosyncline. 249 



that the crest of Appalaclria would have had to be lowered five 

 miles to supply this much of the Upper Devonian sediments. 

 It is seen from the assumptions that this is a minimum estimate 

 and might readily be increased fifty per cent or more. The 

 easiest escape from these quantitative difficulties is to conclude 

 that there is no relation between the Atlantic boundaries of 

 Appalachia and the present coastal shelf. It would seem that 

 the Upper Devonian slopes leading toward the geosyncline 

 was probably as much as 100 miles wide instead of fifty miles 

 as assumed and that the divide of the drainage was at least as 

 far east as the present 100 fathom line southeast of Long 

 Island and New Jersey. The eastern slopes of Appalachia in 

 that case existed where now is deep ocean. The disproportion 

 between the volume of the sediments in the geosyncline and 

 the present area of the continent to the southeast emphasizes 

 the query raised by Gilbert in 1892, whether Appalachia must 

 not have extended into the present ocean basin in order to 

 provide an adequate source for the Paleozoic sediments.* 



The Shelf a Terrace of Post- Paleozoic Construction. 



As another line of investigation bearing upon the problem, 

 the nature of the continental shelf should be considered. 



Ocean waves are known to have a perceptible effect to a depth 

 of about 100 fathoms, planing away the shore and the higher 

 parts of the bottom, carrying the products of fluviatile and 

 marine erosion outward to deeper water. The waves move 

 material along the bottom and prevent the settling of the 

 finest silt until the limit of wave action is reached. Beyond 

 that limit the bulk of the material is rapidly deposited 

 from suspension. In protected situations this depth becomes 

 less and in many places is not over fifty fathoms. There is 

 thus built outwards around the continents a subaqueous terrace, 

 its top gently sloping to the depth of 100 fathoms or less, its 

 front much steeper in comparison, and giving sharpness to 

 the continental margin. The doctrine of the permanence of 

 this line through geologic time implies that the constructional 

 additions are insignificant. But the volumes of sediments 

 which age after age have been poured into continental geosyn- 

 clines must correspond to other volumes swept out to the borders 

 of the ocean basins. Unless isostatic sinking under the load of 

 sediment compensated progressively for the outbuilding of the 

 terraces, the continental shelves, if ancient lineaments, would 

 have widened on the average by some hundreds of miles during 

 the course of geologic time. If there is isostatic sinking it is 



* Continental Problems, Geol. Soc. Am., Bull., iv, 187, 1893. 

 Professor Grabau also has called attention to this significance of the great 

 volume of Paleozoic sediments. 



