THE HUMAN OR PRESENT PERIOD. 529 



breadth and depth to this abetting action of the tides. Particularly 

 may this be true of channels at the outer edge of the continental shelf, 

 where the abysmal slope joins the more nearly horizontal surface of 

 the shelf. We do not find that the subject has been made one of direct 

 investigation, but the following data bear upon it. The speed of the 

 main Atlantic tide is estimated at 520 miles per hour. Computation 

 indicates that on the outer border of the continental shelf the speed 

 is normally about 100 miles an hour. In other words, in passing from 

 the deep ocean across the sloping shelf to the shallow water above the 

 shelf, the velocity is reduced 75%, and a portion of the energy is neces- 

 sarily converted into a wave of translation with erosive power. 



It seems therefore not improbable that the trenches in the outer 

 edge of the continental shelf, and on the abysmal slope, are scoured 

 to greater depths and widths, and extended beyond their original 

 limits, by the tides. Such action might apparently be assigned to 

 any part of the abysmal slope on which the retardation of the tidal 

 wave was sufficient to give rise to a wave of translation. This is con- 

 sistent with the fact that the valleys on the abysmal slopes are broad, 

 and have gradients much higher than those appropriate to river-valleys 

 of like breadth. 



If the foregoing conceptions of the behavior of the continental 

 borders are valid, it is not difficult to understand why the theoretical 

 fringe of sediments is so poorly represented above the sea-level, for 

 it has been borne down and thrust landward by each general defor- 

 mation, and has crept outward and downward with each relaxation. 

 The whole series is to be regarded as present in the continental shelf 

 and the coast-border tract, but as largely concealed by this combina- 

 tion of disturbing processes. When the great depth of the ocean- 

 basins at the edge of the continental shelf is considered, it is obvious 

 that the volume of sediment required to build the shelf seaward is 

 large in proportion to the extension of the shelf, and hence the fringing 

 zone is not very broad. 



If the very general prevalence of harbors and inlets on the con- 

 tinental coasts is due to the foregoing combination of agencies, its 

 importance to commerce is difficult of over-estimation. 



Cooperative agency of the ice-sheets. — In the glaciated regions, 

 especially such as had much relief, like Scandinavia, Greenland, and 

 British Columbia, the ice itself, by its pressure and its own lateral move- 



