Bathymetric surveys have revealed much that is very interesting concerning the ocean floors, not only as regards the deeper portions, but also as concerns the continental shelves. Hull has constructed charts of the North Atlantic and shown that many of the shallower areas are of such a form as to suggest that they were originally portions of existing river-basins, and that the submerged valleys may be traced along the sea bottoms to depths of more than a thousand feet round the present continents. Similar submerged troughs have also been noted along the margin of the Asiatic continent. Two such well-marked channels occur in the ocean at the mouths of the Indus and Ganges. Indeed, the phenomenon would seem to be one common to all the continental shelves.