Physical Geography of the Coast.—It is well known that the North American continent is bordered on both sides by a submarine plateau sloping gently seaward until it attains a depth of about 100 fathoms, and that from this 100-fathom line the bottom drops off rapidly into deep water. This submarine plateau may be regarded as a submerged coastal plain, and its margin as the true boundary between the continent and the ocean-basin, i. e., as the submerged continental margin.
It is also well known that on the eastern coast this submarine plateau is trenched with deep submarine troughs running out from the mouths of the great rivers to the submerged continental margin and there opening into deep water. The best known of these submarine channels are: (1) One running from the mouth of St. Lawrence river through St. Lawrence bay and over the . . .