Page:Popular Science Monthly Volume 53.djvu/34

22 deepened by earth movements, although such is not apparent in the many tributaries. It has been found that the greatest amount of upward warping is in the mountain regions, and the greatest depression is supposed to be along the margins of oceanic abysses. Such movements would have a tendency to somewhat increase terrestrial and submarine declivities; but if transverse upward warping became exaggerated, the valleys would become barricaded so as to form basins, like that of Lake Ontario, or even greater sea basins. Such, however, is not the case with the valleys crossing the submerged coastal plains.

—Since the submarine valleys, wherever traceable to the shores, are found to be continuations of the

buried valleys crossing the coastal plains, they have considerable magnitude where first detected in the soundings. Parenthetically it may be stated that during the late minor oscillations of the continent the old valleys have become filled with sand, etc., for some distance seaward; but beyond this fringe they are always found where the soundings have been taken sufficiently near together. Submerged valleys gradually increase in size until they enter the oceanic embayments indenting the margins of the continental mass. These embayments have been found to vary from perhaps ten to forty miles in width, or within the limit of size shown in the modern Mississippi and St. Lawrence Valleys. Even the upper cañon of the Colorado River has a width of twelve miles. Consequently, the breadth of the drowned valleys