this depression and that a complete tearing and separation of the two blocks must take place long before the sinking portions have reached the level of the deep-sea floor. The foundering of the English Channel, the North Sea, and other former land areas, now transformed into portions of the continental shelf around England, took place, according to our idea, immediately before the breaking away of Newfoundland from Ireland. But they became nevertheless only shallow shelves, for the complete separation of the blocks occurred farther to the west.
If the geographical orientation of the chief rifts
_-_figure_36.jpg)
Fig. 36.—Extensive collapse due to the stretching of the lower crust (diagrammatic).
of the sial crust be considered, the predominance of the meridional direction can be recognized, although many abnormalities occur. Not only does this apply to the rift system of East Africa, to which reference has already been made, and to the Rhine trough formed in the Oligocene, but the Atlantic rift also follows a course which, for the position of the Tertiary poles, is essentially meridional. The same applies to the rift of which one side forms the eastern margin of Africa. The southerly tapering of the continents in South America, South Africa, and India may be traced back to such meridional rifts extending to the pole.
