Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/178

152 be assumed that this strike of the Pacific indicates the old direction of displacement through which the ocean basin opened, or it may be expanded. It is not inconceivable that the ancient folds in the gneiss massives of Brazil, Africa, India, and Australia are the equivalent of this opening of the Pacific; the later, northerly direction of strike in Africa would fit in very well with the direction of the rows of the Pacific islands (that is, it cuts this direction at an angle of 90°).

This compression of the sialsphere must naturally have a thickening as its consequence, and therefore an elevation, whilst at the same time the deep-sea basins increase in size. Hence the flooding of the continental blocks must—without taking into account their change of position—have in general gradually diminished in the course of the earth’s history. This law is generally acknowledged. It also appears very distinctly from the consideration of our three maps of reconstruction.

It is important to observe that the evolution of the sial crust must be one-sided, even if the forces vary, for tension cannot smooth out the folds of a continental block, but at the most can only tear them to pieces. The alternating action of the forces of compression and tension is thus not able to undo its operations, but produces unilateral progressive results: crumpling and dismemberment. The covering of sial becomes continually smaller in area, and thicker, in the course of the earth’s history, but it also becomes increasingly split up. These are complementary phenomena, and are effects of the same causes. The hypsometric curves, which on this basis may be assumed for the past, present and future, are shown in Fig. 29. The present mean-level of the crust coincides with the original surface of the still unbroken sialsphere.