Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/172



this chapter the sialsphere, which to-day occurs only in fragments—the continental blocks—on the earth’s surface, will be considered, and be dealt with—in the first place as a whole.

A map of the continental blocks is given in Fig. 27. Since the shelves belong to the blocks, the outlines deviate considerably at many points from the coast-lines. It is important, for our studies, to emancipate ourselves from the usual map of the earth and to obtain a certain intimacy with the outlines of the complete continental blocks. As a rule the 200 m. line gives the best idea of the limits of these platforms, yet some portions, which belong, without any doubt, to the continental blocks, reach a depth of 500 km.

Fig. 28 shows, further, a section of the earth in its true proportions on a great circle through South America and Africa. Mountains, continents and oceanic depressions form such trifling irregularities that they are to be found within the circular line which denotes the surface of the earth in the figure. On the other hand, the thickness of the continental blocks, amounting to about 100 km., is well shown. The core of the earth, probably composed chiefly of nickel and iron, has been termed the “nife” by E. Suess. For comparison the chief strata of the atmosphere are also introduced; that is, the atmosphere of