is well known, the continents at their surfaces consist frequently not of gneiss, but sediments, and we must on that account be clear as to what rôle these play in the building up of the continental blocks. About 10 km. can be considered as the greatest thickness of the sedimentary strata, a value which the American geologists have calculated for the Palæozoic deposits of the Appalachians; the other limit is nothing, since at many places the primitive rock is bare of any covering of sediments.
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Fig. 28.—Section along the circumference through South America and Africa in true proportions.
Clarke estimated the average thickness on the continental blocks as 2400 m. But since the total thickness of the blocks may be estimated as about 100 km., it is clear that this covering of sediments only means a superficial zone of weathering, the entire removal of which would, moreover, result in the blocks practically ascending to their former altitudes by the restoration of isostasy; so that the relief of the earth’s surface would thus be but little altered.
It is, indeed, not improbable that in the most ancient “pre-geological times” the film of sial covered the whole earth. It could then have been, not 100 km.,
