Professor Kober has attempted to fit all parts of the earth’s crust into a broad structural scheme, recognizing major units of two kinds: great plates or shields, continental in size, which behave as essentially rigid masses; and between them sinuous, comparatively narrow zones, which are relatively weak or labile and in which the effects of deformation are concentrated. These orogenetic zones are marked first by geosynclines, later by great systems of folded mountains. Their positions, as well as the outlines of the plates, have changed somewhat through geologic time. Thus we find within some of the present rigid masses structure lines that mark the sites of mountain zones in the pre-Cambrian or the early Paleozoic. These were belts of weakness which have been healed or welded with the passage of time, while new labile zones have come into existence elsewhere. Therefore, details in the structural . . .