underwent in the course of geological time and which presumably continue even to-day.
According to this idea, to take a particular case, millions of years ago the South American continental plateau lay directly adjoining the African plateau, even forming with it one large connected mass. This first split in Cretaceous time into two parts, which then, like floating icebergs drifted farther and farther apart. Similarly, North America was close to Europe; and, at least from Newfoundland and Ireland northward, they formed with Greenland one connected block, which broke up by a forked rift near Greenland at the end of Tertiary time and farther north even in the Quaternary era; whereupon the constituent blocks moved apart from one another. The shelves, the portions of the continental masses overflowed by shallow seas, will always be considered in this book as parts of the blocks, the boundaries of which for great distances are not given by the coast-lines, but by the steep descent to the deep sea floor.
Similarly, it will be assumed that Antarctica, Australia and India lay adjoining South Africa, and with the latter and South America formed, until the beginning of the Jurassic period, a single large—even if partly submerged at times by shallow water—continental area, which in the course of Jurassic, Cretaceous and Tertiary time split and crumbled into smaller blocks which drifted away from each other in all directions. The three maps of the earth reproduced in Figs. 1 and 2 show these developments during the Upper Carboniferous, Eocene and Lower Quaternary periods. The case of India is somewhat different: it was originally connected by a long continental tract, mostly, it is true, covered by shallow seas, to the Asiatic continent. After, the separation of India from Australia on one side (in the Lower Jurassic) and from Madagascar on
