Page:Popular Science Monthly Volume 80.djvu/397

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Portals Defined.—A study of the distribution and relationships of ancient marine faunas shows that there have been certain critical areas through which these faunas were connected from time to time. These critical areas are depressions on or between continental masses, and are invariably regions of permanent instability of the earth's crust, where mountain-making and the accompanying volcanic and earthquake disturbances have been prevalent.

When these areas were depressed below sea-level, they formed straits, or channels, connecting sea-basins, and affording avenues for intermigration of marine faunas. When they were elevated, they formed barriers impassable to the dwellers in the sea. Thus neither the name strait nor barrier is applicable to them as a general term. Therefore the name portal is selected, as indicating a gateway that may be either open or closed, and still retain its identity.

Of the portals that were important in the ancient world only three are now open: the North Pacific portal, of which Bering Strait is a shrunken remnant; the Iberian, still recognizable in the Strait of Gibraltar; the Malaysian, seen in the inter-island passages in the East Indian Archipelago. And one has shifted its position, growing from an arm of the sea into the noble expanse of the Indian Ocean.

One still shows its nature as a portal in the narrow strip of land joining the two Americas. The others are now concealed as parts of continental masses, namely the Crimean, the Asia Minor portal, and the Bokharan, revealing their nature as former arms of the sea only in the extinct marine fossils now buried in their sediments.

Still other bodies of water that loom up large in our present-day geography, as the North Atlantic and the South Atlantic, too wide to be called merely passages, or to be differentiated from the main ocean by a special name, did not even exist in the ancient days.

These great changes have been wrought chiefly by the world-wide Tertiary mountain building, and the accompanying, or causing, disturbances of the continental areas. Also there was much readjustment in the late Paleozoic topographic revolution, and in the late Jurassic Cordilleran revolution.

Paleogeography.—In recent years the reconstruction of ancient physical geography has been a favorite field of research, not to say of speculation, of geologists; paleontologists, too, have taken their part in