see the periodic rejuvenation of the lands and the renewal of active erosion. During these times of crustal unrest and subsequent internal readjustment the molten portions deep down in the lithosphere are forced to rise as great hot tongues into the cores of the mountain ranges, and in many places they break through and give rise to active volcanoes. Beneath lie great and highly heated masses that subsequently cool into granitic rocks, and when long afterward the mountains are worn away we discern among their roots these cores of granite. However, during all the time of crustal rising and readjustments the volcanoes are more or less active, spewing violently or quietly into the atmosphere tremendous volumes of rocks blown into volcanic ashes and of lavas, and great quantities of gases that increase the volume of the atmosphere, and much juvenile water is added by them and the thermal springs to the vadose water of the hydrosphere. Even though the lithosphere absorbs much vadose water through soaking and mineralization, the oceans are yet constantly growing in volume through this accession of juvenile water. In the same way, even though vast quantities of carbon are taken out of the atmosphere by organic agency and buried in the rocks, there to be transformed into natural gases, petroleum, asphalt, peat, and coal, the air is periodically resupplied by the volcanoes and by the hot-water springs.

Periodic spreading of the oceans. So far we have been describing the alterations that take place in the dry land; let us now turn to the realms of Neptune and note briefly the areal fluctuations of the oceanic waters during the latter half of geologic time. The geography of the earlier times we have not as yet deciphered and therefore cannot relate the movements of the oceans. It has been stated that when the earth shrinks, the lands rise in relation to the strand-line, and that this is especially true at the times of major crustal alterations. It is now definitely known that at the close of all the eras the

continents were largest and at some of these times were even larger than they are now. On the other hand, the present oceans are more than full and for some time have been spilling over the outer edges of the continental platforms, apparently to the extent of nearly ten million square miles or about 5 per cent of the earth's surface. In other words, the present continental platforms occupy 35.5 per cent of the lithosphere, while the total water areas are equal to 69.6 per cent of the earth's surface.

The mean height of the present continents above the sea is given by Sir Archibald Geikie as about 2,400 feet, and if these protuberant masses were deposited in the sea, as they surely will be by the future rivers, the ocean waters would be displaced to the extent of raising the strand-line about 650 feet. We see therefore that the ocean level is also inconstant, and accordingly, even though the North American continent should remain stationary, about half of it would be covered by the displaced seas to depths of several hundred feet.

Between the comparatively short intervals of mountain making and crustal unrest occur the long quiet times of erosion when the lands are planed down to near the sea-level. This transference of rock leads, as has already been stated, to more or less flooding of the continents by the oceans (see Fig. 12). During the Paleozoic, Mesozoic, and Cenozoic eras, North America, and especially the United States, was widely flooded by warm and shallow marine waters at least sixteen times -Waucobian, Croixian, Canadian, Chazyian, Mohawkian, Cincinnatian, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Comanchian, Cretaceous, and in an oscillatory and marginal way during the Cenozoic (see Fig. 13). These shallow-water spreadings begin in a small way, grow to greater dimensions throughout a very long time, and then, declining, recede more rapidly than they came (see Fig. 11). They vary in areal extent up to 4,000,000

square miles. The greatest floods have spread from the Arctic Ocean, others from the Pacific, and those of least amount from the Atlantic. On the other hand, the number of floods was greater from the south than from the north, and the waters were most persistent in the Appalachian, lower Mississippi, and Rocky Mountain areas. Where the greatest thicknesses of sediments accumulate, out of this continental débris there will arise, phoenix-like, a future mountain range. After about 40,000 feet of strata had been deposited in the Appalachian trough, there arose near the close of the Paleozoic era the majestic Appalachian Mountains, some of which may have towered to at least 20,000 feet above the sea. In fact, the original mountains were destroyed by erosion during the Mesozoic era and the reëlevated Appalachians of today are due to later uplifts of more than 2,000 feet; subsequent erosion has developed them into their present interesting forms. Today one may travel comfortably over the roots of these once grand mountains by way of the Pennsylvania, the Baltimore and Ohio, and the Chesapeake and Ohio railways.

Areal variability of organic habitats. We have seen that the dry lands become alternately larger and smaller, and in the same way the shallow-water areas of the oceans are vastly increased when they spread over the lands, and are again greatly decreased when the land areas increase in size. Thus when the lands are widely flooded there is a great increase in the quantity of marine life, but little is originated that is new, while on the lands the climate becomes insular, warm and moist, and productive of the greatest amount of life in the then restricted areas. On the other hand, when crustal readjustments occur, the lands are largest, driest, and coolest, and these changing environments react on the life and bring about great alterations in the composition of the floras and faunas. At these times the shallow-water marine areas are smallest and most variable in turbidity and salinity-conditions that