Periodic reëlevation of the lands. We have thus far presented the sedimentary evidence in the main as if it had continuously and regularly formed, without stoppages or "breaks." The geologic record, however, was not laid down in this way, because the surface of the earth does not remain stationary. Even though to humanity the "everlasting hills" appear to be permanent features of the earth's surface, they are all doomed to be carried away by the rain and transported by the rivers into the seas and oceans. Erosion of the land goes on until all is worn to near sea-level, and then there is but a very sluggish run-off of the rain that falls upon the planed continents. The earth has repeatedly had such a low relief; in fact, this has been the condition during the greater part of its history.

The continents are repeatedly reëlevated in relation to the strand, and this goes on many times in a small way and less often on a large scale (see Fig. 3). We have said that in the course of the geologic ages the earth has shrunk at least 200 and possibly 400 miles in diameter. The earth is still shrinking all the time, and this gives rise on the surface to small warpings whose difference of elevation is usually not more than a few hundred feet. These are but surficial symptoms, due to internal readjustment, remarkable as it may seem, in an earth as rigid as steel-local accommodations of the earth's mass to loss of heat and molecular rearrangements. These alterations finally set up strains in the lithosphere which are too great for its strength to bear, and then there is a time of breaking and greater readjustment between the relatively settling and rising masses. At these times ranges of mountains are slowly raised up near the margins of the continents, due to the shortening, folding, and breaking of the earth's crust, and the ranges have lengths of between 1,000 and 1,500 miles. These are the minor shrinkage movements, the "disturbances," which are coming more and more to be regarded

as the basis for dividing the eras into periods of time, and how often they occurred is one of the things that geologists are trying to ascertain. In North America we know of at least eight of these minor crustal readjustments, but in all the world there are many more than this.

Finally, there comes a time of major shrinking that adjusts all of the strains and stresses set up in the earth's mass by the minor, incompletely adjusted shrinkages. The earth has just passed through one of these major readjustments, and accordingly we see all of the continents standing far higher above sea-level than has been the rule throughout geologic time, and in many of them rise majestic ranges of mountains (see Fig. 3). A grander, more diversified, and more beautiful geography than the present one the earth has never had; this statement is made advisedly and with the knowledge that our planet has undergone at least six of these major readjustments of its mass. These greater movements are the "revolutions" that close the eras. Because the lands are then high they are subject to more active erosion and in the last analysis all of the broken-up detrital and dissolved material is carried away by the streams to the oceans. At these times the continents are also largest and the materials received by the oceans are laid down on the outermost edges of the lands, where subsequent transgressions by the sea cover and hide them from our observation. After a long time the sea again comes to press further and further upon the land and spreads more formations of stratified rocks over those left by the previous floodings, the older geologic formations (see Fig. 11). Therefore there is upon the present continents between each two such successive formations a "break" in deposition, a hiatus in the geologic record, a "time interval" when no record other than erosion is at hand. These "breaks" in sedimentation are representative of loss of record and are called "intervals"; they are regarded as the closing times of the eras.

During these intervals when the continents are highest and largest, the oceans are smallest in areal extent, the separate continents are often united by land-bridges as Panama unites the Americas, such bridges alter the direction of the great ocean streams like the Gulf Stream, the high mountain ranges alter the directions of the air currents and take out of them their moisture so that great desert areas arise like our interRocky Mountain country, and because of these vast changes nearly all of the major movements are accompanied by glacial climates. Such mighty changes in the geography, topography, and climate of the earth react strikingly upon the life of the polar and temperate belts and so disarrange it that the major "intervals" following the "revolutions" are spoken of as the critical times in the earth's history-critical not only for the geography, separating or uniting continental masses, but also for life, since as a result of the lands then becoming high and cool to cold in places and elsewhere moist or dry, vast domains of organisms are forced to adapt themselves to the altered conditions, or to migrate into more favorable areas, or to die out and make room for the rising hordes of fitter types.

In this way cycle after cycle of organisms appear and vanish, and their coming and going is brought about by the changing environment. Thus in the Mesozoic era, we see the lands mastered by the cycads and conifers among the plants and the dinosaurs among the animals, the air by the flying dragons, and the seas and oceans by other reptiles and hordes of ammonites and squid-like molluscs. The Great Reaper then steps in and through struggle and the elimination of countless organisms, resulting in regressive and progressive evolution, the lands in the Cenozoic begin to bloom with more and more flowering plants and grand hardwood forests, the atmosphere is scented with sweet odors, a vast crowd of new kinds of insects appear, and the places of the once dominant reptiles of the lands and seas are taken by the mammals. Out of these

struggles there rises a greater intelligence, seen in nearly all of the mammal stocks, but particularly in one, the monkeyape-man line. Brute-man appears on the scene with the introduction of the last glacial climate, a most trying time for all things endowed with life, and finally there results the dominance of reasoning man over all of his brute associates.

The Cenozoic era was a time of especially marked geographic alteration, as is especially well seen in the evolution and spread of the elephant stock or Proboscidea. These animals arose in Africa early in the era, but there was no means by which they could spread into other continents, because Africa remained isolated. At about the middle of the Cenozoic, the Alps were rising and these crustal alterations also gave birth to a land-bridge across the Mediterranean connecting Europe and Africa. Across this bridge the long-faced elephants, now all gone, spread first into Europe and thence into Asia. A little later much of Asia began to rise, and the culmination is seen in the grandest of all mountains, the present Himalayas. These alterations permitted the elephant stock to spread across Asia and the Nome bridge into Alaska and wider North America, and they had no sooner arrived there than they were on their way across the newly arisen Panama bridge, thence to spread all the way south into distant Argentina. North America was then peopled with many kinds of camels and horses, and they, along with many other animals, migrated into South America. Wanderlust was upon the world, and from South America there migrated into our country great sloths, a claw of one of which was found by Thomas Jefferson in Virginia, and described as that of a huge lion. The great Democrat may be excused for his error, since in those days fossil sloths were unknown in North America; rather should he be praised, for he is the only president who has described a fossil, other than live ones!

Times of volcanic activity. In all that has been said we