CHANGES IN SEDIMENTARY ROCKS.
Emergence of Sedimentary Rocks.—§ 26. Owing to changes which are taking place in the earth's interior, the altitude of most points on the surface has changed repeatedly, and similar changes are now tak ing place. The gulf coast of the United States, the coast of Norway, southern France, northern China, North Africa, and Chili are rising; while the Atlantic coast of the United States, northern France, Netherlands, southern China, Egypt and eastern Australia are sinking. That these changes in level are not confined to the coast is shown by the beach lines of all large lakes. The old beach lines are not parallel to those now being formed. Careful studies made within recent years show that the northern end of Lake Michigan is now rising at a rate which, in the opinion of G. K. Gilbert, would, if continued, cause the lake to find its outlet through the sag and the Illinois river within a few centuries.
Such movements have not been confined to modern times, but have been going on in all geologic ages. That vast areas of our present continents have been covered by the sea is shown by the presence of limestones, sandstones and shales, all stratified and all containing the remains of salt-water animals. It can also be proved that large areas now covered by the sea have once been dry land.
If from most points along the shore line we make a series of soundings running directly away from the shore, we will find that the sea deepens very gradually for a considerable distance and then its bottom plunges suddenly down a steep incline. This sudden drop marks the true border of the continent, and may be traced off the shore of all continental masses. If so traced on a map this Jine would be found to divide the earth's surface into two nearly equal portions, the continental masses and the ocean basins. The earth's surface carries more water than the true ocean basins can hold, and consequently some of the water overflows the lower portions of the continents. As the surface of the continent changes
in altitude this water flows from point to point, and so portions which were once dry land are now covered, and portions which were once under the waters of the ocean are now dry land.
It is probable that every point on the earth's surface has at some time formed part of this submerged continental border, and some have been submerged many times. This will help us to understand the presence of the rocks referred to above at points now far inland, and successive elevations and depressions will help to explain their alter nations.
Metamorphism of Sedimentary Rocks.—§ 27. These same internal changes produce marked wrinkles or folds on the continental areas, which we call mountains. The process of folding develops so much heat and pressure that many of the limestones, sandstones and shales are trans formed into marbles, quartzites and slates. In some places the heat and pressure are so great as to melt and mix the rocks, causing the union of silica with the bases and so changing them again into granitoid rocks. As these changes always occur at great depths, and as the movements once started are apt to recur at the same point for a very long time, the heated rocks do not soon regain their normal temperature and water circulating through them and passing from them to other rocks acquires greater transforming power. Such waters passing through cracks and fissures become powerful agencies in transforming the newly formed granitoid rocks into masses of clay. As these changes are most likely to occur in the walls of fissures, the clay deposits so formed are found in veins and pockets enclosed in crystalline rocks. As these deposits result from the decomposition of granitoid rocks they differ in no material way from the residual clays described above. • Slates are merely hardened and to some extent recrystallized shales which, when ground sufficiently fine, recover the properties of clays.