The cycle of erosion now in progress and recorded in the layers of stratified rock being spread beneath the sea in continental deltas has therefore been preceded by many similar cycles. Again and again movements of the crust have brought to an end one cycle--sometimes when only well under way, and sometimes when drawing toward its close--and have begun another. Again and again they have added to the land areas which before were sea, with all their deposition records of earlier cycles, or have lowered areas of land beneath the sea to receive new sediments. =The age of the earth.= The thickness of the stratified rocks now exposed upon the eroded surface of the continents is very great. In the Appalachian region the strata are seven or eight miles thick, and still greater thicknesses have been measured in several other mountain ranges. The aggregate thickness of all the formations of the stratified rocks of the earth's crust, giving to each formation its maximum thickness wherever found, amounts to not less than forty miles. Knowing how slowly sediments accumulate upon the sea floor (p. 184), we must believe that the successive cycles which the earth has seen stretch back into a past almost inconceivably remote, and measure tens of millions and perhaps even hundreds of millions of years. =How the formations are correlated and the geological record made up.= Arranged in the order of their succession, the formations of the earth's crust would constitute a connected record in which the geological history of the planet may be read, and therefore known as the _geological record_. But to arrange the formations in their natural order is not an easy task. A complete set of the volumes of the record is to be found in no single region. Their leaves and chapters are scattered over the land surface of the globe. In one area certain chapters may be found, though perhaps with many missing leaves, and with intervening chapters wanting, and these absent parts perhaps can be supplied only after long search through many other regions. Adjacent strata in any region are arranged according to the _law of superposition_, i.e. any stratum is younger than that on which it was deposited, just as in a pile of paper, any sheet was laid later than that on which it rests. Where rocks have been disturbed, their original attitude must be determined before the law can be applied. Nor can the law of superposition be used in identifying and comparing the strata of different regions where the formations cannot be traced continuously from one region to the other. The formations of different regions are arranged in their true order by the _law of included organisms_; i.e. formations, however widely separated, which contain a similar assemblage of fossils are equivalent and belong to the same division of geological time.