5 8 EVOLUTION OF THE EARTH 



more quickly. If the erosion on the land is rapid and the 

 rivers are not long, the land debris is apt to be coarser in grain 

 and less well assorted, but of whatever nature the waste of 

 the lands is, the rivers finally deliver it to the seas and oceans; 

 and here the materials are subjected to a long process of re- 

 working and assorting into muds and sandstones. As all of 

 this work is done in waters, as the character of the sediments 

 varies, and as the wave-power is variable with the winds from 

 day to day, it follows that the materials must be laid down in 

 horizontal sheets of different kinds of rock that make up the 

 strata of the stratified rocks. From this it is apparent why 

 the sedimentary rocks of the earth's outer shell not only are 

 of different kinds, but why they so often occur in more or less 

 long cycles of deposits that begin with coarse sandstones or 

 even conglomerates and pass upward into the much finer- 

 grained mudstones and finally into limestones. The limestones 

 are in the main the deposits due to the chemical activities of 

 organisms or the accumulating debris of plants and animals. 

 Geologists therefore speak of limestones and chalks as organic 

 deposits. So throughout the geologic ages, cycle upon cycle 

 of sedimentation succeeded one another, the debris of suc- 

 cessive ranges of once majestic mountains, and in this orderly 

 sequence of deposits lies buried much of the life of the past. 



The United States Geological Survey stated in 1912 that 

 the surface of the United States is being removed by solvent 

 denudation at the average rate of .0013 of an inch a year, 

 or i inch in 760 years. This means a removal by the streams 

 of over 270 million tons of dissolved matter and 513 million 

 tons of suspended matter annually. In other words, there is 

 delivered into the seas and oceans each year from the area 

 of the United States alone 783 million tons of rock materials. 

 The amounts removed from different drainage basins show 

 interesting comparisons. In respect to dissolved matter, the 

 southern Pacific basin heads the list with 177 tons per square 



