24 The Make-Up and the Wor\ings of the Earth 



find shells resembling those of microscopic one-celled plants 

 living in lakes and oceans today. The formation of sandstone, 

 chalk, limestone, diatomaceous earth, and coal, by the sedi- 

 mentation of the materials of which they consist, must have 

 been a very slow process, judging by the rate at which sedi- 

 mentation today takes place in seas and lakes, and by the 

 multitude of living things represented by even a very thin 

 layer of one of these rocks. 



Layers of Crust 



In the gorge of Niagara, in the Grand Canyon of the 

 Colorado, in most other deep valleys we can see layers or 

 strata of soil and rock on opposite sides of the cut. On a 

 smaller scale we can observe the same appearance in an artifi- 

 cial cut made for a railroad, or even for a ditch. In the latter 

 cases we can see at once that the layers on one side of the 

 cut correspond to layers on the other side because continuous 

 layers have been interrupted by the digging. The various 

 layers differ in color, in the coarseness or fineness of the parti- 

 cles, in the character of the materials. These characteristics 

 enable us to identify the corresponding layers on opposite 

 sides. We may also see that there is the same order or ar- 

 rangement of the distinguishable layers. Having concluded 

 that the gorge or valley has been cut through the rocks and 

 soil by the action of moving water, as the railroad cut has 

 been produced by the action of moving picks and shovels, 

 we assume that the layers on one side were at one time con- 

 tinuous with the corresponding layers on the other side, as 

 is the case in the artificial cut. 



These strata happen to be practically horizontal in many 

 parts of the earth, and the character of the material is such 

 that, although nobody ever saw them being formed, we feel 

 safe in assuming that they were produced by the settling of 

 solids out of water and the subsequent compression of the 

 settled sand or mud into the hard layers. They are, at any 

 rate, just what we should expect them to be // they had 

 been produced in this manner. 



