THE EARTH'S SHAPE 



fact that the layers lie one above the other we are 

 furnished with a simple clue which enables us to deter- 

 mine the order of their formation. We may know 

 nothing whatever as to how old they are, measured by 

 years or centuries. But we can be absolutely certain 

 that the bottom layer came first, and the top layer came 

 last. This kind of observation will enable us to find 

 proofs everywhere that the surface of the land has not 

 always been what it is to-day. In some districts, for 

 example, when the dark layer of soil in which vegetables 

 grow is turned up, there may be found beneath it sand 

 and gravel full of smooth, well-rounded stones. Such 

 materials are to be seen in course of formation where 

 water keeps them moving to and fro, as on the beds of 

 rivers, the margins of lakes, or the shallow shores of the 

 sea. Wherever smooth-rolled pebbles occur they point 

 to the influence of moving water, so that we conclude, 

 even though the site is now dry, that water once moved 

 above it. Again, below the soil in other regions lie 

 layers of oysters and other sea shells. 



Pits, quarries, and mines that cut down still deeper 

 into the earth and lay it bare bring before our eyes most 

 impressive testimony regarding the ancient changes of the 

 land. Suppose, by way of further illustration, that un- 

 derneath a bed of sand full of oyster shells there lies a 

 dark brown band of peat. This substance, composed of 

 mosses and other water-loving plants, is formed in boggy 

 places by the growth of marshy vegetation. Below the 

 peat there might occur a layer of soft white marl full of 

 lake shells, such as may be observed on the bottom of 



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