INTERIOR HEAT OF THE EARTH 309 



generally accepted theory that the whole earth was once 

 a glowing liquid mass, like melted iron. As this heated globe 

 continually gave off large amounts of heat into space, a 

 solid crust was formed around the melted interior. When 

 a body cools, it contracts. The contraction of the globe 

 caused its solid crust to be thrown up into huge wrinkles, 

 which are now the mountain ranges ; while the lower places 

 were filled up by the waters of the ocean. Since that time, 

 many shallow seas have been partially filled up by deposits 

 carried into them by rivers and waves, and have later been 

 lifted up, and become dry land. Over three-fourths of the 

 surface rock in North America is stratified, and contains 

 marine fossils, which is positive proof that the ocean once 

 covered all of this area. At the present time, layers of 

 ocean-made rock form the very peaks of many snow-clad 

 mountains. In other places, land, once formed, sank again ; 

 and forests, fields, and towns were buried beneath the 

 waves. Changes like the ones just described have been 

 going on ever since there was land and water on the globe, 

 and they are still going on ; but with the exception of those 

 taking place on the shores of the ocean, and those resulting 

 from severe earthquakes and volcanic eruptions, they take 

 place so slowly that only trained observers notice them. 

 Do rivers raise or lower the land ? Do our lakes become 

 deeper, or can you detect agencies that tend to fill them in ? 

 The teacher should point out how streams and rivers have 

 carved, or eroded, soil and rock. 



REFERENCES 



Tarr. Physical Geography. The Macmillan Company. 

 Tarr. Elementary Geography. The Macmillan Company. 

 Heilprin. The Earth and its Story. Silver, Burdett & Co. 



