INSIDE OF THE EARTH 



49 



"molten lava" they throw out when they are in eruption 

 comes from the depths of the earth. So it seems that, as 

 you go down, the heat increases as well as the pressure. 

 Judged by conditions in deep mines, the thermometer goes 

 up at the rate of about one degree for every hundred feet 

 of depth. This varies a great deal with different localities, 

 and for the first hundred feet or so it usually does not 



apply at all. In . , 



fact, cellars and 

 caves are usually 

 much cooler than 

 the air above. 

 Why is this so? 



3. The earth 

 was once very hot; 

 so hot that noth- 

 ing at the surface 

 was solid. It was 

 all either liquid or 

 gas. The cooling- 

 off process and the 

 formation of solid rocks probably required millions of 

 years. The changes that have occurred to the land areas 

 since they were solidified have been produced chiefly by 

 running water, and by movements of the surface that 

 have changed sea to dry land and dry land back to sea 

 again. To-day we find far inland, and often deep under 

 the surface, sure evidence that the sea was once there. 

 We find accumulations of sea-shells', and formations of 

 limestone that were undoubtedly made from sea-shells 

 (see Fig. 9). We also find under the surface deposits of 

 coal, and coal is a formation due to the accumulation of the 



FIG. ig. Volcano. 



