248 THE EARTH'S CRUST 



Whenever borings have been made into the interior of the 

 earth it has been found, after a depth has been reached where 

 there is no effect from the heat of the sun, that the tempera- 

 ture rises as the depth 'increases. From this gradual in- 

 crease in temperature, it must be that far down within the 

 earth the temperature is very high. The pressure within 

 the earth is so great, however, that rocks at great depths 

 are probably not in a molten condition. If the earth had 

 a liquid interior, the attraction of the other bodies of the 

 solar system would cause changes in its shape ; but it is as 

 rigid as steel. 



The outside cold part of the earth is called its crust. How 

 thick this is, no one knows. This is the part of the earth 

 that is of particular interest to us, for it is the only part 

 that we are able to observe and study. It is impossible 

 for us to conceive the eons of time that passed while the 

 earth's exterior was cooling and changing, and coming into 

 the condition in which we know it. Geologists think in 

 tens and hundreds of thousands of years. The mountains 

 that we see and even the continents we live on are the 

 product of very recent changes, as geologists measure time, 

 in the unimaginably long ages that reach back to the first 

 gathering together of matter forming the earth. 



Experiment 80. When at home measure the greatest and least 

 circumference of a large, smooth apple by winding a string around 

 it and then unwinding and measuring the length of the string. 

 Bake the apple. Measure its circumferences again. Are they 

 greater or less than before? Is the skin of the apple as smooth as 

 it was before? 



There is every reason to believe that the interior of the 

 earth is still cooling and contracting. Since the crust is 

 already cooled, it has ceased to contract. Thus as the 



