GEOLOGICAL TEXT-BOOK. 



CHARACTER AND OBJECTS OF GEOLOGY. 



The science of Geology consists in a systematic arrange- x)efinltion. 

 ment of facts, explaining the structure of the earth. 



Our observations are limited to its exterior rind or coats, observa- 

 We know very Uttle of its interior structure. But the ine- ^'""^^^^ 

 qualities of its surface often give us admission to a consider- 

 able depth ; from which we should be totally excluded were 

 its surface every where smooth like a pacific sea. 



Since the earth's specific gravity has been ascertained by Specific 

 suspending the plumb-line by the side of an insolated mountain f^e garthf 

 rock, we infer that it must be made up of heavier materials 

 interiorly than near its surface. For the whole mass of the 

 earth is found to weigh about five times as much as an equal 

 mass of water. Whereas all its exterior rind, from the sur- 

 face to its greatest ascertained depth, taken in the aggregate, 

 weighs but about half as much. Late experiments by Cordier 



, , ,1 • , . Internal 



and others, seem to lead us to important conclusions respect- heat, 

 ing the internal temperature of the earth. It is proved, that 

 as we penetrate the earth, the temperature diminishes until 

 we are below the limit of solar influence. And from that point 

 the temperature increases at the rate of about one degree of 

 Fahrenheit for every fifty feet. But it is not probable, on any 

 hypothesis, that heat increases in this ratio to the center of 

 the earth. If it did, the whole interior of the earth would be 

 in the state of melted lava ; leaving, according to Cordier, a 

 solid cruBt less than one hundred miles in thickness — not ex- 

 ©eeding in proportion the thickness of an egg-shell. 



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