The Evolution of the Sciences 



the wrinkles of its crust indicate the shrinkage 

 of its volume. 



But then another question arises. What 

 stage of cooling has the earth actually reached? 

 Is it still liquid in its interior, and, if so, how 

 thick is the solidified layer? Or is it only locally 

 that cavities are to be found full of still liquid 

 matter? Or again, is the entire mass solid? 



In order to avoid ambiguity in the examina- 

 tion of this question we intend to confine our- 

 selves to facts, and to inquire into the present 

 means at our disposal for ascertaining what is 

 going on within the earth. Among the chief 

 of these may be mentioned the study of strati- 

 fications, of magnetism, of the variation of 

 temperature with depth, and finally the study 

 of the movements of the ground and of eruptions. 



Hitherto the principal work of the geologist 

 has consisted in differentiating the various 

 formations, ascertaining the chronological order 

 in which they were laid down, their form and 

 superposition. Perhaps the most admirable 

 feature of this work is the certainty with which 

 it has enabled a knowledge of the deep-lying 



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