PART II. 

 STRUCTURAL GEOLOGY. 



CHAPTER I. 



GENERAL FORM AND STRUCTURE OF THE EARTH. 



General Form. 



THE general form of the earth is that of an oblate 

 spheroid flattened a little at the poles. In other words, 

 it is an ellipsoid of revolution about its minor axis. The 

 equatorial diameter is about twenty-six miles greater than 

 the polar diameter. This general form is taken at sea- 

 level, the land-surfaces rising above and the sea-bottoms 

 sinking below. This form is precisely that which a liquid 

 globe would inevitably assume under the influence of ro- 

 tation. It has, therefore, been somewhat hastily concluded 

 that this general form is demonstrative evidence of the 

 early incandescent liquid condition of the earth. It is 

 certain, however, that the earth would have assumed this 

 form by rotation, whether it were originally liquid or 

 solid.* Therefore, while it is almost certain, from other 

 considerations, that the earth was once liquid, and assumed 

 its oblate spheroid form in that condition, yet this gen- 

 eral form alone can not be regarded as proof of that con- 

 dition. 



General Structure. We have already stated (page 



* This subject is more fully explained in the author's " Elements 

 of Geology." 



173 



