PAET II. 

 STRUCTURAL GEOLOGY. 



We have thus far studied causes noiv in operation or dynamical 

 principles. We now study the structure of the earth (which is the 

 effect of the same accumulated throughout all geological time), and 

 the application of the foregoing principles in its explanation. The 

 subject of this part, therefore, is both structural and dynamical ge- 

 ology. 



CHAPTER I. 

 GENERAL FORM AND STRUCTURE OF THE EARTH. 



1. — Form of the Earth, 



The form of the earth is that of an oblate spheroid flattened at 

 the poles. More definitely, it is a solid generated by the revolution 

 of an ellipse on its minor axis. The polar diameter is less than the 

 equatorial diameter by about twenty-six miles, or about 3-^- of the 

 mean diameter.* The highest mountains, being only five miles high, 

 do not interfere greatly with the general form. 



This form, being precisely that which a fluid body revolving freely 

 would assume, has been regarded by many of the most distinguished 

 physicists as conclusive evidence of the former fluid condition of the 

 earth. The argument may be stated as follows : 1. A fluid body 

 standing still, under the influence only of its own molecular or gravi- 

 tating forces, would assume a perfectly spherical form ; but, if rotating, 

 the form which it would assume, as the only form of equilibrium, is 

 that of an oblate spheroid, with its shortest diameter coincident with 

 the axis of rotation. 2s~ow, this is precisely the form not only of the 

 earth, but, as far as known, of all the planetary bodies. 2. In an ob- 

 late spheroid of rotation the oblateness increases with the rapidity of 



* More exactly 3-9-^.3-, Philosophical Magazine, vol. x, p. 121, 1880. 



