CHAPTEE II. 

 GEOLOGY OF AGRICULTURE 



FORM OF THE EARTH— ITS DENSITY— PROPOR- 

 TION OF LAND AND WATER— INEQUALITY OF 

 SURFACE— WEIGHT OF ATMOSPHERE— CRUST 

 OF THE EARTH. 



66. The earth has the form of an oblate spheroid, 

 having an equatorial diameter 26 miles greater than 

 its polar diameter. As this is the form, very nearly, 

 which a fluid body would naturally assume, if revolv- 

 ing on its axis at the same rate, a fair inference is, 

 that the earth was once in a fluid state. Its average 

 weight is about 5 times that of water, and not far 

 from twice and a half that of common rock. 



67. About one fourth of the earth's surface is drv 

 land, and three fourths are water. The land occupies 

 not far from 50 million square miles, and the water 

 about 150 million. The highest peaks of dry land are 

 nearly six miles above tide water, and the lowest 

 depths of the oceans are probably somewhat farther 

 below. These inequalities affect the roundness of the 



