10 



PHYSIOGRAPHIC GEOLOGY. 



oceanic depression ; the subdivision of the land into continents ; 

 the height and kinds of surface of the continents. 



13. (1.) Spheroidal form. — The earth has the form of a sphere 

 with flattened poles, the distance from the centre to the pole being 

 about l-300th (accurately, ^02-5) shorter than from the centre to 

 the equator. The earth's equatorial radius being 3963 miles, the 

 polar is about 13J miles less (exactly 13.2465 miles). 



This is a fact of prime importance in geology, and an appropriate introduction 

 to the science, inasmuch as it is the most obvious proof that the earth has a 

 history, or has been in course of progress under secondary causes ; for this flat- 

 tening is in amount just that which the revolution at its actual rate would pro- 

 duce in a liquid globe having the size and density of the earth. 



14. (2.) General subdivisions of the surface. — Proportion of 

 Land and Water. — In the surface of the sphere there are about 8 

 parts of water to 3 of dry land, or, more exactly, 275 to 100 = 5 2 : 3 2 . 

 The proportion of land north of the equator is nearly three times as 

 great as that south. The zone containing the largest proportion 

 of land is the north-temperate, the area equalling that of the 

 water ; while it is only one-third that of the water in the torrid 

 zone, and hardly one-tenth (2-21ths) in the south-temperate. 



Out of the 197,000,000 of square miles which make up the entire surface of the 

 globe, 144,500,000 are water, and 52,500,000 land. In the northern hemisphere 

 the land covers 38,900,000 square miles, in the southern 13,600,000 square miles. 



15. Land in one hemisphere. — If a globe be cut through the centre 

 by a plane intersecting the meridian of 175° E. at the parallel 

 of 40° N., one of the hemispheres thus made, the northern, will 



Fig. 1. 



165 



is — 5 — 15 1& CT 15 



contain nearly all the land of the globe, and the other be almost 

 wholly water. The annexed map represents the two hemispheres. 



