18 ELEMENTARY GEOLOGY 
When we study it in detail, the crust of the earth is 
found to be very irregular. Considered in a large way, 
it is a sphere, and even the greatest elevations and 
depressions are tiny in comparison with the mass of 
the earth itself. The largest irregularity is that which 
is known as the equatorial protuberance. This depends 
upon the fact that the earth is not a true sphere, .but 
a spherical body somewhat flattened at the poles (an 
oblate spheroid), so that the diameter at the equator 
is 7925.6 miles, and at the poles 7899.1 miles. There 
is nothing in the general appearance of the earth, as 
we see it, to tell of this deviation from the sphere; but 
its existence has been proved by very careful meas- 
urements. 
The continents and ocean basins constitute a second 
great set of irregularities. Between the deepest parts 
of the ocean and the highest portions of the land, there 
is a difference in elevation of more than ten miles. If 
the sea-water were removed from the globe, the site 
of the oceans would be occupied chiefly by broad 
plains, while the continents would rise as great table- 
lands from these low, level areas. The slope between 
these two extensive plains is steep (Figs. 1 and 185). 
Both from the ocean floor and from the platform of 
the land, chains of mountains rise, sometimes extend- 
ing for thousands of miles, and in places reaching an 
elevation of more than five miles above the level of the 
