THE GENERAL FEATURES OF THE EARTH 17 
the dry land is exposed. It also serves to modify cll- 
mate, and is the home of a great abundance and variety 
of animal life. 
In composition, the ocean contains only three or four 
per cent of salty impurities, the remainder being pure 
water. There is a considerable variation in this re- 
spect, for off the mouths of rivers the sea-water is 
freshened, as it is also in regions of heavy rainfall. 
On the other hand, in warm, dry, enclosed seas, like 
the Red Sea and the Mediterranean, nearly constant 
evaporation carries away the fresh water, and makes 
the remainder more salt. 
Usually the waters of the ocean surface are warm, 
being well above freezing-point, though in the Arctic and 
Antarctic zones the temperature is always low, and in 
winter the surface is frozen. Also in the deeper parts of 
the ocean, the temperature of the water near the bottom 
is not far above the freezing-point. This is because of 
a great, slow movement of cold water along the ocean 
bottom, from the polar regions toward the equator. 
The Solid Earth. — Surface Form. Wherever a part 
of the solid earth is exposed above the ocean, it is 
found to be composed of rock; and this is generally 
hard rock with a soil covering of greater or less depth. 
We speak of this as the earth’s crust, and it is the 
only portion of the solid earth with which we are 
familiar from actual examination. 
Cc 
