6 GEOLOGY 



A thermal blanket. The atmosphere is a thermal blanket to 

 the rest of the earth. In its absence the heat of the sun would 

 reach the surface with much greater intensity than now, and it 

 would be radiated back from the surface almost as rapidly as 

 received. During the night, there would be a degree of cold far 

 greater than that now known to any part of the earth. In passing 

 through the atmosphere, certain portions of the radiant energy of 

 the sun are absorbed. Of the remainder which reaches the surface 

 of the earth, a part is transformed into vibrations of lower intensity 

 which are then more effectively retained by the atmosphere. The 

 air thus distributes and equalizes the temperature. The two con- 

 stituents of the atmosphere which are most efficient in this work 

 are water vapor and carbon dioxide, and the climate of the earth 

 is believed to have been very greatly affected by the varying 

 amounts of these constituents in the atmosphere, as well as by 

 variation in the total mass of the atmosphere. 



The function of the atmosphere in sustaining life and promoting 

 all that depends on life is obvious. 



2. The Hydrosphere 



The water which lies upon the surface of the solid earth, is about 

 TVo P ar "k of the earth's mass. Were the solid part of the earth 

 perfectly spheroidal, this amount of water would constitute a 

 universal ocean a little less than two miles deep. Owing to the 

 unevenness of the surface of the lithosphere, the water is chiefly 

 gathered in great basins or troughs, occupying nearly three-fourths 

 (72%) of the earth's surface. These basins are all connected, so 

 that anything which changes the level of the water in one, changes 

 the level in all. 



Oceanic dimensions. The surface area of the ocean is estimated 

 at 143,259,300 square miles. The area of the true oceanic basins 

 is only about 133,000,000 square miles, but these basins are some- 

 what more than full, and the ocean water laps up on the cont incut al 

 shelves to the extent of more than 10,000,000 square miles. If 

 about 600 feet of the upper part of the ocean were removed, the 

 true ocean basins would be just full. Beneath about 20% of 

 the ocean area, the bottom sinks to depths of between 6,000 



