GEOLOGY 



2434 



GEOLOGY 



temperature, and were it not for its influence 

 the burning heat of the day would be followed 

 by intense cold at night. Nevertheless, there 

 is sufficient change in temperature to cause 

 freezing and thawing over large portions of 

 the earth, and these changes in temperature 

 cause the surface of the rock to break into 

 fine particles, or, as the scientists put it, to 

 disintegrate. 



Again, some of the gases in the atmosphere 

 combine with the rocks and form new com- 

 pounds which, because they are not as hard 

 as the original rock, soon crumble. The fine 

 particles of rock and soil found on and around 

 large rocks and at the foot of cliffs are formed 

 chiefly by this quiet work of the atmosphere, 

 which is known as weathering. 



Wind. The atmosphere is constantly in 

 motion, and currents of air, like currents of 

 water, carry along more or less solid matter. 

 An ordinary wind will move fine sand and a 

 strong wind will move gravel ; consequently, 

 over some portions of the earth winds are con- 

 stantly changing its surface. These changes 

 are most striking in sandy regions, like Dune 

 Park, Indiana, on Lake Michigan, where the 

 sand is drifted like snow, and in desert lands, 

 where the surface is very dry. In other re- 

 gions the constant hurling of these particles 

 of sand and dust against the rocks gradually 

 wears them away and carves them into fan- 

 tastic forms, as in Monument Park, Colorado. 

 Moreover, however clear the air may appear 

 to us, it always contains dust, and this dust 

 is so fine that even the slightest current moves 

 it. The particles of dust are so small and this 

 movement is so gentle that we do not perceive 

 them, but geologists estimate that this move- 

 ment going on through all the ages since the 

 earth was formed has caused marked changes 

 on its surface. 



Rainfall. The amount of water vapor in the 

 atmosphere depends upon the temperature. 

 The warmer the air becomes the more water 

 it can contain. When the temperature is low- 

 ered sufficiently, the air becomes saturated with 

 moisture, and as no more can be held some of 

 the vapor falls in the form of rain or snow 

 (see RAIN; SNOW). Some of the rain soaks 

 into the ground and some runs down the 

 slopes into the streams, carrying more or less 

 soil with it. This soil is carried to the lower 

 levels of the streams and there deposited as 

 silt. Thus rainfall is an important agency in 

 wearing down the higher and building up the 

 lower levels. 



The Hydrosphere. The hydrosphere, or 

 water sphere, includes the water in the ocean 

 and on the land. These different bodies of 

 water are described in these volumes under 

 their titles, as LAKE; RIVER; SPRING; OCEAN. 

 Our purpose here is to show how they are 

 causing changes on the earth's surface. 



The Ocean. The ocean covers about three- 

 fourths of the earth's surface, but owing to the 

 position of the continents it is irregularly dis- 

 tributed and .is gathered in a series of great 

 basins. We are here especially interested in 

 the action of the ocean in changing the sur- 

 face of the land; for other particulars, see 

 OCEAN. 



Waves raised by wind and tides wear away 

 the rock in many places, and deposit the sedi- 

 ment on the beach and on the bed of the 

 ocean. In many places waves strike the coast 

 with great force and they often wear caves in 

 the rocks, or cut them into curious forms. 

 They are one of the most powerful agents in 

 wearing away the coasts of the continents. 



While ocean currents move so gently that 

 they do not affect the form of the coast line, 

 they exert a great influence on climate, includ- 

 ing rainfall, and in this way indirectly cause 

 changes on the land. The ocean contains 

 many plants and animals. When these die 

 they settle to the bottom, and in this way the 

 bed of the ocean is being constantly though 

 very slowly raised. However, this change is 

 so slight that in considering the story of the 

 earth we need give no heed to its present 

 influence. 



Water. The effect of running water is de- 

 scribed in the article EROSION. Lakes on a 

 small scale cause the same changes that the 

 ocean does on a large scale. Mineral springs 

 often form solid deposits which are of interest 

 because they show how some rocks have been 

 formed in the past. These formations are so 

 small that they are scarcely worthy of con- 

 sideration when studying the changes on the 

 earth's surface as a whole. Ice and snow are 

 ever wearing down the mountains and filling 

 up the valleys. See GLACIER. 



There is another way also in which water 

 is working changes and which we seldom con- 

 sider because it is out of sight. Whence comes 

 the water that bubbles up in springs, fills our 

 wells and feeds the thirsty plants? It comes 

 from the ground, and is generally known as 

 ground water. The quantity of ground water 

 in a given locality is largely dependent on the 

 rainfall. The height to which it rises, that is, 



