LESSONS IN GEOLOGY. 847 



every wave, and are soon worn to sand and mud. In the river 

 the pieces of rock move forward, and are more nearly spherical 

 in form, while on the beach they move backward and forward 

 and soon become flattened, and are sometimes called shingle. 

 The sand resulting may form sand bars parallel with the shore, 

 or may be transported to other localities by ocean currents, or 

 may be built into dunes by the winds. 



The different forms of life often are of great importance as 

 geological agents. Plants, as mosses and liverworts, by keeping 

 the surface of rocks moist, promote their decay, and in the decay 

 of their leaves and stems organic acids are formed which are 

 active oxidizing agents. These acids, for the most part, become 

 carbon dioxide, and in one form or the other are the great sol- 

 vents of the mineral world, often penetrating rocks to a great 

 depth. Plants and trees also aid erosion in a mechanical way, 

 as their growing roots open cracks and seams in the rocks. 



Turf and trees protect from erosion. Forests protect hillsides 

 and sand dunes; the sand-grass and other plants also protect 

 the same from the wind. Mangrove swamps along the seashore 

 are especially effective in reclaiming and making land. While 

 live plants protect, dead and decaying plants often open the soil 

 to the erosive action of wind and rain. Decaying vegetation en- 

 riches soils, forms muck beds, peat bogs, etc., from which all the 

 coal and much of the rock oil and natural gas were formed. 



Burrowing animals of all kinds, earth worms, cray-fish, open 

 the soil to the action of oxidizing agents, and often to the action 

 of erosive agents as well. Beavers build dams, making ponds 

 and swamps, while cray-fish, rats, etc., open holes through banks 

 which lead to destructive erosion. Some mollusca bore holes in 

 rocks and wood, thus promoting their disintegration. The shells 

 of the mollusca, the framework of corals and other animal 

 organisms have made up most of the calcareous or limestone 

 rocks. And the coral polype, in the tropical sea, is building 

 islands and reefs of coral limestone, while diatoms, and some 

 forms of animal life, are furnishing material for silicious deposits 

 of great extent. 



