GEOLOGICAL HISTORY OF THE EARTH 35 



ing the stony substances together, or depositing them in 

 layers, as the case might be. Water and winds have also 

 done much in this mixing of soils. In moist, low places 

 vegetation has grown and been preserved by water cov- 

 ering it, and thus collected into beds of peat. Peat beds 

 thus made in former times, and afterward covered with 

 clay, in some cases have been gradually transformed 

 into coal. 



In other places, where the surface of the earth is com- 

 posed of solid rock, of hard stones, or even of gravel or 

 sand, which does not easily decay, vegetation has not 

 been able to grow and the surface is still bare and not 

 hospitable to plants. Between these two extremes, 

 where soils have been made of clay or of a mixture of 

 clay and sand and other materials, plants have found a 

 congenial home. The soils have an abundance of plant 

 food ; they are rich in humus or decaying vegetable mat- 

 ter, and, therefore, able to hold moisture, and to provide 

 bacterial and other agencies which aid in producing 

 conditions suitable for the feeding by the plant roots. 

 In some places these congenial soils are formed by the 

 rock decaying where it lies; in other places they are 

 formed by the water washing particles from many 

 places, thus mixing them together and spreading them 

 out in layers ready to form the home of plants. In 

 other places the winds bring together soil particles in a 

 mixture which is adapted to the growth of plants. 



Igneous rocks (rocks that are formed from the molten 

 lava from the interior of the earth, or that at least have 

 been heated) are found at many places on the surface of 

 the earth, as where they were poured through volcanoes 

 or oozed out through openings on the earth's crust. 

 Sand and gravel are found in other places where they 

 have been deposited by the action of water. All these 

 form inhospitable soils. The granitic and other igneous 

 rocks are not open to the penetration of plant roots, and 



