34 FARM DEVELOPMENT 



and animals that have lived at the bottom of the sea. 

 This shows that forces are at work which are con- 

 tinually modifying the physiography of the earth, level- 

 ing some parts and elevating other parts above their 

 original position. The rains falling on the high places 

 of the earth's surface washed the looser particles to the 

 low places and with them the forms of life that then 

 existed, thus leaving a record of the times in the fossils 

 preserved in the layers thus formed. In some cases a 

 deposit of earlier records has been raised and washed 

 down a second time, and here the record is confused, for 

 a layer of rock or clay may thus contain forms that 

 existed centuries apart in point of time. 



We think of the earth as being very stable, but crustal 

 disturbances, as earthquakes and volcanoes, occur fre- 

 quently at many parts of the globe. The formation of 

 mountain ranges is wonderful and can be appreciated 

 only by those who have been on the mountains and have 

 been awed by their stupendous proportions. These 

 mountain ranges may be accounted for, in part, by the 

 fact that the earth is cooling from the outside toward 

 its center, and, as it loses heat, it becomes smaller and 

 the surface, being hard, bulges up in places and settles 

 down in others, as does the rind of an apple when it is 

 drying. 



Development of present land surfaces. The layers of 

 the earth which are now exposed have been modified 

 not only by the action of heat and cold, wind and water, 

 but also by animals and plants. During recent 

 geological times great changes in temperature have 

 caused immense collections of ice, called glaciers, to lie 

 upon and flow or glide very slowly over large areas of 

 land near the north and south poles of the globe. These 

 glaciers, which once pushed much farther out from the 

 poles toward the equator than now, have done much to 

 transport the particles of soil from place to place, mix- 



