THE SOIL MAKERS II 



must contract; because, you know all matter expands 

 when heated and becomes smaller when cooled. With 

 the cooling of the earth its outer clothing was drawn in, 

 with the result that it was wrinkled hills here and high 

 mountains there which continued so long as the con- 

 tractive force was greater than the holding force of the 

 crust. In all this work changes were taking place. Huge 

 beds of rock were thrown up and exposed in an hundred 

 places to air and moisture, where before they were so 

 snugly covered that neither could enter. 



The earth continued to cool and in some places ice 

 formed. Vapor condensed and dropped as rain. For cen- 

 turies rain had fallen, but as it struck the hard earth it was 

 flung back into the air again as vapor and mist. As the 

 earth gradually cooled, water was thrown back with less 

 vengeance and force. Some of it was left for a consider- 

 able time on the earth, where it had collected in basins, or 

 in crevices in the rock. It was caught here at times by 

 wind-storms that were cold enough to freeze this gathered 

 water. As the water froze, it expanded, forcing many 

 crevices wider, breaking many rocks asunder and doing 

 what we are pleased to call its share in soil making. 



It is this change in temperature that assists in soil 

 making that weakens the original rocks that were ages 

 ago forced from the very bowels of the earth. 



Rocks such as the granite type when alternately heated 

 and cooled for a long time gradually weaken and break. 

 Sudden changes in temperature produce similar results. 

 Temperature is more active when moisture is present. 

 Even in the modern world we see stone buildings, that 

 frequently drop a corner or a slab, due to sudden freezing 

 when saturated with water. You recall with what ease 

 the same may be done with a hammer on a cold day. 



Since nearly all rocks, even those deeply imbedded in 



