180 INTRODUCTION TO GENERAL SCIENCE 



4. 1601 : 9-15. The Atmosphere and its Work. 



5. 1601 : 21-26. Over and Over Again. 

 a. 1202 : 255-266. Rate of Weathering. 



fr. 1203 : 13-19. The Decay of Rocks and the Causes. 



c. 1206 : 124-127. Difference in the Rate of Weathering. 



d. 1209 : 16-17. Various Rates of Weathering. 



e. 1301 : 254-257. The Effects of Weathering. 

 /. 1302 : 210-224. Land Sculpture. 



g. 1303 : 136-139. The Wasting of the Lands. 

 h. 1307:111. Weathering Processes. 



128. AGENTS OF WEATHERING 



Without doubt, rain produces the greatest visible change 

 upon rocks and the soil, yet probably the action due to the air 

 and to certain microbes accomplishes an amount not fully 

 realized. The agents of weathering, then, are rain, and, in 

 fact, water in all its forms, for large masses of snow and 

 ice accomplish work not possible to water alone, heat and 

 cold, the oxygen of the air, and various microorganisms, as 

 well as some plants and trees. 



We learned when we were studying heat that it causes the 

 expansion of all material, and the expansion is pretty nearly 

 proportional to the temperature. Where the sun shines on 

 exposed rocks, their surface becomes hot much faster than 

 their interior. This causes their surface to expand faster 

 than their interior, making the outside of the rock too large to 

 fit on the inside, so that there is a separation of a thin layer. 

 When the rock becomes cool at night, the reverse action 

 takes place. The outside contracts more rapidly than the 

 inside, since radiation takes place from the surface. Then the 

 outside is too small for the inside, and peels off. We call this 

 exfoliation, and if we are on the watch, we may see-* many 

 evidences of this action. 



