Nature-Study Agriculture 



The 



breaking 



effect of 



expansion 



and 



contraction 



FIG. 1 6. Chemical action, changes in temperature, vegetation, and the action 

 of the stream are all at work in forming soil in this Rocky Mountain canon. 



night to day and from summer to winter, the uneven 

 expansion and contraction of the different mineral 

 particles tends to break the stones. Nature's process 

 was imitated, we are told, when some of the roads were 

 constructed in the Yosemite Valley : fires were built 

 upon the obsidian rock or " volcanic glass " that was 

 to be removed; then cold water was thrown upon the 

 heated surface, cracking it. 



