THE FORMATION OF SOIL 13 



simple in character. These so-called weather- 

 ing agents may be grouped as follows : 



First, variations of temperature in other 

 words, the lower temperature of winter as 

 compared with summer, and the greater 

 warmth of the day as compared with the night 

 have had much to do with the breaking 

 up of rocks and the disintegration of stones. 

 This result would not have followed, to any 

 great extent, if rocks and stones were ab- 

 solutely uniform throughout ; but this is 

 far from being the case, most stones, and the 

 rocks from which they have been derived, 

 being complex in composition and containing 

 many substances. One of the simpler rocks 

 is granite, the main constituents of which 

 are quartz, felspar, and mica, and these three 

 substances show varying degrees of expansion 

 and contraction under changes of temperature, 

 that is to say, they do not all expand and 

 contract equally. It is evident, therefore, 

 that, even if a rock hold no more than two 

 or three substances, there must be a strong 

 tendency for the constituents to work apart, 

 so that cracks or fine fissures are formed 

 where the different substances come into con- 

 tact. Whenever a rock or stone is no longer 

 solid, but is traversed more or less by a net- 

 work of fissures, even if they are so fine as 



