178 THE LURE OF THE LAND 



ROCKS DIFFER IN SPEED OF WEATHERING. 



" It is also a matter of observation that certain kinds 

 of rocks are much more sensitive to weathering than 

 others. In the selection of permanent building stones 

 the property of rocks to withstand weathering is ex- 

 tremely important. The breaking down of building 

 stones is usually ascribed to the freezing and thawing 

 process. Those stones which permit the freest entry 

 of moisture, thus storing up water which, when frozen, 

 expands, are as a rule those that most rapidly give way. 

 The action of freezing and thawing, however, has been 

 perhaps somewhat over-rated. In warm or even tropi- 

 cal climates, where these agents are never active in the 

 disintegration of rocks, some of the deepest and best 

 weathered soils are produced. In point of fact, a long 

 hard winter, where there is no thawing at all, tends 

 to preserve the rocks from decay rather than to favor it. 



" There are many facts which have been observed by 

 geologists confirming the idea that a warm climate s , 

 especially if it be a wet one, is more favorable to rock 

 decay than a cold climate. The progress of decay, 

 therefore, is distinctly affected by latitude. Extensive 

 investigations carried on along the Atlantic side of the 

 country show wide differences in the rate of decay in 

 the same kind of rocks in different latitudes. In gen- 

 eral, the progress of decay is more marked toward the 

 south. The same fact is observed in the great interior 

 valleys of the country; at least, everywhere except in 

 the arid and semi-arid regions. Wherever there is a 

 deficiency of water the processes of decay have been 

 arrested. Where the rock strata have been displaced 

 from a horizontal position the progress of decay has 

 been more rapid. This is easily understood. The 



