158 THE PRINCIPLES INVOLVED IN ROCK-WEATHEEING 



to the various parts of the United States, would give us for the 

 Atlantic coast states 3.75 mm. ; for the upper Mississippi val- 

 ley, 2.50 mm. ; for the lower Mississippi valley, 4.50 mm. ; and 

 for the northern Pacific states 6.25 mm. As it is mainly when 

 this carbonic acid is thus brought to the surface by the rain 

 and snows that its effects become of direct significance in the 

 present work, the matter may be dropped here, to be taken up 

 again when considering the chemical action of water. 



(3) Oxygen. — Under ordinary conditions oxygen is the most 

 active principle in atmospheric air, and to it is due the process 

 of oxidation which almost invariably characterizes the decom- 

 position of silicates and other minerals containing iron in the 

 protoxide state. Such oxidation is, however, almost inactive 

 unless aided by moisture, and a further discussion of the subject 

 may well be deferred, to be taken up again when discussing the 

 action of water. 



(4) Heat and Cold. — The ordinarily feeble action of the air 

 is greatly augmented through natural temperature variations. 

 That heat expands and cold contracts is a fact too well known 

 to need elaboration. That, however, the constant expansion 

 and contraction due to diurnal temperature variations may be 

 productive of weakness and ultimate disintegration in so inert 

 a body as stone, seems not so generally understood, or is, at 

 least, less well appreciated; hence a little space is devoted to 

 the subject here. Rocks, it must be remembered, as the writer 

 has noted elsewhere,^ are complex mineral aggregates of low 

 conducting power, each individual constituent of which possesses 

 its own ratio of expansion, or contraction, as the case may be. 

 In crystalline rocks these various constituents are practically 

 in contact. In clastic rocks they are, on the other hand, sepa- 

 rated from one another by the interposition of a thin layer of 

 calcareous, ferruginous, or siliceous matter which serves as a 

 cement. As temperatures rise, each and every constituent ex- 

 pands and crowds against its neighbor; as temperatures fall, a 

 corresponding contraction takes place. Since in but few regions 

 are surface temperatures constant for any great period of time, 

 it will be readily perceived that almost the world over there 

 must be continuous movement within the superficial portions of 

 the mass of a rock. 



The actual amount of expansion and contraction of stone 

 ^ Stones for Building and Decoration, Wiley & Sons, New York. 



