THE WEATHBEINa OF BOOKS {Continued) 



IV. TIME CONSIDEEATIONS 



Concerning the rate of decomposition of rocks of various 

 kinds, only very general rules can be laid down, since much 

 depends upon climatic conditions and the position of rock 

 masses relative to the action of frost, moisture, and the various 

 growing organisms. 



(1) Rate of Weathering influenced by Texture. — Prom the 

 study of building materials it has become apparent that a 

 coarsely crystalline rock will, all other conditions being the 

 same, disintegrate more rapidly than one of finer grain. This 

 is doubtless owing in part to expansion and contraction from 

 ordinary temperature varijations, which act the more energetic- 

 ally the larger the crystalline particles/ 



It has already been remarked {ante, p. '40) that crystalline 

 rocks have a greater density than do glassy forms of the same 

 chemical composition. This indicates a contraction during the 

 processes of crystallization, which manifests itself, according to 

 at least one authority, in the development of minute interspaces 

 between the individual crystals. The coarser the crystalliza- 

 tion, then, the greater the amount of interstitial space, and 

 hence the greater the absorptive power. 



*Tlie coefficient of cubical expansion for several of tlie more common 

 rock-forming minerals has been determined as follows: — 



Quartz 0.0000360 Tourmaline 0.000022 



Orthoclase 0.0000170 Garnet . 0.000025 



Hornblende 0.0000284 Caleite 0.000020 



Beryl 0.0000010 Dolomite 0.000035 



The strain brought to bear npon a mass of rock through the unequal 

 rate of expansion of its various constituents is further complicated through 

 the unequal expansion of the individual minerals along the direction of 

 their various axes. Thus quartz gives a coefl&cient of 0.00000769 parallel 

 to the major axis, and of 0.000001385 at right angles thereto. Adularia 

 gives 0.0000156, 0.000000659, and 0.00000294 for its three axes, and horn- 

 blende 0.0000081, 0.00000084, and 0.0000095 (Stones for Building and 

 Decoration, p. 410). ' 



255 



