

TIME CONSIDEEATIONS 



Fig. 20. 



These coarser rocks, owing to their tendency to undergo a 

 mechanical disintegration, or disaggregation, may also yield to 



the decomposing agencies 

 more readily than those 

 of finer grain, though 

 from the fact that they 

 first fall away to coarse 

 sand, whereby the rock- 

 like character is lost, one 

 might, on casual inspec- 

 tion, be led to the oppo- 

 site eonclusiom It need 

 scarcely be said that, 

 among rocks haying the 

 same composition, whe- 

 ther fragmental or crys- 

 talline, siliceous or cal- 

 careous, those of a granu- 

 lar structure will un- 

 dergo disintegration more 

 quickly than will those 

 in which the individual 

 minerals are closely com- 

 pacted or interknit, as in 

 many quartzites and dia- 

 ba*ses. 



(2) Rate of Weather- 

 ing influenced by Com- 

 position. — Among rocks 

 of the same structure as 

 regards crystallization 

 and size of particles, the 

 basic varieties, such as the 

 diabases and gabbros, as 

 Mierostriicture of sandstone (Mg. 20), show- ^ rule SUCCUmb more read- 

 ing relatively large amount of interstitial .-, ,-, -, .-t --, 

 space and absorptive power, and (Fig. 21) ^^^ ^^^^ ^^ ^^^ ^^^® ^^^^ 

 of diabase, with relatively little. varieties like the granites. 



This for the reason that the 

 iron-magnesian as well as the soda-lime minerals are more suscep- 

 tible than are the potash silicates and other essential constituents 



Fig. 21. 



