WEATHEEINa OF CALCABEOUS EOCKS 



219 



Analysis of Fresh Limestone and Residual Clat, Staunton, Virginia 



SiOj + TiOa^ 



FeA(2) 

 CaO . 

 MgO. 

 Kfl . 

 Na^O. 

 C02 . 



PA. 

 H2O . 



o 



Eh 



w 



7.41% 



1.91 



0.98 

 28,29 

 18.17 



1.08 



0.09 

 41.57 



0.03 



0.57 



100.00 



o 



57.57% 

 20.44 



7.93 



0.51 



1.21 



4.91 



0.23 



0.38 



0.10 



6.69 



100.00 



11 



Percentage op 

 Each Constitu- 

 ent Saved 



2.03% 



0.00 



0.29 

 28.24 

 18.06 



0.62 



0.07 

 41.53 



0.02 



O.55J 



72.61% 

 100.00 



75.11 

 0.17 

 0.62 



42.51 



23.96 

 0.85 



31.22 



90.86 





O H 



£ O H 



27.39% 

 0.00 

 24.89 

 99.83 

 99.38 

 57.49 

 76.04 

 99.15 

 69.78 



This leaching out of the lime carbonates and the accumula- 

 tion of insoluble residues is a strikingly conspicuous feature in 

 regions abounding in limestone caverns, and to it is due the 

 tenacious ferruginous clays which cover their floors. So rich 

 indeed are some of these residual deposits in iron oxide that 

 in some instances they are locally used for pigments, under the 

 name of ochre or mineral paint, or again, where occurring in 

 large quantities, as ores of iron. (See p. 100.) 



It is possible that loosely consolidated beds of shell limestone 

 may undergo a process of change, perhaps more nearly akin to 

 alteration than decomposition, through agencies quite different 

 from those we have been considering. Darwin found the shells 

 in the raised sea-beaches of San Lorenzo, South America, altered 

 to the condition of a white powder without trace of organic 

 structure, and consisting of carbonate, sulphate and chloride 

 of lime with sulphate and chloride of sodium. This alteration he 

 believed to be due to a mutual reaction taking place between the 

 original sodium chloride derived from the sea-water and the 

 lime carbonate of the shells, and he speaks of it as an interesting 

 illustration of the fact that the dry climate of the west coast of 



^ Gain. 



