722 STUDIES FOR STUDENTS 



(lo) Apatite was readily acted upon, as could be detected by 

 its appearance under the microscope. 



(ii) Olivine was the most readily attacked of all the sili- 

 cates tested, probably twice as easily decomposed as the ser- 

 pentine. 



(12) Magnesian silicates were attacked by the carbonated 

 waters. Hence serpentine cannot be considered a final product 

 of decomposition.^ 



Of all the materials forming any essential part of the earth's 

 crust the limestones are most effected by the solvent power of 

 water. It is stated that pure water will dissolve one part in 

 10,800 when cold and one part in 8.875 when boiling of lime 

 carbonate. 



Since rock weathering is, as already stated, a superficial phe- 

 nomenon, we have to do only with waters of ordinary tempera- 

 tures and under ordinary conditions of pressure though this 

 expression must not be taken as necessarily meaning cold waters, 

 since during the rainy season in tropical countries the waters 

 falling upon the heated rocks may have their temperatures raised 

 as high as 140° F. or even 150° according to A. Caldeleugh.^ 



It is almost wholly to this solvent action that is due the 

 formation of the multitudinous caverns of limestone regions. 

 Even where caverns are not apparent the corrosive action is 

 evident to the practiced eye. In the quarry regions of Tennes- 

 see surface blocks of limestone are often grooved to a depth of 

 an inch or more with wonderful sharpness, simply from the 

 water of rainfalls with its acids absorbed from the atmosphere 

 and surface soils, while in the quarry bed the stone is found no 

 longer in continuous layers, but in disconnected bowlder-like 

 masses. In such cases casual examinations give very little clue 

 to the rapidity of the destruction going steadily on, since all is 

 removed in solution excepting the comparatively small amount 



' Serpentine, however, cannot be properly considered a decomposition product. 

 It is rather a product of alteration. 



^ On the Geology of Rio Janeiro, Trans. Geol. Soc. of London, 2d Sen Vol. II, 

 1829. 



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