PRINCIPLES OF ROCK WEATHERING 723 



of insoluble matter (usually clay or silica) existing as an 

 im[)urity. Jn these limestone regions the solvent action has not 

 infrequently gone on so extensively as to leave its imprint upon 

 the topographic features of the landscape. The drainage is no 

 longer wholly superficial but by subterranean streams sinking 

 entirely into the ground to reappear again at lower levels, it may 

 be miles away, having traversed the intervening distance in some 

 of the numerous passages (fissures enlarged by solution) with 

 which the rocks abound. Entire landscapes are not infrequently 

 undulating through the abundance of sinkholes — shallow depres- 

 sions down through which the water has percolated and escaped 

 into the underground passages. An idea of the amount of 

 material thus dissolved may be gained when I state that some 

 275 tons have been calculated' as annually removed from each 

 square mile of Calciferous (Lower Silurian) limestone exposed in 

 the Appalachian region alone, while a well-known English auth- 

 ority^ has calculated that with an annual rainfall of 32 inches per- 

 colating only to a depth of 18.3 inches, there are annually 

 removed by solution from the superficial portions of England 

 and Wales an average of 143.5 tons per square mile of area. He 

 further calculates that the average amount of carbonate of lime 

 alone annually removed from each square mile of the entire 

 globe amounts to 50 tons. It is to this corrosive action of 

 meteoric waters that still another authority ^ would attribute the 

 slight thickness and nodular condition of man}^ beds of Palze- 

 ozoic limestone. He argues that originally thick bedded lime- 

 stones have, during the ages subsequent to their formation and 

 uplifting become so impoverished through the dissolving out and 

 carrying away in solution of the lime carbonate, as to have been 

 quite obliterated or reduced to mere nodular bands, and given 

 rise to important palaeontological breaks in the geological record. 

 Other than organic acids may locally exert a potent influence. 



'A. L. EwiNG, Am. Jour, of Science, 1885, p. 29. 



'T. Mellard Reade (Chemical Denunciation in Relation to Geological Time). 



3F. RuTLEY, the Dwindling and Disappearance of Limestones, Quar. Jour. 

 Geol. Soc. of London, Aug. 1893. 



