xix. WASTE OF CALCAREOUS ROCKS. 487 



in proportion to their inclination, and thus will waste the rocks 

 in those directions underground more than elsewhere. It will also 

 in some degree waste the surface more in those directions, because 

 of the convergence of the small rain-streams thither. Thus con- 

 tinuous hollows, small valleys in short, will be produced over 

 many parts of the surface, with a tendency to converge to the 

 directions of least continuity and least resistance in the masses of 

 the rocks. 



3. Let the rocks be calcareous. In this case the rain-water, by 

 means of the carbonic acid which it brings, will dissolve the rock 

 along the subterranean fissures and channels. This action will be 

 accelerated by the additional solvent power imparted to the water 

 by decomposing vegetable matter in the soil. The whole surface 

 of the country will undergo slow dissolution ; carbonate of lime 

 and carbonate of magnesia will be carried downwards, to be de- 

 livered in the springs, or else in particular places to be separated 

 and deposited as stalactite, stalagmite, &c. Phosphate of lime, 

 sulphate of lime, chloride of sodium, &c. are also dissolved out 

 of many rocks by the rain. 



4. This process continued through a very long period of time 

 must necessarily cause subterranean channels and cavities along 

 the course of the descending streams : in the lapse of time the roofs 

 of these channels, weakened and unsupported, fall in, and the series 

 of caverns becomes a rugged cliff-bordered vale, such as are common 

 in the great limestone districts of Yorkshire and Derbyshire, where 

 they may be seen in progress of extension ; and in the Mendip 

 hills, where, as at Wokey-hole, the ancient channels are partly 

 choked with bones and mud and partly swept clear by the strong 

 underground stream. 



5. If the rocks be not calcareous the effect is different ; they are 

 not dissolved, but fretted away by the sandy matter suspended in 

 the stream : and this is observed much more about the surface, 

 especially in countries where the rocks are argillaceous, than in 

 the interior. But where the line of weakness is determined by a 

 ' fault,' that divisional plane is usually marked by the free passage 

 of water and its mechanical and chemical effects. In the lines of 

 such faults mineral springs frequently arise. 



6. These things, now put as depending on the supposition of 

 an elevated tract of strata, are perfectly real, and are in daily 



