Feb.. 1893. THE UNDERGROUND WASTE OF THE LAND. 125 



large amount of mineral ingredients compared with some other 

 saline waters, daily carry away between three and four tons of solid 

 matter, and the removal of this must cause cavities. Perhaps the 

 landslips that have occurred from time to time at Bath may be to 

 some extent influenced by subterranean movements, caused by the 

 filling-in of cavities by the overlying strata. The waters of Burton-on- 

 Trent may be cited in reference to chemical erosion, and it has been 

 estimated that about 150 tons of gypsum are annually imbibed in 

 potations of Burton beer. 



In the more hilly and mountainous regions, where bare rocks 

 frequently jut out at the surface, the evidences of destruction are 

 apparent. Among the gentler hills and vales of the midland and 

 southern counties, the evidences of erosion are to be judged mainly 

 by the solid matter carried away by the streams. The matter held 

 in solution is due chiefly to subterranean erosion ; that held in 

 suspension is due almost entirely to superficial erosion. The 

 hills are, for the most part, composed of limestones, sands, and sand- 

 stones, through which the rain-waters may percolate, until arrested 

 by a band of clay, when they issue as springs. Hence the hills are 

 not so much subject to superficial denudation as are the vales, for 

 these lie mainly in tracts of clay that are directly acted upon by the 

 streams that flow across them. The main features of hills have, 

 therefore, remained permanent for long ages, though their general 

 level may have been continuously, if imperceptibly, lowered. 



The question arises whether the underground erosion may not 

 be partly mechanical, even if it be mainly chemical. Underground 

 waters that flow on the top of a mass of clay must form channels in 

 that material. The permanence of springs that issue here and there 

 along the foot of escarpments, indicates plainly that the underground 

 waters follow definite courses ; and the actual outlet of such springs 

 may be found at varying levels beneath the plane of division that 

 separates the porous and impervious strata, because the springs 

 have eroded their channels in places, perhaps, to depths of five or six 

 feet. 



Referring to the Central Himalayan region, Mr. C. L. Griesbach 

 (4) mentions a limestone which rests conformably on calcareous 

 shales, and this, like most limestones, is much jointed ; consequently, 

 all the drainage finds its way through the joints into the underlying 

 shales. These become disintegrated, and are gradually carried away, 

 while the thick limestone band sinks down to the level which was 

 formerly occupied by the shales. 



In an article in which I discussed the origin of the Scenery of 

 Norfolk, I remarked that on the clayey strata the rainfall must 

 accumulate or flow away at once towards lower levels ; on the sandy 

 and gravelly strata it will sink down until arrested by impervious 

 beds beneath. Hence the earliest exposed channels no doubt com- 

 menced on the clayey areas that formed the surface. Beneath the 



