AQUEOUS CAUSES OF CHANGE. 207 



CH.A PTE R, IV. 

 Springs* 



" Thou dost wear 



No stain of thy dark birthplace ; gushing up 

 From the red mould and slimy roots of earth, 

 Thou flashest through the sun." 



Bryant* 



IN the present chapter we shall consider another aqueous cause 

 of change, springs, or as they have been termed " subterranean 

 drainage." Every one is familiar with the fact, that the water 

 which is deposited upon the loose soil, easily percolates through 

 it, and makes its way downward to a certain depth according to 

 the nature of the underlying strata. Whilst it easily penetrates 

 through the gravelly, and sandy formations, it is arrested by the 

 almost impervious beds of clay, and sometimes collected into 

 large sheets of water, which are often subjected to intense press- 

 ure, upon the well known hydraulic principle so often employed 

 in the arts under the form of the hydrostatic press. Mr. Lyell 

 mentions that the transmission of water is so rapid through the 

 loose gravelly soil over which the river Thames flows, and which 

 is upon an impervious sub-stratum of clay, that the wells in this 

 vicinity alternately ebb and flow, with the tides of the river. It 

 is from this cause, that wherever on the side of a hill, strata of 

 clay are found below sandy soils, the water oozes out, not indeed 

 in a continuous sheet, but, probably from me slight difference 

 in the constitution of the clay, or from natural fissures or cracks, 

 in the form of little streams. The effect of such minute streams 

 in finally undermining hilly tracts of country is surprising ; con- 

 stantly running, they bear out the light sand, and thus the sub- 

 terraneous reservoir extends its surface gradually, until, at length, 

 the superincumbent mass gives way, and sliding upon the slip- 

 pery cly is precipitated yito the valley below. 



