GEOLOGY. 125 



circumstance that the deposit throughout presents no appearance 

 of stratification. 



Springs derived from water whicli percolates through the 

 earth, always become more or less charged with matter cha- 

 racteristic of the strata from which they issue. If from lime- 

 stone rocks, such as the Purbecks, the water as it passes through, 

 becomes impregnated with lime, held in solution by carbonic acid 

 gas, which is thrown off when in contact with the atmosphere, 

 and the free lime precipitated. This occurs in many localities 

 both in this and foreign coimtries. In Scotland, for instance, 

 several of the lochs are remarkable for the production of a cal- 

 careous marl ; and in the cliff of Tollard's bay on the south coast 

 of the Isle of Wight, is a bed of calcareous tufa. Sir Charles 

 Lyell describes an extensive deposit in the Valley of the Rhine, 

 provincially termed loess, which in some instances presents a 

 close analogy to that at Blashenwell, being of freshwater origin, 

 and containing calcareous concretions enclosing land shells of the 

 same species as those which exist at the present time in the im- 

 mediate neighbourhood. After heavy rains, much surface water 

 would flow into the stream, bringing land shells from the hills 

 and conveying them down its course, until some check to the 

 current allowed them to sink to the bottom; and thus they 

 became embedded in the constantly increasing deposit, in com- 

 pany with the freshwater testacea which inhabited the lake. 

 But although the agency to which the presence of these 

 shells are thus to be attributed, must of necessity be termed 

 mechanical y it does not follow that the same term must, or even 

 can be applied to the production of the calcareous mass in which 

 they are enclosed. On the-contrary, the visitor at Blashenwell 

 will at once perceive that, with the exception of the shells and 

 flints, it contains nothing which in any probability was conveyed 

 there in a solid state, not even as silt. There is no vein, how- 

 ever thin, of any foreign matter, nothing appears besides the 

 calcareous tufa, which is evidently of chemical origin. The upper 

 portion indeed may be more correctly termed trctvertine, it being 

 much harder and more stalagmitic in its character. This has 

 been accounted for by a supposition that the stream, which is 

 made use of for irrigating the super-imposed meadows, con- 

 tinues to deposit the carbonate of lime with which it is highly 

 impregnated. I cannot however agree in this opinion, although 



