THE MAJOR SOIL-FORMING MINERALS, 41 



or " heavy " soils are always calcareous. On the other hand, 

 the blue-grass region of Kentucky, and most of the lands of the 

 arid regions are prominent examples of " light " calcareous 

 soils. 



Caves, Sinkholes, Stalactites. Perhaps the most striking exempli- 

 fication of the solvent power of carbonated water is seen in the form- 

 ation of limestone caves. As a matter of fact, the vast majority of all 

 existing caves is found in limestone formations ; and such formations, 

 as will be more fully discussed hereafter, nearly always bear a luxuriant 

 vegetation. The water filtering through the vegetable mold, in which 

 carbonic acid is constantly being formed, becomes charged with it, and 

 on reaching the underlying rock, dissolves to a corresponding extent 

 the lime carbonate of which this rock wholly or chiefly consists. When 

 penetrating crevices it soon enlarges these, to an extent proportioned 

 to the length of time and the strength of the solvent ; and thus gradually 

 subterranean passages or caves are formed, which at first are almost 

 always the bed of a stream, the mechanical action of which accelerates 

 the process of enlargement, until after some time the water is perhaps 

 drained off through some crevice to a lower level, where the same pro- 

 cess is repeated. 



Sometimes the ceiling gives way, forming the funnel-shaped " sink- 

 holes" or " lime-sinks " so familiar in some of the Mississippi Valley 

 States. Sometimes the lime solution on reaching the ceiling of the 

 cave, instead of dropping down, evaporates there and eventually forms 

 icicle-like " stalactites " out of the dissolved substance ; while when 

 dropping on the floor and thus growing upwards, the corresponding 

 formation is called "stalagmite" These caves, subterranean rivers, 

 sinkholes, natural bridges and tunnels, etc., mostly owe their origin to 

 this solvent action of carbonated water on limestone formations. 1 



The same occurs on a small scale, when calcareous land is 

 underdrained : the lime carbonate dissolved from the soil is 

 partially deposited in the drain pipes, which it frequently ob- 

 structs. Similarly, an impure, porous deposit of calcareous 

 tufa is frequently formed on the surface, at the foot or in rills 



1 T. M. Keade (in his treatise on Chemical Denudation in Relation to Geological 

 Time) calculates that 143.5 tons f lime carbonate are annually removed by solu- 

 tion from each square mile of land in England and Wales, and that the average 

 amount thus removed annually from each square mile of the earth's surface 

 is about fifty tons. 



