88 



of the lime that this residual soil requires the addition of lime to render 

 it fertile. A handful of soil may be treated with acid without giving an 

 appreciable effervescence, even though the soil be taken from within an 

 inch of the limestone. Analysis of this clay reveals about 67 per cent, to 

 SO per cent, silica, 8 per cent, to 14 per cent, aluminum, G per cent, or 7 

 per cent, iron oxide (Fe^0 3 ), and very small percents of lime, magnesia, 

 soda and potash, etc. The iron is responsible for the intensely red 

 color of the clay. The process which has produced this soil is the solution 

 of the limestone with oxidation of the iron which exists in minute quan- 

 tities in the original rock as a protoxide. The surface of the limestone 

 beneath the soil, besides being rough and ragged as explained above, is 

 usually minutely roughened, though sometimes fairly smooth, especially 

 in the Mitchell limestone. In some cases, especially in the Salem lime- 

 stone, the rock in contact with the overlying soil is rotted and discolored 

 beyond recognition and shows a graded passage from sound unmodified 

 rock below to soil above. Where layers of shaly rock occur, as in the 

 Mitchell, they are often so rotted that while they retain much of their 

 original appearance and stratification, they may be removed with pick 

 and shovel as easily as any clay. Sometimes a layer of limestone over- 

 lying a layer, of shale is left as an isolated chain of boulders in the gen- 

 eral mass of residual soil. The deepest accumulation of residual soil 

 seen by the writer is in the cut on the Illinois Central Railroad in the 

 northwest edge of Bloomington, where it is 30 feet deep. Usually 

 it is not more than five or six feet deep. Over the Mitchell and Harrods- 

 burg limestones the soil contains chert, and, in the latter rock, geodes 

 in abundance, because of the relative insolubility of these substances. 

 Where blocks of Salem limestone are exposed at the surface to the 

 rain they become deeply furrowed by the solvent action of the rain- 

 water running over their flanks. The faces of old ledges, long exposed to 

 the weather, are scarred and seamed by this action and extensively 

 honeycombed, owing to the unequal solubility of the rock. In these holes 

 and pockets on the rock surface small plants find lodgment and by the 

 mechanical action of their roots and the chemical action of the pro- 

 ducts of their decay, greatly aid the process of disintegration. 



The effects thus far described are seen to best advantage in the 

 exposures of the Salem limestone. The Mitchell shows to a pre-eminent 

 degree the deeper-seated effects of solution in the formation of caverns 

 and underground streams. Everywhere the surface of the country occu- 



