CEMENTING MATERIALS IN SOIL 29 



Like humus, the hydrated ferric oxide diminishes the tenacity 

 of clay, while increasing that of sand. 



Calcium carbonate is one of the commonest of the 

 cementing materials occurring in rocks. When deposited 

 from its solution in carbonic acid by the escape of this 

 gas, it encrusts and unites the particles on which it is 

 precipitated. This action is familiar to us in the case of 

 petrifying springs. 



Calcium carbonate is a common ingredient in soils, and is 

 usually the most abundant of the solid constituents held in 

 solution in soil water. Under the varying conditions of the 

 supply of water and carbonic acid, and with alterations in 

 temperature ; calcium carbonate will at times enter into 

 solution in the soil, and at other times be redeposited. By 

 the action of rain and vegetation a gradual removal of 

 carbonate of calcium from the surface soil is generally in 

 progress. In districts in America having a deficient rainfall, 

 a hard pan frequently forms in the subsoil at the depth to 

 which the drainage water usually penetrates ; the cementing 

 matter in this pan is carbonate of calcium. The lime-pan, 

 which sometimes forms in English soils which have been 

 dressed with quicklime, is probably due to the conversion 

 into carbonate in the subsoil of the calcium hydrate carried 

 down in the drainage water. The effect of calcium carbonate 

 in increasing the coherence of surface soils is most plainly 

 seen after a dressing of rnarl or chalk has been applied to 

 a sandy soil. Calcium carbonate is a crystalline and not 

 a colloid body. As already mentioned, it diminishes the 

 tenacity of clay. 



Soils usually contain various hydrated silicates, and some- 

 times hydrated alumina ; these colloid substances are doubtless 



