THE VARIOUS ROCKS AS SOIL-FORMERS. 55 



It appears from the above table that in the change from the 

 original limestone to the soil mass as found at three feet depth, 

 81.5% of the lime carbonate has been eliminated by leaching, 

 leaving behind somewhat less than one fifth of the original 

 mass. Taking the average depth of the soil mass at 8 feet, 

 this thickness of material has required about 45 feet of the 

 rotten limestone. Considering that notwithstanding the ten- 

 acity of the clay soil, some of it must in the course of time 

 have been washed away, we may safely assume that the orig- 

 inal rock surface was from 50 to 60 feet higher than at 

 present. 



Sandstone Soils. The indefiniteness of the nature of " sand- 

 stones " as such renders generalizations in regard to the soils 

 formed from them rather difficult, save as to their physical 

 qualities, which in the nature of the case are always " light." 

 In the Old World and in the humid region generally, sandstone 

 and sandy soils are usually spoken of as being poor, because 

 there the sand almost always consists of quartz grains only, and 

 hence the fine portions alone can be looked to for plant nutri- 

 tion. Consequently, the more sand is seen in a soil, the poorer 

 it is usually presumed to be. But this presumption would be 

 wholly erroneous in the arid regions. (See chapt. 6, p. 86). 



Clearly, the nature of the soils produced by the weathering 

 of sandstones depends upon two points : first, the nature of the 

 cement binding the sand grains, and second the character of 

 the latter themselves. 



Varieties of Sandstones. As has been stated above, the 

 cements may be roughly classified into five kinds, and their 

 intermixtures, to wit: qnartzose or siliceous,, calcareous, fer- 

 ruginous, aluminous or clayey, and ccolitic. As regards the 

 first, it is obvious that siliceous sandstones will disintegrate 

 with great difficulty, since neither the cement nor the grains 

 are susceptible of material change by weathering. Such sand- 

 stones frequently pass insensibly into quartz rock, and the 

 light, unsubstantial soils they produce are of the poorest, con- 

 taining often mere traces of the plant-food ingredients. This 

 of course, is true, not only of the soils formed by the actual 

 weathering of sandstones, but equally of those consisting of 

 quartz-sand deposited by water or drifted by winds. 



