THE VARIOUS ROCKS AS SOIL-FORMERS. 57 



Sandstones with purely zcolitic cement are on the whole not 

 of frequent occurrence, the zeolites forming, more commonly, 

 the hard portion of a clay-sandstone cement, which disinte- 

 grates by their weathering-out. 



In regions where the tufaceous rocks of eruptives prevail, we not 

 uncommonly find the " volcanic ash " solidly cemented by a zeolitic 

 mass, which is then usually apparent in cavities or crevices in the form 

 of crusts or crystals. Such tuffs are commonly rich in alkalies and 

 lime, but mostly poor in phosphates, and in disintegration form soils of 

 a corresponding nature. They are largely represented in the valleys 

 off Puget Sound, as well as in portions of central Montana, and north- 

 ward. 



Clay-Sandstones (argillaceous sandstones) when soft, as is 

 mostly the case, form as a rule desirable loam soils, of a gener- 

 alized composition, difficult to predict. It is here that the com- 

 position of the sand grains themselves most frequently comes 

 into play in modifying the soil quality. From clay-sandstones 

 to claystones of various degrees of sandiness there is, of course, 

 every grade of transition, the soils ranging correspondingly 

 in the scale of lightness or clayeyness. As a general rule, the 

 potash contents of such soils are sensibly proportioned to the 

 clayey ingredient, at least in the humid regions. 



Claystones (i. e., clays hardened by some one or more of 

 the cements mentioned in connection with sandstones), will 

 in the nature of the case, when disintegrated from the condi- 

 tion in which they lie in the geological formations, make cor- 

 respondingly clayey, heavy soils, which as experience shows 

 are usually rich in the ingredients of plant food, but frequently 

 too heavy and intractable in tillage to be readily utilized. 



There are, of course, exceptions ; such as soils formed from pipe- 

 clays, in which little if any mineral plant-food remains, and which are 

 best used for other purposes than agriculture, unless under special con- 

 ditions it may be worth while to reclaim them by fertilization. 



Natural Clays. Clays occur in nature in a great variety of 

 modifications that have received designations known in com- 

 mon life. Such are porcelain clay, pipe-clay, fire-clay, potters' 



