Sandstones with purely zeolitic cement are on the whole not of frequent occurrence, the zeolites forming, more commonly, the hard portion of a clay-sandstone cement, which disintegrates 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 northward.
Clay-Sandstones (argillaceous sandstones) when soft, as is mostly the case, form as a rule desirable loam soils, of a generalized composition, difficult to predict. It is here that the composition 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 condition in which they lie in the geological formations, make correspondingly 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 conditions 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 common life. Such are porcelain clay, pipe-clay, fire-clay, potters’ clay, brick-clay, and many others of more or less local use only. As these materials practically concern the farmer in very many cases, they may properly find a brief discussion here.
The variety-names enumerated above in the order of the actual contents of the materials in true clay substance (“colloidal clay”), are partly based upon that fact, partly upon the degree of plasticity attained by that substance, and essentially upon the nature and amount of foreign admixtures associated with it. Thus, porcelain clay is chalky kaolinite, sometimes associated with enough of pure white plastic clay to render it workable in the potter’s lathe, but more commonly requiring to be molded in porous molds; it is very refractory to heat. Pipe-clay is also white, but more plastic and usually less refactory. Fire-clay is a refractory pipe-clay commingled with some coarse infusible material, such as quartz sand (or the same clay burnt and crushed), in order to prevent excessive contraction and change of shape in drying and burning. Potters’ clay is a much less pure, and from that cause more fusible clay, which when burnt forms at a moderate heat a semi-fused, more or less hard mass, such as crockery and pottery ware. Brick-clay is a still more impure clay, or loam, containing considerable sand and usually iron oxid, and largely falls already within the limits of tillable soils or subsoils, rendered fusible by the presence of relatively considerable amounts of iron, magnesia and lime.
Iron colors natural clays either red, yellow, green or blue; the latter two colors turning to yellow or red on exposure to the air, and to red on burning. Black color is usually due to carbon, such clays often turning white on heating.
Clays containing much lime are usually of a gray or whitish tint, and like the soft crumbly limestones are often called marls, and are used as such for land improvement. But it should be understood that the colors of clays, mostly derived from some iron compound, have little to do with their uses in the arts, except that no deeply colored clay (black excepted) is refractory in the fire.