CHAPTER IV.. The Various Rocks As Soil-Formers.

THE VARIOUS ROCKS AS SOIL-FORMERS.

Rock-weathering in arid and humid Climates.—From what has been said in the preceding chapters of the physical and chemical agencies concerned in rock-weathering, it is obvious that climatic differences may materially influence the character of the soils formed from one and the same kind of rock. Since kaolinization is also a process of hydration, the presence of water must greatly influence its intensity, and especially the subsequent formation of colloidal clay; so that rocks forming clay soils in the region of summer rains may in the arid regions form merely pulverulent soil materials. Many striking examples of these differences may be observed, e. g., in comparing the outcome of the weathering of granitic rocks in the southern Alleghenies with that of the same rocks in the Rocky Mountains and westward, especially in California and Arizona. The sharpness of the ridges of the Sierra Madre, and the roughness of the hard granitic surfaces, contrasts sharply with the rounded ranges formed by the “rotten” granites of the Atlantic slope, where sound, unaltered rock can sometimes not be found at a less depth than forty feet; while at the foot of the Sierra Madre ridges, thick beds of sharp, fresh granitic sand, too open and pervious to serve as soils, cover the upper slopes and the “washes” of the streams, causing the latter to sink out of sight. A general discussion of the kinds of soils formed from the various rocks must, therefore, take these differences into due consideration.

GENERAL CLASSIFICATION OF ROCKS.

Rocks may be broadly classified into three categories, viz:

1. Sedimentary rocks, formed by deposition in water and hence more or less distinctly stratified.

2. Metamorphic rocks, formed from rocks originally sedimentary, by subterranean heat in presence of water. Usually crystalline, that is, composed of more or less distinct (large or minute) crystals of one or several of the minerals mentioned above.

3. Eruptive rocks, ejected in the molten state from volcanoes or fissures; crystalline or not, according to slow or rapid cooling.

Sedimentary Rocks.—Sedimentary rocks are forming to-day by deposition from either sea or fresh water, precisely as they were in the most remote geological times; the oldest clearly sedimentary rocks being sometimes undistinguishable in their nature and composition from the very latest immediately preceding our present time. They may for the purposes of the present work be simply classified as follows:

1. Limestones, formed in comparatively shallow seas, or fresh water basins, from the calcareous shells or skeletons of various organisms.

2. Sandstones, and conglomerates (sometimes called pudding-stones) formed from the debris of pre-existing rocks disintegrated by the agencies described above, (chap. 1-2), re-cemented by means of solutions of one or several substances, such as silex, carbonate of lime, ferric hydrate and others. Loose sands and gravels are the initial stages of such rock formation as well as the results of their disintegration.

3. Clays, Claystones and Clay shales, consisting of clay substance with more or less sand, and soft or hard according to the nature of the waters or solutions that may have acted upon them, with or without the aid of heat. These rocks can only be formed in comparatively quiet or “back” waters, since clay would not ordinarily be deposited in moving water.

Metamorphic Rocks.—The effects of subterranean heat or metamorphism upon the sedimentary rocks may be roughly stated as follows:

Limestones are transformed into marbles of various degrees of purity, according to the nature of the original rocks.

Sandstones when cemented by silex are transformed into quartzite, of greater or less purity according to the nature of the “sand” entering into its composition. When cemented by materials other than quartz, these also will be segregated in the form of various minerals in the body of the rock.

The clay rocks form the most varied products under the influence of (aqueo-igneous) metamorphism; granites, gneiss, syenite and hornblendic schist are among the most common. The great variations in the composition of clayey materials account for the correspondingly great variations in the nature of the resultant metamorphic rocks.

Igneous or Eruptive Rocks.—These are usually divided into two groups; the one characterized by a large proportion of free quartz (silicic acid), and hence designated as acidic, and usually of a light tint; the other the basic, containing little or no free quartz, and commonly of a dark tint caused by the presence of a large amount of iron (contained in pyroxene, more rarely in hornblende).

Of the latter class are the dark “basaltic” rocks constituting the mass of the enormous eruptive sheet of the Pacific Northwest, covering the greater part of Washington, Oregon and northeastern California. The lavas of the Hawaiian islands are of the same class and even more basic; while the eruptives of Nevada, middle and southern California, and eastward to the Rocky Mountains, are mostly of the light-colored, acidic type. The same is largely true of the rocks of the Andes of Central and South America, the gray “Andesites,” also represented in the Caucasus.

As one and the same eruptive material may, according to the greater or less rapidity of cooling, appear as a glassy mass (obsidian, pumice, volcanic ash, tuff, etc.,) or as a crystalline rock resembling coarse granite in structure, it is not easy to identify them in all their various forms. This can frequently be done only by ascertaining their component minerals by the microscope, or by chemical analysis. The same is sometimes true of metamorphic rocks; and as in the latter, the several feldspars and quartz, with pyroxene instead of hornblende, constitute the predominant soil-forming minerals. More rarely, garnet, chrysolite, leucite and other silicates require consideration.

Generalities regarding the Soils derived from various Rocks.

It is hardly necessary to insist that as in the case of the rocks composed of single minerals, already referred to above, the predominant mineral or minerals of compound rocks determine the facility of weathering, as well as the quality of the soil resulting therefrom. Since rocks are named essentially in accordance with the kinds of minerals that constitute their regular mass, the proportion in which the several constituents stand to each other may vary greatly. Thus a granite may consist, over considerable areas, mainly of a mixture of potash feldspar and quartz; in others, mainly of quartz and mica with little feldspar. Very frequently, hornblende replaces mica partially or wholly. The latter will weather much more slowly than feldspar or hornblende, and will produce an inferior soil when decomposed. Allowing for such variations, a fairly approximate general estimate of the quality and peculiarities of soils from crystalline rocks may nevertheless be made. To some extent such estimates must make allowance not only for the chief ingredients, but also for those which are called “accessory” or characteristic, and which while not present in large amount, may nevertheless exert a considerable influence upon the quality of the soil.

Soils from granitic and crystalline rocks.—In the case of the (potash-feldspar) granite soils it is generally admissible to expect that they will be fairly supplied with phosphoric acid, because in the great majority of cases, minute crystals of apatite (phosphate of lime) are more or less abundantly scattered through it. From the potash feldspar present, granite soils may always be relied on for a good supply of potash for plant use; on the other hand, unless hornblende be present, they are pretty certain to be deficient in lime, since neither lime, feldspar nor calcite are probable accessory ingredients of this rock.

Granite is exceedingly apt to weather by mechanical disintegration far in advance of its chemical decomposition. It is therefore common to find in sedentary soils overlying granite, a gradual increase of grains of its component crystalline minerals as we descend in the subsoil; until finally the latter grades off into rock almost unchanged save in lacking coherence. This is seen strikingly in the southern Appalachians, as well as in the Sierra Nevada and Sierra Madre of California; at Cintra in Portugal, at Heidelberg in Germany, and elsewhere.

But of the rocks that resemble granite and are popularly so called, a good many are not “true to name” and therefore form soils differing materially from the type just mentioned.

Thus the so-called granite areas of the Sierra Nevada of California are largely occupied by a rock containing, besides quartz, chiefly soda-lime feldspar and some hornblende, and scarcely any mica. It is more properly a diorite (grano-diorite); the soils formed from it are rather poor in potash, not strongly calcareous, and quite poor in phosphoric acid. On account of the small proportion of hornblende (unusual in diorites), these soils are light-colored (not “red”), and bear a growth of small pine instead of the usual oak growth of the lower Sierra slopes.

What is said of granite soils is also generally true of those formed from Gneiss, which is composed of the same minerals as granite, but has a slaty cleavage and on that account when upturned on edge, weathers rather more rapidly than most granites. Owing to the frequent occurrence of lenticular masses of quartz in gneiss, its soils are more commonly of a siliceous nature than are those of the true granite regions, and not as “strong” as the latter. This is the more true since gneiss often passes gradually into mica schist, which, being a mixture of quartz and mica only, not only weathers very slowly but also supplies but little of any importance to plants, to the soils formed from it. Such soils would mostly be absolutely barren but for the frequent occurrence in the rock, of accessory minerals that yield some substance to the soil. Yet it remains true that inasmuch as gneiss and mica-schists are among the rocks in which mineral veins most commonly occur, the proverbial barrenness of mining districts is very frequently traceable to these rocks. The same may be said of some of the related rocks, such as gabbro, minette and others.

Normal diorite consists of hornblende and soda-feldspar, with more or less quartz.

The soils derived from certain diorites of the Sierra Nevada of California have just been referred to. But these granite-like diorites are on the whole exceptional; it should be added that the (diabasic) “greenstones” of the Eastern United States and of the Old World, which are usually much finer-grained, do not form the mass of fine, angular debris constituting the subsoil in the Sierra Nevada, but weather into rounded masses and fine-grained soils possessing, on the whole, a fair fertility, though liable to contain an excessive proportion of silex in various forms.

Of the eruptive rocks as a class it is often said that they form very productive soils; yet, as these rocks differ widely from each other in composition, this statement must be taken with a great deal of allowance. Very many of them decompose with extreme slowness on account of their glassy nature; this is particularly true of obsidian, pumice stone, and the “volcanic ash” derived from its pulverization, and which is found unchanged, in sharp scales, among the decayed minerals of other rocks in complex soils. Other volcanic ash, however, being formed by the pulverization of crystalline or of basic lavas, weathers rather readily, as already stated; so that certain plants take possession in the course of a few years. The general classification into basic and acidic rocks, given above, is of importance in connection with soil formation from eruptive masses; for the basic rocks are much more easily attacked by the atmospheric agencies than the acidic class.

A broad distinction must, however, be made between the basic rocks of the basaltic class, which contain black pyroxene as a prominent ingredient, and those which, like many trachytes, are rich in feldspathic minerals. The latter are naturally rich in alkalies (potash and soda) which they impart to the corresponding light-colored soils; while the black basaltic rocks and lavas weather into “red” soils, sometimes containing extraordinary amounts of iron (ferric hydrate) and (from the lime-feldspars they contain) a fair supply of lime, but oftentimes very little potash. Experience seems to prove that the red basalt soils are mostly rather rich in phosphoric acid; this is especially true of the country covered by the great eruptive sheet of the Pacific Northwest, in the rocks of which the microscope readily detects the presence of numerous needles of apatite (lime phosphate). The same is true of the highly iron-bearing soils from the black basaltic lavas of the Hawaiian islands, even though they have been leached of all but traces of lime and potash. All these soils are physically “light” and easily workable, since the rocks in question contain but little alumina from which to form clay; they are sometimes extremely rich in iron, even to the extent of being capable of serving as iron ores.

The soils derived from trachytes and trachytic lavas are generally light-colored and light in texture; the latter from the presence of a large proportion of volcanic glass, together with undecomposed crystalline minerals. These are usually rich in potash, but poor in lime and phosphates. The high quality of the wines of the lower Rhine has been ascribed to these soils, which however vary greatly within the areal limits of the production of the high-grade wines, not only from gray trachytes to dark colored, highly augitic basalt, but also to acidic quartz porphyries or rhyolites, and clay-slates.

The rhyolites on the whole yield the poorest soils among the eruptive rocks; they are slow to weather at best, and the soils produced are poor and unsubstantial, largely from the predominance of quartz and undecomposable, glassy material; of which the phonolites are the extreme type, resisting the influence of the atmospheric agencies just as would so much artificial glass. Soils consisting largely of volcanic glass may be found covering considerable areas in the Sierra Nevada of California. Such “volcanic ash” soils are usually very unthrifty, and bear a growth of small pines.

Soils from sedimentary rocks.—Limestones, when pure and hard, are very slow to disintegrate, and are also very slowly attacked by carbonated water (see chap. 3, page 41). Soft impure and vesicular limestones are, however, very rapidly attacked, especially when underlying a surface clothed with the luxuriant vegetation that usually flourishes on soils rich in lime. The popular adage that “a limestone country is a rich country,” is of almost universal application and stamps lime, from the purely practical standpoint, as one of the most important soil ingredients.

Residual Limestone Soils.—Striking examples of the formation of large, fertile soil areas by the leaching out of limestones are found in the States of Alabama, Mississippi, Louisiana and Texas, where the fertile black prairies have been largely thus formed. The “blue-grass” country of Central Kentucky is another case in point.

The following table shows a representative example of the relative composition of the (cretaceous) “Rotten Limestone” of Mississippi, and the “residual” soil-stratum derived from it. The average thickness of the layer of residual clay above the limestone is about eight feet, but ranges from seven to ten; the upper layers of the limestone are somewhat softened, but the rock is always fresh at twelve feet, from which depth the sample analyzed was taken, in a cistern adjoining the field from which the soil and subsoil were procured. The black soil varies in depth from 8 to 15 inches; then there is a change to a brownish subsoil, reaching down to about two feet, and in drying cleaving into prismatic fragments. The black soil has here in the highest degree the peculiarity of crumbling in drying from its water-soaked condition, so that it may be plowed when wet without injury, although in the roads it works up into the toughest kind of mud. The prairie is sparsely timbered with compact, fair-sized black-jack oak, accompanied originally by red cedar.

The limestone derives its popular name of “rotten” from its being usually soft enough to be cut with a knife or hatchet, and is therefore somewhat used for building, and for burning lime.

COMPOSITION OF LIMESTONE, AND RESIDUAL SOIL AND SUBSOIL, FROM BLACK PRAIRIE, MONROE CO., MISSISSIPPI.

================================+============+=========+======== | “ROTTEN | SUBSOIL | SOIL |LIMESTONE.” |(YELLOW).|(BLACK). --------------------------------+------------+---------+-------- FINE EARTH. |Depth 12 ft.| 2-3 ft. |15 ins. Chemical analysis of fine earth.| | | --------------------------------+------------+---------+-------- Insoluble matter | 10.90 | 71.54 | 78.29 Soluble silica | | | Potash (K₂O) | .25 | .54 | .33 Soda (Na₂O) | .32 | .23 | .08 Lime (CaO) | 45.79 | 1.08 | 1.37 Magnesia (MgO) | .88 | .77 | .36 Br. Ox., of Manganese (Mn₃O₄) | | .05 | .14 Peroxide of Iron (FeO) | 1.42 | 5.42 | } Alumina (Al₂O₃) | 1.96 | 13.15 | } 14.22 Phosphoric Acid (P₂O₅) | | .05 | .10 Sulfuric Acid (SO₃) | | .04 | .03 Carbonic Acid (CO₂) | 35.73 | | Water and Organic matter | 2.84 | 6.99 | 5.75 | ------ | ----- | ------ Total | 100.09 | 99.86 | 100.67 | | | Humus | | | 1.93 “ Ash | | | 4.38 Hygroscopic moisture | | 10.35 | 12.82 absorbed at °C | | 19° | 19° --------------------------------+------------+---------+--------

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 tenacity of the clay soil, some of it must in the course of time have been washed away, we may safely assume that the original rock surface was from 50 to 60 feet higher than at present.

Sandstone Soils.—The indefiniteness of the nature of “sandstones” 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 nutrition. 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: quartzose or siliceous, calcareous, ferruginous, aluminous or clayey, and zeolitic. 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 sandstones frequently pass insensibly into quartz rock, and the light, unsubstantial soils they produce are of the poorest, containing 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.

Of this character are the pine-forest soils of the coast region of the Gulf of Mexico, particularly the “Sand hammocks” of the immediate Gulf border, from Mississippi Sound to Charlotte Harbor, Florida; the sandy lands of the Grand Traverse region of Michigan, and many other minor areas in the United States, usually characterized by a pine growth, often more or less stunted, according to the nature of the sand grains.

Calcareous sandstones usually form a very much better class of soils, partly for the intrinsic reason given above as regards limestones as soil-formers. The calcareous cement is very rarely pure calcite; in most cases it is very impure, as, most commonly, is also the “sand” itself. This is explained from the fact that such rocks (mostly soft and often quite unconsolidated) are, like limestones themselves, the result of deposition in shallow seas or lakes, receiving deposits from the land drainage, and enriched by the animal and vegetable life of such waters. Not uncommonly they contain, disseminated through them, grains of the mineral glauconite (a hydrous silicate of iron and potash), which readily supplies available potash; while the remnants of animals and plants furnish more or less of available phosphates. Thus the general presumption regarding calcareous sandstones is that the derived soils are of good quality, frequently of the very best. The same, however, does not appear to be true of sandstones cemented by dolomite; the soils derived from magnesian sandstones are in many cases noted for their unproductiveness. (See chapt. 3, p. 42).

Ferruginous Sandstones manifestly derive no important soil ingredients from their cement when the latter is measurably pure ferric hydrate; and when in addition the sand itself is purely siliceous, the soils resulting from the disintegration of the rocks are very poor.

Such are, e.g., the soils derived from the ferruginous sandstones of the Lafayette formation in a part of northern Mississippi and adjacent portions of Tennessee and Alabama, characterized by small scrubby oak or dwarfed pine. On the whole, however, such purely ferruginous quartz sandstones are exceptional, and should not detract from the favorable inferences usually to be drawn from the iron-rust tint of soils (see chapter 15).

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.

“Colloidal” Clay.

In connection with soils, clay may be defined, in the most general terms, as being the substance which imparts plasticity and adhesiveness to soils when wetted and kneaded, and which, when heated to redness, loses this property completely and permanently, becoming hard and coherent in proportion to the degree of heat to which it is exposed.

This term was first employed by Th. Schloesing, in communications to the French Academy of Sciences, and reported in the Comptes Rendus of that body; first in 1870. Unaware of Schloesing’s work, the writer began a full investigation of the subject of mechanical soil analysis in 1871, and published the results in 1873 (Am. Jour. Sci., Oct. 1873). Up to that time the limited resources of the library of the University of Mississippi had not given him an opportunity to see Schloesing’s publication. The two independent investigations, though conducted on somewhat different lines, gave of course practically the same results, and complement each other.

In common life, however, the name is applied to the whole of any naturally occurring earth which on wetting and kneading assumes a reasonable degree of plasticity and adhesiveness. When the latter property becomes nearly or quite insensible, the earth is designated as a “loam,” more or less “clayey” according to the amount of the pure, plastic and adhesive material associated with the mineral powders and sand that form the bulk of most soils.

Chemically, the pure clay substance probably consists (as has been stated above) of silica and alumina in the proportion of nearly 46 to 40, the rest (14%) being water of hydration, which is lost on burning the clayey material. But while it is true that such is the composition of the plastic substance of clays, plasticity and adhesiveness are by no means invariable properties of this compound. In its purest state, as kaolinite, it is readily mistaken for chalk, (and is sometimes used as such), being powdery to the touch and entirely devoid of plasticity when wetted and kneaded. The microscope shows this chalky kaolinite to consist of minute, mostly rounded, originally six-sided, thin plates, which when pure resemble to the touch powdered talc (soapstone) or even black-lead, rather than any clay known to common life. But being exceedingly soft, the kaolinite substance is easily ground or triturated into an extremely fine powder; and Johnson and Blake succeeded in producing sensible plasticity and adhesiveness by long-continued trituration of kaolinite with water in a mortar. A similar process, but continued much longer by the mechanical agencies concerned in soil-formation (see chapt. I), is unquestionably the chief factor concerned in the formation of natural plastic clays; but whether this is the only process by which the powdery kaolinite may be transformed into plastic clay, is a question not definitely settled. It is at least possible that repeated freezing and thawing, as well as the action of hot water, may take a part in the transformation, beyond that by which they destroy the crumbly (flocculated) structure of soils and clays, and render them plastic; as is done in the maturing of clays by potters.

There is still some discussion as to the chemical identity of colloidal clay with Kaolinite; but the objections are not convincing.

It has of late been attempted to extend the meaning of this word to the behavior of all powders when wetted with water. But the adhesive plasticity of clay stands almost alone, in that (aside from contraction) it preserves in drying the form into which it may have been molded while wet, even when struck, whereas other powdery substances similarly treated at once collapse back into the original powder. The exclusive use of clay in modeling offers the typical example of plasticity as generally understood. The addition of any powdery substance, however fine, diminishes the plasticity of clay.

American Journal of Science, 2d Ser., Vol. 43, p. 357.

Causes of Plasticity.—In any case the property of plasticity and adhesiveness is restricted to the particles so fine that they fail to settle, in the course of 24 hours, through a column of pure water eight inches (200 m) high, while some are so extremely minute that they will not settle for many months, and even for several years. Such turbid “clay water” may sometimes be found existing in nature, in moist, secluded places, for weeks after the subsidence of the overflows of rivers whose water is exceptionally free from dissolved mineral matter.

Williams (Forsch. Agr. Phys. Vol. 18, p. 225 ff.) claims that the diameter of the minutest clay particles is one-thousandth of a millimeter, their form being that of scales showing continual (Brownian) motion in water. He maintains that the plasticity of clay is due to this minute size, and this view has gained wide acceptance in late works on the subject. But this assumption cannot be maintained in the face of the fact that nothing like the adhesive plasticity of clay can be attained even by the finest powders of other substances, least of all by those having the closest mineralogical resemblance to kaolinite, such as graphite and talc. Above all, the most persistent trituration with water utterly falls to restore plasticity to clay once baked so as to expel its water of hydration, although the fineness of the particles is thereby not only not diminished, but actually increased, by contraction in heating. No powders however fine can replace the functions of clay in soils, viz. the maintenance of floccules, and tilth dependent thereupon; and they distinctly impair the plasticity of clay. The fine “slickens” of quartz mills merely render soils containing them more close and impervious, and more difficult to flocculate. Even gelatinous masses like hydrated ferric and aluminic oxids fail to replace clay in its adhesive functions.

Separation of Colloidal Clay.—This property of the plastic clay substance, of diffusing in pure water, furnishes the means of separating from it the coarser, sandy and silty portions of soils and natural clays, and observing its characteristic properties, so far as the almost unavoidable admixture of some other substances, presently to be considered, permits.

In natural soils the clay particles usually incrust the powdery ingredients, cementing them together; or themselves form complex aggregates (floccules) of large numbers of individual particles. These may be loosened from their adhesion or cohesion either by prolonged, gentle kneading of the wet clay, or by more or less prolonged digestion (soaking) in hot water, or more expeditiously, by lively boiling with water. The boiling should not, however, be prolonged beyond the time actually required for disintegration, since (as Osborne has shown) long-protracted boiling tends to render the clay permanently less diffusible.

From the turbid clay-water the diffused clay may be obtained either by evaporating the water (which as the bulk is very large, is usually inconvenient), or, more conveniently, by throwing it down from its suspension by the action of certain substances which possess the property of curdling (coagulating) the clay substance into flocculent masses that settle quickly. Of all known substances, lime, in the form of lime-water, acts most energetically in producing this change; but other solutions of lime, as well as most salts and mineral acids, produce the same effects when used in sufficient quantity. Common salt is among the most convenient, because it can most readily be leached out of the clay precipitate thus thrown down. This when white, resembles boiled starch, but being usually colored by iron might be easily mistaken for the mixed precipitate of ferric hydrate and alumina so commonly obtained by chemists in soil analysis. When separated from the water and dried, the jelly-like substance (“colloidal clay”) shrinks as extravagantly as would so much boiled starch, into hard, shiny crusts or flakes, which when struck in mass are sometimes even resonant, and bear more resemblance to glue than to the clay of everyday life. Like glue, too, but much more quickly and tenaciously, the dried colloidal clay adheres to the tongue, so as to render the separation painful; when wetted it quickly bulges with great energy, and in a short time resumes its former jelly-like condition. When moistened with less water it assumes a highly plastic and adhesive condition, so that it is difficult to handle and almost as sure to soil the operator’s hands as so much pitch.

Rep. Conn. Agr. Expt. Stn., 1886, 1887.

Effects of Alkali Carbonates upon Clay.—The carbonate of potash and soda, when in very dilute solution (.01 to .05%) exert upon diffused clay an effect the reverse of the acids and neutral salts. They destroy the flocculent aggregates formed by precipitation with these, or naturally existing in the soil, and tend to puddle the clay so as to render it impervious to water. It is thus that in the alkali lands of the arid regions we often find the soil or subsoil consolidated into a very refractory “hardpan,” difficult to break even with a sledge hammer and impossible to reduce to tilth until the alkali carbonate is destroyed by means of a lime salt, such as gypsum. (See chapt. 23). Ammonia water also helps to cause the diffusion of clay in water, but its effect of course disappears upon drying. It is probable that this property of sodic carbonate can be utilized in rendering earth dams firmer and more secure against the penetration of water.