Principles and Practice of Agricultural Analysis

27. Types Of Rocks.—Rocks may be divided in reference to their structure into four types: First, crystalline; second, vitreous; third, colloidal; fourth, fragmental.

Of these classes there may be selected, as types of the first order, granite and crystalline limestone.

The second class is typically represented by obsidian. Rocks of this kind are confined to a volcanic origin.

The third class of rocks is completely amorphous in its structure and is less common than the others. It is found only in rocks of chemical origin. Types of this class are the siliceous sinters, opals, flint nodules, and many serpentines.

Of the fourth class of rocks, sandstone is typical, being comprised wholly of fragments of rocks pre-existing. The particles may be held together either by cohesion or by a cement composed of silica, iron oxids, carbonate of lime or clayey matter.

28. The Microscopical Structure of Rocks.—A great deal more light is thrown upon the nature of rock materials by microscopical study than by their study in bulk. The requisites for a microscopical study of rock are that the material should be cut into extremely thin laminae with parallel sides and polished so as to transmit the light freely. The study of the crystalline structure of the material is then conducted by means of a microscope furnished with polarizing and analyzing appliances. The light before passing through the mineral film is polarized by a Nicol prism. After passing through the film it is analyzed by a second Nicol prism. In this way the crystalline structure of the rock as affecting polarized light is distinctly brought out. The thickness of the films examined should be from ¹⁄₅₀₀ to ¹⁄₆₀₀ of an inch.

Fig. 1. Microstructure of granite.
Fig. 2. Microstructure of micropegmatite.
Fig. 3. Microstructure of quartz porphyry.
Fig. 4. Microstructure of porphyritic obsidian.
Fig. 5. Microstructure of trachyte.
Fig. 6. Microstructure of serpentine.

The method of rock study by thin microscopic sections is one of comparatively recent origin. It is scarcely more than a dozen years since the process was fairly adopted by mineralogists. The value of the method is based upon the fact that every crystalline mineral has certain definite optical properties. Therefore, when a crystalline mineral is distorted or misshapen so as to be incapable of identification by the ordinary method, it can be at once identified by its optical examination in the manner just described. In this way not only can one mineral be distinguished from another, but the crystalline system to which it belongs can be accurately pointed out. The value of the method is well summed up by Merrill,^[21] who says that it is not merely an aid in determining the mineralogical composition of a rock, but also, which is often much more important, its structure and the various changes which have taken place in it since its first consolidation. Rocks are not the definite and unchangeable mineral compounds they were once considered, but are rather ever varying aggregates of minerals which even in themselves undergo structural and chemical changes almost without number.

Another valuable result of such a study is illustrated by the discovery that the structural features of a rock are not dependent upon its chemical composition or geologic age, but upon the conditions under which it cooled from the molten magma. Portions of the same rock may vary all the way from a wholly crystalline to a pure vitreous form.

Some typical microstructures of crystalline rocks are shown in the accompanying figures 1–6.^[22]

Although this method of study has thus far been confined mainly to crystalline rocks, its efficiency is by no means limited to them. The fragmental rocks and their decomposed débris to which the name soil is given are equally worthy of study by this method. Indeed, the full value of a chemical analysis of any rock or soil can not be ascertained unless such an analysis is accompanied by a microscopic examination. It is desirable to know not merely what there is in any soil, but in what form these compounds exist. To this latter question the chemical analysis as ordinarily made will give no clew. In Germany a beginning has been made in this line of work, and American scientists are beginning to realize its importance. An outline of this method of analysis will be given in the proper place.

29. Specific Gravity.—Much information in regard to the properties of a rock, or mineral constituent thereof, may be derived from its specific gravity.

The internal structure of a rock may have much to do with its apparent specific gravity. As an instance of this, it may be stated that an obsidian pumice will float upon water, buoyed up by the air contained in its vesicles, while a compact obsidian of the same composition will sink immediately. A careful discrimination must, therefore, be made between apparent and true specific gravity. In general it may be said that crystalline rocks have a higher specific gravity than those of a vitreous nature. The specific gravity is, therefore, largely dependent upon chemical and crystallographic properties; for instance, among siliceous rocks those which contain the largest amount of silica are the lightest, while those with a comparatively small amount, but rich in iron, lime, and magnesia, are heaviest.

30. Chemical Composition of Rocks.—Rocks are often classified with respect to the chief mineral constituent which they contain. Rocks which are composed largely of lime are termed calcareous; of silica, siliceous; of iron, ferruginous; and of clay, argillaceous. In respect of eruptive rocks, it is customary to speak of those which show above sixty per cent of silica as acidic, while those containing less than fifty per cent of silica and a correspondingly larger amount of iron, lime, and magnesia, are spoken of as basic. Illustrations of the classification of rocks on the above principles are given below.^[23]

Stratified Rocks.
Kind.	Specific Gravity.	Composition.
Calcareous:
	Compact limestone	2.6 to 2.8	Carbonate of lime.
	Crystalline limestone

	Compact dolomite	2.8 to 2.95	Carbonate of lime and magnesia.
	Crystalline dolomite

Siliceous:
	Gneiss	2.6 to 2.7	Same as granite.
	Siliceous sandstone	2.6	Mainly silica.
	Schist	2.6 to 2.8	60 to 80 per cent silica.

Argillaceous:
	Clay slate (argillite)	2.5	Mainly silicate of alumina.

Eruptive Rocks.
		Specific Gravity.	Per cent silica.
Acidic Group:
	Granite	2.58 to 2.73	77.65 to 62.90
	Liparite	2.53 to 2.70	76.06 to 67.61
	Obsidian	2.26 to 2.41	82.80 to 71.19
	Obsidian pumice	Floats on water.	82.80 to 71.19

Intermediate Group:
	Syenite	2.73 to 2.86	72.20 to 54.65
	Trachyte	2.70 to 2.80	64.00 to 60.00
	Hyalotrachyte	2.4 to 2.5	64.00 to 60.00
	Andesite	2.54 to 2.79	66.75 to 54.73

Basic Group:
	Diabase	2.66 to 2.88	50.00 to 48.00
	Basalt	2.90 to 3.10	50.59 to 40.74
	Peridotite	3.22 to 3.29	42.65 to 33.73
	Peridotite (iron rich)	3.86	23.00
	Peridotite (meteorite)	3.51	37.70

31. Color Of Rocks.—The color of rocks is determined chiefly by the oxids of metals which they contain and the degree of oxidation of the mineral in each particular case. There are, however, many colors of rocks which seem to depend not upon any particular mineral ingredient which they contain, but upon some particular crystalline structure or physical condition.

The chief coloring matters in minerals are those which form colored bases such as iron, manganese, chromium, etc. The yellow, brown, and red colors, common to fragmental rocks, are due almost wholly to free oxids of iron. The gray, green, dull brown, and even black colors of crystalline rocks are due to the presence of free iron oxids or to the prevalence of silicate mineral rich in iron, as augite, hornblende, or black mica. Rarely copper and other metallic oxids than those of iron are present in sufficient abundance to impart their characteristic hues. As a rule, a white or light-gray color denotes an absence of an appreciable amount of iron in any of its forms. The bluish and black colors of many rocks, particularly the limestones and slates, are due to the presence of carbonaceous matter.

In still other cases, and particularly the feldspar-bearing rocks, the color may be due in part to the physical condition of the feldspar.

Inasmuch as the color of rocks is due so largely to metallic oxids, it is easy to see that they may undergo changes when exposed to weathering, or the degree of oxidation may change, and either, together with changes in the physical structure of the rock, may cause a distinct change in color. Luster is often considered in connection with color, and is due almost exclusively to physical conditions.

32. Kinds of Rocks.—The rocks which form any essential part of the earth’s crust are grouped under four main heads, the distinction being based upon their origin and structure.^[24] Each of the main divisions may be subdivided into groups or families, the distinction being based mainly upon chemical composition, structure, and mode of occurrence. The four chief families are:

First, aqueous rocks, formed mainly through the agency of water as chemical precipitates or as sedimentary beds.

Third, metamorphic rocks, changed from their original condition through dynamic or chemical agencies, and which may have been partly of aqueous and partly of igneous origin.

Fourth, igneous or eruptive rocks, which have been brought up from below in a molten condition, and which owe their present structural peculiarities to variations in composition and conditions of solidification.

33. Aqueous Rocks.—Aqueous rocks may be divided into the following general classes:

Second, rocks formed as sedimentary deposits and fragmental in structure. The second class may again be subdivided into rocks formed by mechanical agencies and mainly of inorganic materials; and second, rocks composed mainly of the débris of plant and animal life.

In regard to the first form of aqueous rocks, namely, those formed as chemical precipitates, it may be said that while their quantity is not large they are yet of considerable importance from an agricultural point of view. They embrace those substances which, having once been in a condition of vapor or aqueous solution, have been deposited or precipitated, either by cooling or by the evaporation of the liquor holding them in solution, or by coming in contact with chemical substances capable of precipitating them. The influence of water as a solvent is perhaps not fully appreciated. Its solvent influence will be noted particularly under the head of weathering or decay of rocks. Its importance, however, in producing stratified rocks has been very great. Water, especially when under great pressure and at a high temperature, has the power of dissolving many minerals. This power is often greatly increased by the mineral matter previously in solution in the water or by the gases which it may contain. As an illustration of the latter property, the solvent action of water charged with carbon dioxid on limestone may be cited.

When mineral matters have been dissolved by the water in the ways mentioned and carried with the water beyond the condition where the solution has taken place, new conditions are found favorable to the precipitation of the dissolved matters. The water, which before may have been very hot, may reach a place where it cools, and being a supersaturated solution, the excess of the material is thrown down as the water cools.

On the other hand, if the solution be due to the presence of carbon dioxid and the water reach a place where it is exposed to the air or where the pressure under which the abundance of the gas has been due is diminished, the carbon dioxid will escape and the mineral matters which have been dissolved thereby will be precipitated.

The incrustations which often appear round the mouth of springs and the occurrence of stalagmites and stalactites in caves are illustrations of this action.

In respect of the formation of rocks as precipitates from a state of vapor we have scarcely any illustrations excepting in volcanic regions. Rocky materials with which we are generally acquainted are practically non-volatile at the highest temperature which can be secured on the earth’s surface, but it is possible that in the interior of the earth the temperature may be so high as to maintain many substances in a state of vapor.

They may, in this case, become disassociated so that the compounds or elements exist distinctly in a vaporous condition. Such a vapor transported to regions of diminished temperature would first of all on cooling permit a union of the chemical elements forming new compounds less volatile, which, of course, would be at once precipitated.

The rocks and minerals formed in this way which are of some agricultural importance may be classified as follows:

Oxids, carbonates, silicates, sulfur, sulfids, sulfates, phosphates, chlorids, and hydrocarbon compounds, the most important from an agricultural point of view being the phosphates.

The second group of rocks, namely those formed as sedimentary deposits, differ from those just described in that they are comprised mainly of fragmental materials derived from the breaking down of pre-existing rocks. The formation of fragmental rocks includes, therefore, the same processes as are active in the formation of arable soil. They are deposited from water, and are as a rule distinctly stratified.

Through the action of pressure and the heat thereby generated, or simply through the chemical action of percolating solutions, such rocks pass over into the crystalline sedimentary forms known as metamorphic. All metamorphic rocks, however, are not of a sedimentary origin. For instance, by pressure, heat, and the chemical changes thereby induced, granite may be changed into gneiss and the latter would then be a metamorphic rock.

This group of sedimentary rocks and of sedimentary material, either unchanged or metamorphosed, is of vast extent and includes materials of widely varying chemical and mineralogical nature. They form by far the greater portion of the present surface of the earth, even the mountain ranges being composed mainly of this sedimentary material. Indeed, in the whole of this country there is only a comparatively very small extent of igneous or irruptive rocks. They are of great importance from a purely scientific, as well as agricultural standpoint, since they contain the fossil records of past geologic ages. From them it is possible to study the variations in climate, the meteorological conditions in circumstances and periods far remote, and thus form some idea of the process by which the crust of the earth has been modified by natural forces from its original form to the present time.

The sedimentary rocks may be divided, with sufficient accuracy for our purposes, into two great classes: First, rocks formed by mechanical agencies and mainly of inorganic materials. These are subdivided again as follows:

The second class of sedimentary rocks is formed largely, or in part at least, by mechanical agencies, but is comprised chiefly of the débris of plant and animal life. It may be subdivided as follows:

(c) The carbonaceous group, such as peat, lignite, coals, etc. The different classes of rock described above are distinguished by special qualities represented largely by the name. The first division, the arenaceous group, is composed mainly of the siliceous or coarsely granular materials derived from the disintegration of older crystalline rocks, which have been rearranged in beds of varying thickness through the mechanical agency of water. They are, in short, consolidated or unconsolidated beds of sand and gravel. In composition and texture they vary almost indefinitely. Many of them having suffered little during the process of disintegration and transportation are composed essentially of the same materials as the rocks from which they were derived.

The sandstones, which are the type of these rocks, vary greatly in structure as well as in composition, in some the grains being rounded while in others they are sharply angular.

The material by which the individual grains of a sandstone are bound together is usually the material of some of the other classes. The calcareous, ferruginous, and siliceous cements being the chief ones. This cementing substance is deposited among the granules forming the sandstone by percolating water.

The colors of sandstone are dependent usually upon iron oxids. Especially is this true of the red, brown, and yellow colors. In some of the light grey varieties, the color is that of the minerals comprising the stone. Some of the darker colored sandstones contain organic matter.

Fig. 7.

Microscopic Structure of Sandstone.

The rocks of the argillaceous group are composed essentially of a hydrous silicate of alumina, which is the basis of common clay, and varying amounts of free silica, oxids of iron and manganese, carbonates of lime and magnesia, and small quantities of organic matter. They may have originated in situ from the decomposition of feldspars or as deposits of fine mud or silt at the bottom of large bodies of water. The older formations of these rocks are known as shales, argillites, and slates and the fissile structure which enables this to be split into thin sheets is probably due to the conditions under which they have been formed and not to any properties of the clays themselves.

One of the purest forms of this rock is kaolin, which is almost a pure hydrous silicate of alumina formed from the decomposition of feldspathic rocks from which the alkalies, iron oxids and other soluble constituents have been removed by water.

Under the volcanic group are included the materials ejected from volcanic vents in a more or less finely comminuted condition and which through the drifting power of atmospheric currents may be scattered over many miles of territory. Various names are applied to such products, names dependent in large part upon their state of subdivision. Volcanic dust and sand, or ashes, includes the finer dust-like or sand-like materials, and lapilli, or rapilli the coarser. The general name tuff includes the more or less compacted and stratified beds of this material, while trass, peperino, and pozzuolano are local varietal names given to similar materials occurring in European volcanic regions.

The second division, namely sedimentary rocks composed of the débris of plant and animal life includes many forms of great agricultural importance.

The first subdivision of this group is the infusorial or diatomaceous earth. It forms a fine white or yellowish pulverulent rock composed mainly of minute shells, or tests of diatoms, and is often so soft and pliable as to crumble readily between the thumb and fingers. According to Whitney the beds are of comparatively limited extent and for this reason are of little agricultural value, although the weathering of this diatomaceous material gives rise to a light yellow clay forming very fertile agricultural lands.

The second subdivision of this group includes the rocks of a calcareous nature derived from animal life; that is to say, what are properly called limestones. They vary in color, structure, and texture almost indefinitely, and include all possible grades of materials from those which can be used only as a flux, or for lime burning, through ordinary building materials to the finest marbles. These rocks are world-wide in their distribution and limited to no one particular geologic horizon, but are found in stratified beds among rocks of all ages from the most ancient to the most recent.

Owing to the fact that their chief constituent, carbonate of lime, is soluble in ordinary meteoric waters, the rocks have undergone extensive decomposition, their lime being removed, while their less soluble constituents or impurities remain to form soil. A single ton of residual soil represents not infrequently a loss of 100 tons of original rock matter. As this mass of lime carbonate is removed by solution the residual soil settles, and as the limestone rocks are more soluble than the adjacent rock formations limestone formations usually form valley lands with ridges on either side. Caves are frequently found in such formations. Furthermore, as the lime is almost all in the form of the easily soluble lime carbonate it can be very completely removed and the fertile “limestone soils” are often very deficient in lime and respond readily to an application of burnt lime, which, not infrequently, is quarried from the same field. From an agricultural standpoint this group is of very great interest and importance.

The third subdivision of this group, namely, that of vegetable origin, includes peat, lignite, coals, etc. Rocks of this group are made up of more or less fragmental remains of plants. In many of them, as the peats and lignites, the traces of plant structure are still apparent. In others, as the anthracite coals, these structures have been wholly obliterated by metamorphisms.

Plants when decomposing on the surface of the ground give off their carbon to the atmosphere in the shape of carbon dioxid gas leaving only the strictly inorganic or mineral matter behind. When, however, they are protected from the oxidizing influence of the air, by water or by other plant growth, decomposition is greatly retarded, and a large portion of the carbonaceous and volatile matters is retained, and by this means together with pressure from the overlying mass, the material becomes slowly converted into coal. When this process goes on near the surface of the earth, and without much pressure, peat or muck is the product.

The fourth subdivision of this group, the phosphatic, forms a class of rocks limited in extent but of the greatest economic importance. Guano, coprolites, and phosphatic rocks (the phosphorites) come under this head.

34. Aeolian Rocks.—This class of rocks is of less importance than the others, either geologically or agriculturally. It is formed from materials drifted by the winds and this material has various degrees of compactness. Usually the components of these drifts form rocks or deposits of a friable texture and of a fragmental nature. The very extensive deposits of loess in China, forming their most fertile lands, are admitted now to have been formed in this way, but it is now generally admitted that similar deposits in this country are of subaqueous origin.

Chief among these rocks, are the volcanic ashes which are often carried to a long distance by the wind before they are deposited and consolidated into rock masses. Many loose soils may be carried to great distances by the wind, the deposits forming new aggregates. This is particularly the case in arid regions.

35. Metamorphic Rocks.—This class of rocks includes all sedimentary or eruptive rocks, which, after their deposition and agglomeration, have been changed in their nature through the action of heat, pressure, or by chemical means. Sometimes these changes are so complete that no indication of the character of the original rocks remains. At other times the changes may be found in all the stages of progress, so that they can be traced from the original fragmental or irruptive to the completely metamorphosed deposit. This is especially true of rocks containing large quantities of lime. In those containing silica, the changes are less readily traced.

Fig. 8.

Microstructure of Crystalline Limestone.

(West Rutland, Vermont.)

The metamorphic rocks may be divided into two subclasses, namely, stratified or bedded, and foliated or schistose.

The rocks of the first class are represented by the crystalline limestones and dolomites. The microstructure of a crystalline limestone is shown in Fig. 8.^[25] When lime and magnesia occur together in combination with carbon dioxid, the substance is known as dolomite. The chemical nature of these rocks and their soil-forming properties are the same as those of the ordinary, non-metamorphosed limestones and dolomites to which reference has already been made. The subject need not, therefore, be further dwelt upon here.

Fig. 9.

Microstructure of Gneiss.

(West Andover, Massachusetts.)

At a a are shown plagioclase crystals broken and rounded by the sheering force producing the foliation.

The second variety of metamorphic rocks is represented by the gneisses and crystalline schists. Gneiss has essentially the same composition as granite and can frequently hardly be distinguished from it, except by a microscopic study of its sections, and even thus it is sometimes difficult to determine. Frequently a number of new minerals is formed in the metamorphic changes. The microstructure of a gneiss is shown in Fig. 9.^[26] The schists include an extremely variable class of rocks, of which quartz is the prevailing constituent, and which, as a rule, are deficient in potash and other important ingredients.

36. Rocks Formed Through Igneous Agencies, or Eruptive Rocks.—This group includes all those rocks, which, having been at some time in a state of igneous fusion, have been solidified into their present form by a process of cooling. It may be stated, as a general principle, that the greater the pressure under which a rock solidifies and the slower and more gradual the cooling the more perfect will be found the crystalline structure. Hence, it follows that the older and more deep-seated rocks which are forced up in the form of dikes, bosses, or intrusive sheets, into the overlying masses, and which have become exposed only through erosion and removal of the overlying rocks, are the more highly crystalline.

Among the more important of the first division of the plutonic form, from an agricultural point of view, are the granites. The essential constituents of granite are quartz, potash feldspar, and plagioclase. One or more minerals of the mica, amphibole or pyroxene groups are also commonly present, and in microscopic proportions apatite and particles of magnetic iron. The more valuable constituents, from an agricultural standpoint, are the minerals potash feldspar, and apatite, which furnish by their decomposition the essential potash and phosphoric acid.

In addition to the granites, which have already been mentioned, the group includes the syenites, the nepheline syenites, the diorites, the gabbros, the diabases, the theralites, the peridotites, and the pyroxenites.

The second group, the effusive or volcanic rocks, includes those igneous rocks, which, like the first group, have been forced up through the overlying rocks, but which were brought to the surface, flowing out as lavas. These, therefore, represent, in many cases, only the upper or surface portions of the first group, differing from them structurally, because they have cooled under little pressure more rapidly, and hence are not so distinctly crystalline. These comprise the following groups:

It is, in most cases, impossible to state which of the above classes is of most importance from an agricultural standpoint, since, in the process of soil formation, both chemical and physical processes are involved, whereby the character of the resultant soil is so modified as to but remotely resemble its parent rock. In most soils, the prevailing constituent is but the least soluble one of the rock mass from which it was derived. Thus a limestone soil may contain upwards of ninety per cent of silica and alumina, while the original limestone itself may not have carried more than one or two per cent of these substances. Of course, if a rock mass contains none of the constituents essential to plant growth, its resultant soil must by itself alone be quite barren. It does not absolutely follow, however, that those rocks containing the highest percentages of valuable constituents will yield the most fertile soils, since much depends on the manner in which they have been formed, the amount of leaching, etc., they may have undergone. Nevertheless, the study of the composition of the rocks in their relation to soils, is an extremely interesting and by no means unimportant one.

A comparative table of rock compositions is here given. It will be observed that there is a considerable range of variation even among rocks of the same class, a fact due to the varying abundance of their mineral constituents. The figures given are not those of actual analyses on any one particular rock, but are selected from a number of comparatively typical cases; and, it is thought, fairly well represent the composition of the class of rocks indicated.

Composition of Rocks.—The Figures Indicate Parts per Hundred.
	SiO₂.	Al₂O₃.	Fe₂O₃. FeO.	MgO.	CaO.	Na₂O.	K₂O.	P₂O₅.
Granite Quartz poryhyries} Liparite	63–78	10–15	2–3	0.3–0.5	1–2	2–3	4–5	0.05–0.15

Syenite Orthoclase porphyries} Trachyte	55–73	12–16	5–7	2–6	3–5	2–6	4–7	trace.

Nepheline syenites} Phonolites	54–56	16–22	4–6	0.4–0.88	2–4	3–7	4–6	0.15

Diorites Porphyrites} Andesites	52–65	16–20	7–10	5–7	5–7	2–4	1–2	0.1–0.3

Gabbros Norites} Melaphyrs	48–55	12–20	8–15	2–7	6–10	2–4	0.5–2	0.1–0.33

Theralites Tephrites} Basanites	43–47	15–23	9–18	1–6	6–10	5–7	2–4	trace.

Peridotites Picrite porphyrites} Limburgites	23–43	1–10	10–15	15–45	1–4	0–4	trace.	0.0

Pyroxenites} Augitites	50–55	0.5–4	4–10	20–25	8–15

Leucite rocks	48–50	15–20	7–10	1–2	5–10	3–5	5–7	0.5–2

Nepheline rocks	40–45	8–20	10–20	1–13	4–10	4–8	1–3	0.2

37. Origin of Soils.—The soils in which crops grow and which form the subject of the analytical processes to be hereinafter described have been formed under the combined influences of rock decay and plant and organic growth. The mineral matters of soils have had their origin in the decay of rocks, while the humic and other organic constituents have been derived from living bodies. It is not the object of this treatise to discuss in detail the processes of soil formation, but only to give such general outlines as may bear particularly on the proper conceptions of the principles of soil investigation.

38. The Decay of Rocks.—The origin and composition of rocks are fully set forth in works on geology and mineralogy. Only a brief summary of those points of interest to agriculture has been given in the preceding pages. The soil analyst should be acquainted with these principles, but for practical purposes he has only to understand the chief factors active in securing the decay of rocks and in preparing the débris for plant growth.

The following outline is based on the generally accepted theories respecting the formation of soils.[7]

39. The Action of Freezing and Thawing.—In those parts of a rock stratum exposed near the surface of the earth the processes of freezing and thawing have perhaps had considerable influence in rock decay. The expansive force of freezing water is well known. Ice occupies a larger volume than the water from which it was formed. The force with which this expansion takes place is almost irresistible. The phenomenon of bursted water pipes which have been exposed to a freezing temperature is not an uncommon one. While the increase in volume is not large, yet it is entirely sufficient to produce serious results. The way in which freezing affects exposed rock is easily understood. The effect is unnoticeable if the rock be dry. If, on the other hand, it be saturated with water, the disintegrating effect of a freeze must be of considerable magnitude. This effect becomes more pronounced if the intervals of freezing and thawing be of short duration. The whole affected portion of the rock may thus become thoroughly decayed. But even in the most favorable conditions this form of disintegration must be confined to a thin superficial area. Even in very cold climates the frost only penetrates a few feet below the surface, and therefore the action of ice cannot in any way be connected with those changes at great depths, to which attention has already been called. Nevertheless, certain building stones seem very sensitive to this sort of weathering, and the crumbling of the stone in the Houses of Parliament is due chiefly to this cause.

On the whole it appears that the action of ice in producing rock decay has been somewhat overrated, although its power must not by any means be denied. But on the other hand a freeze extending over a long time tends to preserve the rocks, and it therefore appears that the entire absence of frost would promote the process of rock decay.

At best it must be admitted that frost has affected the earth’s crust only to an insignificant depth, but its influence in modifying the arable part of the soil is of the utmost importance to agriculture.

40. The Action of Glaciers.—The action of ice in soil formation is not confined alone to the processes just described. At a period not very remote geologically, a great part of our Northern States was covered with a vast field of moving ice. These seas of ice crept down upon us from more northern latitudes and swept before them every vestige of animal and vegetable life. In their movement they leveled and destroyed the crests of hills and filled the valleys with the débris. They crushed and comminuted the strata of rocks which opposed their flow and carried huge boulders of granite hundreds of miles from their homes. On melting they left vast moraines of rocks and pebbles which will mark for all time the termini of these empires of ice. When the ice of these vast glaciers finally melted the surface which they had leveled presented the appearance of an extended plain. No estimate can be made of the enormous quantities of rock material which were ground to finest powder by these glaciers. This rock powder forms to-day no inconsiderable part of those fertile soils which are composed of glacial drift. The rich materials of these soils probably bear a more intimate relation to the rocks from which they were formed than of any other kinds of soil in the world. The rocks were literally ground into a fine powder, and this powder was intimately mixed with the soils which had already been formed in situ. The melting ice also left exposed to disintegrating forces large surfaces of unprotected rocks in which decay would go on much more rapidly than when covered with the débris which protected them before the advent of the ice. The area of glacial action extended over nearly all of New England and over the whole area of the northern tier of States. It extended southward almost to the Ohio river, and in some places crossed it. The effect of the ice age in producing and modifying our soil must never be forgotten in a study of soil genesis. It is not a part of our purpose here to study the causes which produced the age of ice. Even a brief reference to some of the more probable ones might be entirely out of place. Before the glacial period it is certain that a tropical climate extended almost, if not quite, to the North Pole. The fossil remains of tropical plants and animals which have been found in high northern latitudes are abundant proofs of this fact.

In the opinion of Sterry Hunt,^[27] rock decay has taken place largely in preglacial and pretertiary times. The decay of crystalline rocks is a process of great antiquity. It is also a universal phenomenon. The fact that the rocks of the southern part of this country seem to be covered with a deeper débris than those further north is probably due to the mechanical translation of the eroded particles towards the south. The decay and softening of the material were processes necessarily preceding the erosion by aqueous and glacial action.

It is possible that a climate may have existed in the earlier geologic ages more favorable to the decay of rocks than that of the present time.

41. Progress of Decay as Affected by Latitude.—Extensive investigations carried on along the Atlantic side of the country show wide differences in the rate of decay in the same kind of rocks in different latitudes. In general, the progress of decay is more marked toward the south. The same fact is observed in the great interior valleys of the country; at least, everywhere except in the arid and semi-arid regions. Wherever there is a deficiency of water the processes of decay have been arrested. Where the rock strata have been displaced from a horizontal position the progress of decay has been more rapid. This is easily understood. The percolation of water is more easy as the displacement approaches a vertical position.

A most remarkable example of this is seen in the rocks of North Carolina.^[28] A kind of rock known as trap is found in layers called dikes in the Newark system of rocks in that State. These dikes have been so completely displaced from the horizontal position they at first occupied as to have an almost vertical dip. The edges thus exposed vary from a few feet to nearly 100 feet in thickness. The trap rock in those localities is composed almost exclusively of the mineral dolerite, which is so hard and elastic in a fresh state as to ring like a piece of metal when struck with a hammer. In building a railroad through this region these dikes were in some places uncovered to a depth of forty feet and more. At this depth they were found completely decomposed and with no indications of having reached the lower limit of disintegration. The original hard bluish dolerite has been transformed into a yellowish clay-like mass that can be molded in the fingers and cut like putty. Similar geologic formations in New Jersey and further North do not exhibit anything like so great a degree of decomposition, thus illustrating in a marked degree the fact that freezing weather for a part of the year is a protection against rock decay. The ice of winter at least protects the rocks from surface infiltration, although it can not stop the subterranean solution which must go on continuously.

Other things being equal, therefore, it appears that as the region of winter frost is passed the decay of the rocks has been more rapid than in the North, because the chief disintegrating forces act more constantly.

42. The Solvent Action of Water.—The water of springs and wells is not pure. It contains in solution mineral matters and often a trace of organic matter. The organic matter comes from contact with vegetable matter and other organic materials near the surface of the earth. The mineral matter is derived from the solvent action of the water and its contents on the soil and rocks.

The expressions “hard” and “soft” applied to water indicate that it has much or little mineral matter in suspension, as the case may be. When surface and spring waters are collected into streams and rivers they still contain in solution the greater part of the mineral matters which they at first carried.

When well or spring waters have more than forty grains of mineral matter per gallon they are not suitable for drinking waters. Mineral waters, so called, are those which carry large quantities of mineral matter, or which contain certain comparatively rare mineral substances which are valued for their medicinal effects.

The analysis of spring, well, or river waters will always give some indication of the character of the rocks over which they have passed.^[29] The vast quantities of mineral matters carried into oceans and seas are gradually deposited as the water is evaporated. If, however, these matters be very soluble, such as common salt, sulfate of magnesia, etc., they become concentrated, as is seen with common salt in sea waters. In small bodies of waters, such as inland seas, which have no outlet, this concentration may proceed to a much greater extent than in the ocean. As an instance of this, it may be noted that the waters of the Dead Sea and Great Salt Lake are impregnated to a far greater degree with soluble salts than the water of the ocean. The solvent action of water on rocks is greatly increased by the traces of organic or carbonic acids which it may contain. When surface water comes in contact with vegetable matter it may become partially charged with the organic acids which the growing vegetable may contain or decaying vegetable matter produce. Such acids coming in contact with limestone under pressure will set free carbon dioxid. Water charged with carbon dioxid acts vigorously on limestone and other mineral aggregates. If such solutions penetrate deeply below the surface of the earth their activity as solvents may be greatly increased by the higher temperature to which they are subjected. Hence, all these forces combine to disintegrate the rocks, and through such agencies vast deposits of original and secondary rocks have been completely decomposed.

The gradual passing of the firm rock into an arable soil is beautifully shown in Fig. 10, a print from a negative taken by Mr. Geo. P. Merrill, near Washington, D. C.

2a.	Orthoclase.	Anhydrous silicate of alumina with varying amounts of lime, potash, or soda and rarely barium.
2b.	Microcline.
2c.	Albite.
2d.	Oligoclase.
2e.	Andesite.
2f.	Labradorite.
2g.	Bytownite.
2h.	Anorthite.

3a.	Hornblende.	Anhydrous silicates of lime and magnesia with iron and alumina in the dark varieties.
3b.	Tremolite.
3c.	Actinolite.
3d.	Arfvedsonite.
3e.	Glaucophane.
3f.	Smaragdite.

4a.	Malacolite.	Anhydrous silicates of magnesia and lime with alumina and iron in the dark varieties.
4b.	Diallage.
4c.	Augite.
4d.	Acmite.
4c.	Aegerite.

5a.	Enstatite (bronzite).	Silicates of magnesia and iron.
5b.	Hypersthene.	Silicates of magnesia and iron.

6a.	Muscovite.	Anhydrous silicates of alumina with potash, soda, and iron.
6b.	Biotite.
6c.	Phlogopite.

ROCKS AND ROCK DECAY.