Another great group of ores to be considered under this head are the

iron ores of Lake Superior,--which were originally deposited as

sediments, called jaspers or iron formations, with too low a percentage

of iron to be of use, and which have required a secondary concentration

by surficial agencies to render them valuable. The process of

concentration has been a simple one. The iron minerals have been

oxidized in place and the non-ferrous minerals have been leached out,

leaving iron ores. This process contrasts with the concentration

described above, in that there is little evidence of collection of iron

minerals from disseminated sources. The Lake Superior iron ores are

essentially residual concentrations in place. The outstanding problems

of secondary concentration relate to the structural features which

determined the channels through which the oxidizing and leaching waters

worked, and to the topographic and climatic conditions which existed at

the time the work was done. As with many other classes of ores, it was

first assumed that these processes were related to the present erosion

surface; but it is now known that concentration happened long ago under

conditions far different from those now existing. These deposits

contribute to the rapidly accumulating evidence of the _cyclic_ nature

of ore concentration.

Our least satisfactory knowledge of the Lake Superior ores relates to

the peculiar conditions which determined the initial stage of

sedimentation of the so-called iron formation. As in the case of the

Clinton iron ores, no present-day sedimentation gives an adequate clue.

Students of the problem have fallen back on the association of the iron

formation with contemporaneous volcanic rocks, as affording a possible

explanation of the wide departure from ordinary conditions of

sedimentation evidenced by these formations.[9]

Coal deposits are direct results of sedimentation of organic material.

They are mainly accumulations of vegetable matter in place. To make them

available for use, however, they undergo a long period of condensation

and distillation. Conditions of primary deposition may be inferred from

modern swamps and bogs; but, as in the case of sediments described under

the preceding heading, we are sometimes at a loss to explain the

magnitude of the process, and especially to explain the maintenance of

proper surface conditions of plant growth and accumulation for the long

periods during which subsidence of land areas and encroachment of seas

are believed to have been taking place. The processes of secondary

concentration are also understood qualitatively, but much remains to be

learned about the influences of pressure and heat, the effect of

impervious capping rocks, and other factors.

Various oil shales and asphaltic deposits are essentially original

sediments which have subsequently undergone more or less decay and

distillation. The migration of the distillates to suitable underground

reservoirs is responsible for the accumulation of oil and gas pools.

Oil and gas are distillates from these oil shales and asphaltic

deposits, and also from other organic sediments such as carbonaceous

limestones. The distillates have migrated to their present positions

under pressure of ground-waters. The stratigraphic horizons favorable to

their accumulation are generally recognized. The geologist is concerned

in identifying these horizons and in ascertaining where they exist

underground. He is further concerned in analysis of the various

structural conditions which will give a clue to the existence of local

reservoirs in which the oil or gas may have been accumulated. So

capricious are the oil migrations that the most intensive study of these

conditions still leaves vast undiscovered possibilities.

ANAMORPHISM OF MINERAL DEPOSITS

Mineral deposits formed in any one of the ways indicated above may

undergo repeated vicissitudes, both at the surface and deep below the

surface, with consequent modifications of character. They may be

cemented or replaced by introduction of mineral solutions from without.

They may be deformed by great earth pressures, undergoing what is called

dynamic metamorphism (pp. 25-27), which tends to distort them and give

them schistose and crystalline characters. They may be intruded by

igneous rocks, causing considerable chemical, mineralogical, and

structural changes. All these changes may take place near the surface,

but on the whole they are more abundant and have more marked effects

deep below the surface.

In general all these changes of the deeper zone tend to make the rocks

more crystalline and dense and to make the minerals more complex.

Cavities are closed. The process is in the main an integrating and

constructive one which has been called _anamorphism_, to contrast it

with the disintegrating and destructive processes near the surface,

which have been called _katamorphism_ (see also pp. 27-28). There is

little in the process of anamorphism in the way of sorting and

segregation which tends to enrich and concentrate the metallic ore

bodies. On the contrary the process tends to lock up the valuable

minerals in resistant combinations with other substances, making them

more difficult to recover in mining. Later igneous intrusions or the

ordinary ground-waters may bring in minerals which locally enrich ores

under anamorphic conditions, but these are relatively minor effects. An

illustration of the general effect is afforded by a comparison of the

Cuban iron ores, which are soft and can be easily taken out, with the

Cle Elum iron ores of Washington, which seem to be of much the same

origin, but which have subsequently been buried by other rocks and

rendered hard and crystalline. In the first case the ores can be mined

easily and cheaply with steam shovels at the surface. In the second,

underground methods of mining are required, which cost too much for the

grade of ore recovered.

On the other hand, the same general kind of anamorphic processes, when

applied to coal, result in concentration and improvement of grade. The

same is true up to a certain point in the concentration of oil; but

where the process goes too far, the oil may be lost (pp. 140-141).

CONCLUSION

Mineral deposits are formed and modified by practically all known

geologic processes, but looked at broadly the main values are produced

in three principal ways:

(1) As after effects of igneous intrusion, through the agency of aqueous

and gaseous solutions given off from the cooling magma.

(2) Through the sorting processes of sedimentation,--the same processes

which form sandstone, shale, and limestone. Organic agencies are

important factors in these processes.

(3) Through weathering of the rock surface in place, which may develop

values either by dissolving out the valuable minerals and redepositing

them in concentrated form, or by dissolving out the non-valuable

minerals and leaving the valuable minerals concentrated in place. The

latter process is by far the more important.

The overwhelming preponderance of values of mineral deposits as a whole

is found in the second of the classes named.

Under all these conditions it appears that the maximum results are

obtained at and near the surface. On the scale of the earth even the

so-called deep veins may be regarded as deposits from solutions reaching

the more open and cooler outer portions of the earth. However, valuable

mineral deposits are found in the deepest rocks which have been exposed

by erosion, and the question of what would be found at still greater

depths, closer to the center of the earth, is a matter of pure

speculation.

Ultimately all minerals are derived from igneous sources within the

earth. The direct contributions from these sources are only in small

part of sufficient concentration to be of value; for the most part they

need sorting and segregation under surface conditions.

We can only speculate as to causes of the occurrence of valuable

minerals in certain igneous rocks and not in others. Many granites are

intruded into the outer shell of the earth, but only a few carry

"minerals"; also, of a series of intrusions in the same locality, only

one may carry valuable minerals. It is clear that in some fashion these

minerals are primarily segregated within the earth. Causes of this

segregation are so involved with the problem of the origin of the earth

as a whole that no adequate explanation can yet be offered. Our

inductive reasoning from known facts is as yet limited to the

segregation within a given mass of magma, and even here the conditions

are only dimly perceived. A discussion of these ultimate problems is

beyond the scope of this book.

FOOTNOTES:

[4] Ransome, Frederick Leslie, Copper deposits near Superior, Arizona:

_Bull. 540, U. S. Geol. Survey_, 1914, pp. 152-153; The copper deposits

of Ray and Miami, Arizona: _Prof. Paper 115, U. S. Geol. Survey_, 1919,

p. 156; Discussion: _Econ. Geol._, vol. 8, 1913, p. 721.

[5] For more specific definitions of vertical zones of ore deposition in

association with igneous rocks see Spurr, J. E., Theory of ore

deposition: _Econ. Geol._, vol. 7, 1912, pp. 489-490; Lindgren, W.,

_Mineral deposits_, McGraw-Hill Book Co., 2d ed., 1919, Chapters

XXIV-XXVI; and Emmons, W. H., _The principles of economic geology_,

McGraw-Hill Book Co., 1918, Chapters VI-VIII.

An excellent discussion of a case of vertical and areal zoning of

minerals is contained in _Ore deposits of the Boulder batholith of

Montana_, by Paul Billingsley and J. A. Grimes, Bull. Am. Inst. Min.

Engrs., vol. 58, 1918, pp. 284-368.

[6] Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore

deposits of Utah: _Prof. Paper 111, U. S. Geol. Survey_, 1920, p. 201.

[7] Leith, C. K., and Mead, W. J., _Metamorphic Geology_, Pt. 2, Henry

Holt and Company, New York, 1915.

[8] Butler, B. S., Loughlin, G. F., Heikes, V. C., and others, The ore

deposits of Utah: _Prof. Paper 111, U. S. Geol. Survey_, 1920, pp.

152-158.

[9] Van Hise, C. R., and Leith, C. K., Geology of the Lake Superior

region. _Mon. 52, U. S. Geol. Survey_, 1911, pp. 506-518; and references

there given.

CHAPTER IV

MINERAL RESOURCES--SOME GENERAL QUANTITATIVE CONSIDERATIONS

Of the 1,500 known mineral species, perhaps 200 figure in commerce as

mineral resources.

For the mineral substances used commercially, the term "mineral" is used

in this chapter with a broad significance to cover any or all of the

materials from which the needed elements are extracted,--whether these

materials be single minerals or groups of minerals; whether they be

rocks or ores; whether they be liquid or solid.

The following figures are generalizations based on the miscellaneous

information available. The purpose is to indicate the general

perspective rather than the detail which would be necessary for precise

statement.

WORLD ANNUAL PRODUCTION OF MINERALS IN SHORT TONS

Exclusive of water, but inclusive of petroleum, the world's annual

output of mineral resources amounts to two billions of tons. This figure

refers to the crude mineral as it comes from the ground and not to the

mineral in its concentrated form.

Of this total extraction, coal amounts to nearly 70 per cent, stone and

clay 10 per cent, iron ore about 9 per cent, petroleum 4 per cent,

copper ore 3 per cent, and all the remaining minerals constitute less

than 6 per cent.

If spread out on the surface in a uniform mass with an estimated average

density based on relative proportions of the crude minerals, this annual

production would cover a square mile to a depth of 2,300 feet.

Of the total annual production 85 per cent comes from countries

bordering the North Atlantic basin; 75 per cent is accounted for by the

United States, England, and Germany; the United States has 39 per cent

of the total, England 18 per cent, and Germany 18 per cent. By

continents, Europe accounts for nearly 51 per cent, North America for

nearly 42 per cent, Asia for nearly 4 per cent, and the remaining

continents for nearly 4 per cent. The United States mineral production

in recent years has been about 900,000,000 tons.

According to the United States census of 1920, nearly half of all the

establishments or businesses engaged in quarrying or mining operations

in this country are operating in oil and gas.

Of the crude materials extracted from the ground perhaps 10 per cent,

including gold, silver, copper, lead, zinc, nickel, and other ores, are

concentrated mainly at the mine, with the result that this fraction of

the tonnage in large part does not travel beyond the mine. About 90 per

cent of the total production, therefore, figures largely in the

transportation of mineral resources.

It is estimated that roughly two-thirds of the annual world production

is used or smelted within the countries of origin, the remaining

one-third being exported. Of the minerals moving internationally, coal

and iron constitute 90 per cent of the tonnage.

The metal smelting capacity of the world in terms of yearly production

of crude metal is estimated at nearly 100,000,000 short tons. Of this

amount about 80 per cent is located in the United States, England, and

Germany. The United States alone has over half of the total. Of the

oil-refining capacity the United States controls nearly 70 per cent.

One of the significant features of the situation above summarized is the

concentration of production and smelting in a comparatively few places

in the world. This statement applies with even more force to the

individual mineral commodities.

Water may be regarded as a mineral resource in so far as it is utilized

as a commodity for drinking, washing, power, irrigation, and other

industrial uses. For purposes of navigation and drainage, or as a

deterrent in excavation, it would probably not be so classed. While it

is not easy to define the limits of water's use as a mineral resource,

it is clear that even with a narrow interpretation the total tonnage

extracted from the earth as a mineral resource exceeds in amount all

other mineral resources combined.

WORLD ANNUAL PRODUCTION OF MINERALS IN TERMS OF VALUE

In terms of value, mineral resources appear in different perspective.

The annual world value of mineral production, exclusive of water, is

approximately $9,000,000,000. This figure is obtained by dividing the

annual value of the United States output of each of the principal

minerals by the percentage which the United States output constitutes in

the world output, and adding the figures thus obtained. The values here

used are mainly selling prices at the mines. It is impossible to reduce

the figures absolutely to the value of the mineral as it comes from the

ground; there are always some items of transportation included. This

method of figuring is of course only the roughest approximation; the

values as obtained in the United States cannot be accurately

exterpolated for the rest of the world because of locally varying

conditions. However, the figures will serve for rough comparative

purposes.

Of this total value coal represents roughly 61 per cent, petroleum 12

per cent, iron 6 per cent, copper 5 per cent, and gold 3 per cent.

In terms of value, about 25 per cent of the world's mineral production

is available for export beyond the countries of origin. Of this

exportable surplus the United States has about 40 per cent, consisting

principally of coal, copper, and formerly petroleum.

The value of the United States annual mineral production in recent years

has been from about $3,500,000,000 to $5,500,000,000. Annual imports of

mineral products into the United States have averaged recently in the

general vicinity of $450,000,000, the larger items being copper, tin,

fertilizers, petroleum, gems and precious stones, manganese, nickel, and

tungsten.

Again the perspective is changed when the value of water resources is

considered. As a physiologically indispensable resource, the value of

water in one sense is infinite. There is no way of putting an accurate

value on the total annual output used for drinking and domestic

purposes,--although even here some notion of the magnitude of the

figures involved may be obtained by considering the average per capita

cost of water in cities where figures are kept, and multiplying this

into the world population. This calculation would not imply that any

such amount is actually paid for water, because the local use of

springs, wells, and streams can hardly be figured on a cash basis; but,

if human effort the world over in securing the necessary water is about

as efficient as in the average American city, the figures would indicate

the total money equivalent of this effort.

SIGNIFICANCE OF GEOGRAPHIC DISTRIBUTION OF MINERAL PRODUCTION

The remarkable concentration of the world's mining and smelting around

the North Atlantic basin, indicated by the foregoing figures, does not

mean that nature has concentrated the mineral deposits here to this

extent. It is an expression rather of the localized application of

energy to mineral resources by the people of this part of the world. The

application of the same amount of energy in other parts of the world

would essentially change the distribution of current mineral production.

The controlling factor is not the amount of minerals present in the

ground; this is known to be large in other parts of the world and more

will be found when necessary. Controlling factors must be looked for in

historical, ethnological, and environmental conditions. This subject is

further discussed in the chapters on the several resources, and

particularly in relation to iron and steel.

THE INCREASING RATE OF PRODUCTION

The extraction of mineral resources on the huge scale above indicated is

of comparatively recent date.

From 1880 to the end of 1918 the value of the annual mineral production

of the United States has increased from $367,000,000 to more than

$5,500,000,000, or nearly fifteen times; measured in another way, it has

increased from a little over $7 per capita to more than $52.[10]

More coal has been mined in the United States since 1905 than in all the

preceding history of the country. More iron ore has been mined since

1906 than in all the preceding history. The gold production of the

United States practically started with the California gold rush in

1849. The great South African gold production began in 1888. Production

of diamonds in South Africa began about 1869. The large use of all

fertilizer minerals is of comparatively recent date. The world's oil

production is greater now each year than it was for any ten years

preceding 1891, and more oil has come out of the ground since 1908 than

in all the preceding history of the world. The use of bauxite on a large

scale as aluminum ore dated practically from the introduction of

patented electrolytic methods of reduction in 1889.

In one sense the world has just entered on a gigantic experiment in the

use of earth materials.

The most striking feature of this experiment relates to the vast

acquisition of power indicated by the accelerating rate of production

and consumption of the energy resources--coal, oil, and gas (and water

power). Since 1890 the per capita consumption of coal in the United

States has trebled and the per capita consumption of oil has become five

times as great as it was. If the power from these sources used annually

in recent years be translated roughly into man power, it appears that

every man, woman, and child in the United States has potential control

of the equivalent of thirty laborers,--as against seven in 1890. Energy

is being released on a scale never before approximated, with

consequences which we can yet hardly ascertain and appraise. This

consideration cannot but raise the question as to the ability of modern

civilization to control and coodinate the dynamic factors in the

situation.

CAPITAL VALUE OF WORLD MINERAL RESERVES

It is impossible to deduce accurately the capital value of mineral

resources from values of annual output, but again some approximation may

be made. The profit on the extraction of mineral resources on the whole,

considering the cost of exploration, is probably no greater than in

other industries (p. 330). If we assume a 6 per cent return, which

perhaps is somewhere near the world-wide standard of interest rate for

money, and capitalize the value of the world's annual output at this

rate, we obtain a world capital value for mineral resources, exclusive

of water, of 150 billions of dollars. This assumes an indefinitely long

life for reserves. This assumption may need some qualifications, but

it is the writer's view (Chapter XVII) that it is justified for a

sufficiently long period to substantiate the above method of

calculation.

[Illustration: FIG. 2. Commercial (financial) control of the

mineral resources of the world.]

[Illustration: FIG. 3. Political (territorial) control of the

mineral resources of the world.]

POLITICAL AND COMMERCIAL CONTROL OF MINERAL RESOURCES

The occurrence of a mineral resource within a country does not

necessarily mean control by that particular political unit. A citizen of

the United States may own a mineral resource in South America.

Commercial control of this sort was demonstrated during the war to be of

more far-reaching significance than had been supposed, and it became

necessary to ascertain, not only the output of the different countries,

but the commercial control of this output. Investigation of this subject

for twenty-three leading commodities shows that the political and

commercial control are by no means the same. These are partly summarized

in the accompanying graphs from Spurr.[11] It is to be noted that the

graphs show the control of many commodities as it existed in 1913, the

last normal year before the war. Changes during and since the war have

of course largely altered the situation for certain commodities, notably

for iron, coal, and potash. These developments are summarized in the

discussion of the individual resources. It is also to be noted that the

commercial or financial control of the world's minerals, under the

influence of the fostering and protective policies of certain

governments discussed in Chapter XVIII, is at present in a state of

flux. Considerable changes are taking place today and are to be looked

for in the future.

RESERVES OF MINERAL RESOURCES

Annual production figures are only to a very partial extent an

indication of the distribution of the great reserves of mineral

resources. For instance, there are enormous reserves of coal in China

which are not yet utilized to any large extent. The minerals of South

America and Africa are in a very early stage of development. The total

world reserves will of course not be known until exploration and

development of the world's resources are complete--a time which will

probably never come. Figures of reserves represent only our present

partial state of knowledge and are likely to be considerably modified in

the future. Furthermore, the quantitative accuracy of knowledge of

reserves is so variable in different parts of the world that it is

almost impossible to make up world figures which have any great

validity. There are, however, certain broad facts ascertainable.

Every country in the globe is deficient in supplies of some minerals.

The United States is better off than any other country, but still lacks

many mineral commodities (see pp. 396-399.) No single continent has

sufficient reserves of all mineral commodities.

For the world, however, it may be stated with reasonable certainty that

the reserves of the principal minerals are now known to be ample with

the exception of those of oil, tin, and perhaps gold and silver. By

_ample_ we mean sufficient to give no cause for worry for the next few

decades. For many mineral commodities the amounts now actually in sight

will not last long, but the possibilities of extension and discovery are

so great that a long future availability of these commodities can be

counted upon with reasonable safety.

The present shortages in oil, tin, and other minerals mentioned may be

only temporary. There is a large part of the world still to be explored,

and the present reserves merely mark a stage in this exploration.

Nevertheless, the ratio of reserves and discovery on the one hand to

accelerated use on the other gives cause for much concern. Looking

forward to the future, the problem of mineral reserves in general is not

one of the possible ultimate amount which the earth may

contain--presumably in no case is this deficient--but of the success

with which the resource may be found and developed to keep up with the

rapid acceleration of demand. In the chapter on conservation the

suggestion is made that future difficulties are more likely to arise

from failure to coodinate the dynamic factors of supply and demand, than

from absolute shortage of material in the earth.

FOOTNOTES:

[10] Bastin, Edson S., and McCaskey, H. D., The work on mineral

resources done by the U. S. Geological Survey: _Min. Res. of the United

States for 1918, U. S. Geol. Survey_, pt. 1, 1920, p. 3a.

[11] Spurr, J. E., Who owns the earth?: _Eng. and Min. Jour._, vol. 109,

1920, pp. 389-390.

CHAPTER V

WATER AS A MINERAL RESOURCE

GENERAL GEOLOGIC RELATIONS

With the solid earth as the special care of geology, it may seem

presumptuous for the geologist to claim the waters thereof, but he does

not disclaim this inheritance. Water is so all-pervasive that it is more

or less taken for granted; and so many and so intricate are its

relations that it is not easy to make an objective survey of the water

problem in its relation to geology.

The original source of water, as well as of air, is in molten magmas

coming from below. These carry water and gases,--some of which are

released and some of which are locked up in the rocks on cooling, to be

later released during the alterations of the rocks. It is supposed,

whatever theory of the origin of the earth we favor, that in its early

stages the earth lacked both hydrosphere and atmosphere, and that during

the growth of the earth these gradually accumulated on and near the

surface in the manner stated.

During alterations at the surface water is added to the mineral

constitution of the rocks, and by alterations deep below the surface it

may be subtracted. Water is the agent through which most mineral and

chemical changes of rocks are accomplished. It is the agent also which

is mainly responsible for the segregation of mineral deposits. Water,

both as running water and in the solid form of ice, plays an important

part in determining the configuration of the earth's surface. Water is

the medium in which most sedimentary rocks are formed. It is an

important agent in the development of soil and in organic growth. These

various influences of water on geological processes touch the economic

field at many points, especially in relation to the concentration of

ores and to the development of soils and surface forms.

Water comes even more directly into the field of economic geology as a

mineral resource. Water supplies, for the greatest variety of purposes,

involve geologic considerations at almost every turn.

Finally, water may be an aid or a hindrance to excavation and to a great

variety of structural operations, both in war and in peace; and in this

relation it again affords geologic problems.

The part played by water in geologic processes, such as that of mineral

segregation, is more or less incidentally discussed in other chapters.

We may consider more fully in this chapter the application of geology to

the general subject of water supplies.

From the geological point of view, water is a mineral,--one of the most

important of minerals,--as well as a constituent of other minerals. It

becomes a mineral resource when directly used by man. It is ordinarily

listed as a mineral resource when shipped and sold as "mineral water,"

but there is obviously no satisfactory line between waters so named and

water supplies in general, for most of them are used for the same

purposes and none of them are free from mineral matter. Water which is

pumped and piped for municipal water supply is as much a mineral

resource as water which is bottled and sold under a trade name. Likewise

water which is used for irrigation, water power, and a wide variety of

other purposes may logically be considered a mineral resource.

Notwithstanding the immense economic importance of water as a mineral

resource its value is more or less taken for granted, and considerations

of valuation and taxation are much less in evidence than in the case of

other mineral resources. Water must be had, regardless of value, and

market considerations are to a much less extent a limiting factor.

Economic applications of geology to this resource are rather more

confined to matters of exploration, development, total supply, and

conservation, than to attempts to fix money value.

DISTRIBUTION OF UNDERGROUND WATER

Free water exists in the openings in rocks where it is sometimes called

_hygroscopic_ water. There is also a large amount of water combined

molecularly with many of the minerals of rocks, in which form it is

called _water of constitution_. This water is fixed in the rock so that

it is not available for use, though some of the processes of rock

alteration liberate it and contribute it to the free water. The

immediate source of underground water, both free and combined, is mainly

the surface or rain waters. A subordinate amount may come directly from

igneous emanations or from destruction of certain hydrous minerals.

Ultimately, as already indicated, even the surface water originates from

such sources.

The openings in rocks consist of joints and many other fractures, small

spaces between the grains of rocks (pore space), and amygdaloidal and

other openings characteristic of surface volcanic rocks. Many of these

openings are capillary and sub-capillary in size. Most rocks, even dense

igneous rocks, are porous in some degree, and certain rocks are porous

in a very high degree. The voids in some surface materials may amount to

84 per cent of the total volume. In general the largest and most

continuous openings are near the surface,--where rocks on the whole are

more largely of the sedimentary type and are more fractured,

disintegrated, and decomposed, than they are deep within the earth. The

largest supplies of water are in the unconsolidated sediments. The water

in igneous and other dense rocks is ordinarily in more limited quantity.

APPROXIMATE QUANTITY OF WATER WHICH WILL BE ABSORBED BY SOILS AND

ROCKS[1]

------------------------------------------------------

_Volume of water asborbed

_Material_ per 100 of material_

------------------------------------------------------

Sandy soil[2] 45.4

Chalk soil[2] 49.5

Clay[2] 50-52.7

Loam[2] 45.1-60.1

Garden earth [2] 69.0

Coarse sand [2] 39.4

Peat subsoil[2] 84.0

Sand 30-40

Sandstone 5-20

Limestone and dolomite 1-8

Chalk 6-27

Granite 03.-.8

------------------------------------------------------

1: Mead, Daniel W., _Hydrology_: McGraw-Hill Book Co.,

New York, 1919, p. 393.

2: Woodward, H. B., _Geology of soils and substrata_:

Edward Arnold, London, 1912.

Immediately at the surface, the openings of rocks may not be filled with

water; but below the surface, at distances varying with climatic and

topographic conditions, the water saturates the openings of the rocks

and forms what is sometimes called the _zone of saturation_ or the _sea

of underground water_. The top surface of this zone is called the _water

table_, or the _ground-water level_. The space between the water table

and the earth's surface is sometimes referred to as the _vadose zone_ or

the _zone of weathering_, since it is the belt in which weathering

processes are most active. The zone of weathering is not necessarily

dry. Water from the surface enters and sinks through it and water also

rises through it from below; it may contain suspended pockets of water

surrounded by dry rocks; it is not continuously and fully saturated.

The water table or ground-water level may be near or at the surface in

low and humid areas, and it may be two thousand feet or more below the

surface in arid regions of high topographic relief. Because of the

influence of capillarity, the water table is not a horizontal surface.

It shows irregularities more or less following the surface contours,

though not nearly so sharply accentuated.

The lower limit of the ground-water is more irregular than the upper

surface and is less definitely known. In general, openings in rocks tend

to diminish with depth, due to cementation and to closing of cavities by

pressures which are too great for the rock to withstand. But rocks

differ so widely in their original character, and in their response to

physical and chemical environment, that it is not unusual to find dense

and impervious rocks above, and open and porous rocks below. The lower

limit of the zone of abundant underground water varies accordingly. A

well may encounter nearly dry rock at a comparatively shallow depth, or

it may reach a porous water-bearing stratum at considerable depth. At

the greater depths pockets of water are sometimes found which have a

composition different from that of the surface water, and which

evidently are isolated from the surface water by zones of non-pervious

rock.

Attempts have been made to calculate the total volume of underground

water by measuring the openings of rocks and making assumptions as to

the depth to which such openings may extend. In this manner it has been

estimated that, if all the ground-water were assembled in a single body,

it would make a shell between eighty and two hundred feet thick

(depending on the assumptions) over all the continental areas.

MOVEMENT OF UNDERGROUND WATER

Availability of water supplies is determined by the movement or flow of

water as well as by its distribution and amount. The natural flow of

water underground is caused by gravity in the larger openings, but in

the smaller openings adhesion and capillarity are also important forces.

Of all the water falling on the surface, some may not go below the

surface at all but may immediately evaporate or join the runoff--that

is, the surface streams. Another part may penetrate a little distance

into the zone of weathering and then join the runoff. Of the water which

reaches the zone of saturation, a part may soon come to the surface in

low areas and join the runoff, and a part may penetrate deeply.

Above the zone of saturation gravity carries the water downward in

devious courses until it reaches the water table. Thereafter its course

is determined largely by the lowest point of escape from the water

table. In other words, the water table is an irregular surface; and

under the influence of gravity the water tends to move from the high to

the low points of this surface. Between the point of entrance and the

point of escape from the water table, the water follows various courses,

depending upon the porosity and the openings in the rocks. In general it

fills all of the available openings, and uses the entire available cross

section in making its progress from one point to another. The difference

in height or the "head" between the point of entrance and the point of

escape, together with the porosity of the rock and other factors,

determine the general speed of its movement (see p. 73). With equal

porosity the flow is at a maximum along a line directly connecting the

two points, and on more devious courses the flow is less.

The surface water first enters the ground through innumerable small

openings. Soon, however, it tends to be concentrated into channels of

easiest flow, with the result that in the later part of its underground

course it may be much concentrated in large trunk channels. These

channels may consist of joints, or frequently of very coarse and

pervious beds. The sedimentary rocks as a whole contain the most voids,

and therefore the largest flow and largest supply of water is often

localized in them. Of the sedimentary rocks, sandstones and limestones

usually contain the largest and most continuous openings, and thus

afford the freest circulation for water. The voids in fine-grained

shales may equal in volume those in sandstones and limestones, but the

openings are so small and discontinuous that the water does not flow

freely. Regardless of total amount of water, unless there are continuous

openings of some size the flow may be small.

The relations of more porous rocks to containing impervious strata also

profoundly affect the flow of underground water. Between impervious

strata the circulation may be concentrated and vigorous within the

porous bed. Where the porous bed is not so contained, the movement may

be more dispersed and less vigorous locally. The inclination of the

beds, of course, also affects the direction and amount of the flow.

The influence of gravity upon underground water may locally tend toward

a state of equilibrium in which there is little movement. In such a case

the water is substantially ponded, and moves only when tapped by

artificial openings.

WELLS AND SPRINGS

Underground water becomes available for use by means of springs and

through wells or bore holes. Water rises to the surface in natural

springs at points where the pressure or _head_, due to its entrance into

the ground at a higher level, is sufficient to force it to the surface

after a longer or shorter underground course. The movement may be all

downward and lateral to the point of escape, or it may be downward,

lateral, and upward. Ordinarily, the course of spring waters does not

carry them far below the surface. Heat and gases may be added beneath

the surface by contact with or contributions from cooling igneous rocks.

These may accelerate the upward movement of spring waters, and yield

thermal and gas-charged waters, as in the springs and geysers of

Yellowstone Park.

When a well is sunk to tap the underground water supply, the water may

not rise in the artificial opening but may have to be lifted to the

surface.

If, however, the water is confined beneath an impervious stratum and is

under pressure from the water of higher areas, a well opening may simply

allow it to move upward under its own pressure or head. This pressure

may carry it upward only a few feet or quite to the surface or beyond,

in which latter case the well is called an _artesian_ well. The

essential condition for an artesian circulation is a porous zone,

inclining downward from the surface beneath an impervious stratum which

tends to confine and pond the water. The water at any point in the

water-bearing rock is under pressure which is more or less equivalent to

the weight of the column of water determined by the difference in height

between this point and the point of entrance or feeding area of the

water. If the feeding area is higher than the collar of the well, the

water will rise quite to the surface; if not, it will rise only part

way. Capillary resistance, however, may and usually does lessen the

theoretical pressure so figured.

The flow in deep artesian circulations is ordinarily a slow one. For the

artesian wells of southern Wisconsin, it has been calculated that waters

entering the outcrop of the southward dipping sandstone and limestone

layers in the northern part of the state have required two or three

hundred years to reach a point in the southern part of the state where

they are tapped. Because of this slow movement, a large number of wells

in any one spot may exhaust the local supply faster than it is

replenished from the remainder of the formation. The drilling of

additional wells near at hand in such cases does not increase the total

yield, but merely divides it among a larger number of wells.

The porosity of the rocks, and therefore the flow of an artesian

circulation, may in some cases be artificially increased by blasting and

shattering.

COMPOSITION OF UNDERGROUND WATERS

Underground waters are never entirely free from dissolved mineral

substances, and seldom are they free from suspended particles. Some

waters are desired because they contain very small quantities of

dissolved mineral matter. Others are prized because they have an

unusually high content of certain mineral substances. In determining the

deleterious or beneficial effect of dissolved substances, much depends

on the purpose for which the water is to be used,--whether for drinking,

washing, steam boilers, or irrigation. Near the surface underground

waters may carry bacteria, as well as animal and vegetable refuse,

which from a sanitary standpoint are usually objectionable. Deeper

waters are more likely to lack this contamination because of filtration

through rocks and soils.

The dissolved mineral substances of underground water are derived for

the most part from the solution of rocks with which the waters come in

contact, particularly at or near the surface. Through the agency of

underground water most of the mineral and chemical changes of rocks are

produced. The dissolved substances in solution at any time and place may

therefore be regarded as by-products of rock alterations. Locally they

may to some extent be derived from direct emanations from cooling

igneous masses.

The most common mineral substances contained in waters are lime and

magnesia. Less common, but abundant locally, are soda, potash, iron, and

silica. Waters contain also certain acid and gaseous substances, the

most common of which is carbon dioxide; and less widespread, but locally

abundant, are chlorine and sulphur dioxide. Where lime and magnesia are

abundant the water is ordinarily classed as a hard water. Where absent,

or subordinate to soda and potash, the water is ordinarily classed as a

soft water. Large amounts of the acid substances like chlorine and

sulphur are detrimental for most purposes. Where there are unusual

amounts of carbon dioxide or other gases present, they may by expansion

cause the water to bubble.

If we were to attempt to describe and define the characteristics, with

reference to dissolved mineral content and temperature, which make a

given water more desirable than another, we should enter a field of the

most amazing complexity and one with many surprising contradictions. For

the most widespread use, the most desirable water is a cold water as

free from mineral content as possible, and especially one lacking an

excess of lime and magnesia which make it hard; also lacking an excess

of acid constituents like sulphur dioxide, carbon dioxide, or chlorine,

which give the water a taste, or which make impossible its use in

boilers. Locally and for special reasons, waters of other qualities are

in demand. Waters so excessively carbonated as to bubble, sulphureted

waters, chlorine waters, waters high in iron, high in silica, high in

potash, high in soda, or high in magnesia, or waters of high

temperature, may come to be regarded as desirable. It is an interesting

fact that any water with unusual taste, or unusual mineral content, or

unusual temperature, is likely to be regarded as having medicinal value.

Sometimes this view is based on scientific knowledge; sometimes it is an

empirical conclusion based on experience; and again it may be merely

superstition. In one case the desirable feature may be the presence of a

large amount of carbon dioxide; in another case it may be its absence.

In one case the desirable feature may be high temperature; in another

case low temperature. The same combination of qualities which in a

certain locality may be regarded as highly desirable, may be regarded as

highly detrimental somewhere else where certain other types of waters

are in vogue.

Proprietary rights and advertising have brought certain waters into use

for drinking purposes which are not essentially different from more

widely available waters which are not regarded as having special value.

Two springs located side by side, or a spring and a deep well, whose

waters have exactly the same chemical characteristics, may be used and

valued on entirely different scales. Any attempt to classify mineral

waters sold to the public in any scientific way discloses a most

intricate and confused situation. One can only conclude that the

popularity of certain waters is not based alone on objective qualities

of composition, but rather on causes which lie in the fields of

psychology and commerce.

The part played by sentiment in putting value on water is well

illustrated by the general preference for spring waters as compared with

well waters. In the public mind, "spring water" denotes water of unusual

purity and of more desirable mineral content than well water.

Illustrations could be cited of districts in which the surface or spring

waters have a composition not different from that of the deeper well

waters, and are much more likely to be contaminated because of proximity

to the surface; and yet people will pay considerable sums for the spring

water in preference to the cheaply available well water.

RELATION OF GEOLOGY TO UNDERGROUND WATER SUPPLY

It is obvious that a knowledge of geology is helpful in locating an

underground water supply. Locally the facts may become so well known

empirically that the well driller is able to get satisfactory results

without using anything but the crudest geologic knowledge; but in

general, attention to geologic considerations tends to eliminate

failures in well drilling and to insure a more certain and satisfactory

water supply.

In drilling for water, it is essential to know the nature, succession,

and structure of the rocks beneath the surface in order to be able to

identify and correlate them from drill samples. The mere identification

of samples is often sufficient to determine whether a well has been

drilled far enough or too far to secure the maximum results. In order to

arrive at any advance approximation of results for a given locality, a

knowledge of the general geology of the entire region may be necessary.

Especially for expensive deep artesian wells it is necessary to work out

the geologic possibilities well in advance. It is useless, for instance,

to look for artesian water in a granite; but in an area of gently

inclined strata, with alternations of porous and impervious layers, the

expert may often figure with a considerable degree of certainty the

depth at which a given porous stratum will be found, and the pressure

under which the water will be in this particular stratum at a given

point. Even the mineral content of the water may in some cases be

predicted from geologic study.

One of the most obvious and immediately useful services of the geologist

in most localities is the collection and preservation of well samples

for purposes of identification and correlation of rock formations, and

as a guide to further drilling. Failure to preserve samples has often

led to useless and expensive duplication of work.

The problem of water supply in some localities is comparatively simple

and easy. In other areas there is an infinite variety of geologic

conditions which affect the problem, and the geologist finds it

necessary to bring to bear all the scientific knowledge of any sort

which can be used,--particularly knowledge in relation to the type of

rock, the stratigraphy and the structure.

SURFACE WATER SUPPLIES

Where underground water is not abundant or not cheaply available, or

where larger amounts of water are needed, as in large cities or for

irrigation purposes, surface water is used. In general, surface waters

are more likely to be contaminated by vegetable and animal matter and

to require purification for drinking purposes.

Surface waters are also used for irrigation, water power, drainage, the

carrying of sewage, etc. This great variety of uses brings the

consideration of surface waters into many fields other than geology, but

an understanding and interpretation of the geological conditions is none

the less fundamental. This is evidenced by the inclusion of geologic

discussions in most textbooks of hydrology, and in the reports of the

Hydrographic Branch of the U. S. Geological Survey. The very fact that

this important branch of governmental investigation is in a charge of

the U. S. Geological Survey indicates its close relation to geology.

The principles of geology used in the study of surface waters relate

chiefly to physiography (see Chapter I). It is usually necessary to know

the total quantity of flow, its annual and seasonal variation, and the

possible methods of equalization or concentration; the maximum quantity

of flow, the variation during periods of flood, and the possibilities of

reduction or control; the minimum flow and its possible modification by

storage or an auxiliary supply. These questions are obviously related to

the size and shape of the catchment area, the topography, the rock

structure, the relation between underground flow or absorption and the

runoff, and other physiographic factors. Quoting from D. W. Mead:[12]

Geological conditions are frequently of great importance in

their influence on the quantity and regularity of runoff. If

the geological deposits of the drainage area are highly

impervious, the surface flow will receive and transmit the

water into the mass only through the cracks and fissures in

the rock. Pervious materials, such as sandstones, sands,

gravels, and cracked or fissured rocks, induce seepage, retard

runoff, and, if such deposits are underlaid with an impervious

bed, provide underground storage which impounds water away

from the conditions which permit evaporation, and hence tends

to increase runoff and equalize flow. On the other hand, if

such pervious deposits possess other outlets outside of the

stream channel and drainage area, they may result in the

withdrawal of more or less of the seepage waters entirely from

the ultimate flow of the stream. Coarse sands and gravels

will rapidly imbibe the rainfall into their structure. Fine

and loose beds of sand also rapidly receive and transmit the

rainfall unless the precipitation is exceedingly heavy under

which conditions some of it may flow away on the surface.

Many of the highly pervious indurated formations receive water

slowly and require a considerable time of contact in order to

receive and remove the maximum amount.

In flat, pervious areas, rainfalls of a certain intensity are

frequently essential to the production of any resulting stream

flow. In a certain Colorado drainage area, the drainage

channel is normally dry except after a rainfall of one-half

inch or more. A less rainfall, except under the condition of a

previously saturated area, evaporates and sinks through the

soil and into the deep lying pervious sand rock under the

surface which transmits it beyond the drainage area. Such

results are frequently greatly obscured by the interference of

other factors, such as temperature, vegetation, etc.

* * * * *

The natural storage of any drainage area and the possibilities

of artificial storage depend principally upon its topography

and geology. Storage equalizes flow, although the withdrawal

of precipitation by snow or ice storage in northern areas

often reduces winter flow to the minimum for the year. Both

surface and sub-surface storage sometimes hold the water from

the streams at times when it might be advantageously used.

Storage, while essential to regulation, is not always an

advantage to immediate flow conditions.

UNDERGROUND AND SURFACE WATERS IN RELATION TO EXCAVATION AND CONSTRUCTION

Scarcely more than a mention of this subject is necessary. In mining,

the pumping charge is one of the great factors of cost. A forecast of

the amount and flow of water to be encountered in mining is based on the

geologic conditions. The same is true in excavating tunnels, canals, and

deep foundations. Detailed study of the amount and nature of water in

the rock and soil of the Panama Canal has been vital to a knowledge of

the cause and possibilities of prevention of slides. Rock slides in

general are closely related to the amount and distribution of the water

content.

The importance of ground-water as a detriment in military operations was

shown during the recent war in trenching and other field works. At the

outset, with the possible exception of the German army, a lack of

scientific study of ground-water conditions led to much unnecessary

difficulty. It soon became necessary to study and map the water

conditions in great detail in advance of operations. Much of this work

was done by geologists (see Chapter XIX).

Geological considerations are involved in a great variety of engineering

undertakings related to river and harbor improvements, dam sites, etc.,

mentioned in Chapter XX.

FOOTNOTES:

[12] Mead, Daniel W., _Hydrology_: McGraw-Hill Book Co., New York, 1919,

pp. 447-448, 456.

CHAPTER VI

THE COMMON ROCKS AND SOILS AS MINERAL RESOURCES

ECONOMIC FEATURES OF THE COMMON ROCKS

Under the general heading of common rocks are included the ordinary

igneous, sedimentary, and "metamorphic" rocks, and the unconsolidated

clays, sands, and gravels characteristic of surface conditions, which

are mined and quarried for commercial use. Soils are closely related to

this group; but since they present special problems of their own, they

are discussed under a separate heading at the end of the chapter. Names

of the common rocks will be used with the general commercial

significance given them by the United States Geological Survey in its

mineral resource reports.

Because of their inexhaustible quantity and ready availability, the

value of the common rock products is not large per unit of weight; but

in the aggregate it ranks high among mineral products. In respect to

tonnage, common rocks constitute perhaps 10 per cent of the world annual

output of all mineral commodities (exclusive of water).

The greater tonnage of the common rocks is used commercially in crushed

or comminuted forms for road material, for railroad ballast, and for

cement, brick, concrete, and flux. In blocks and structural shapes, of

less aggregate tonnage, they are used as building stone, monumental

stone, paving blocks, curbing, flagging, roofing, refractory stone, and

for many other building and manufacturing purposes.

The common rocks are commodities in which most countries of the globe

are self-sufficing. International trade in these commodities is

insignificant, being confined to small quantities of materials for

special purposes, or to local movements of short distances, allowed by

good transportation facilities.

The common rocks are so abundant and widespread that the conservation

of raw materials is not ordinarily a vital problem. Conservational

principles do apply, however, to the human energy factor required for

their efficient use. In the valuation of common rocks, also, the more

important factors are not the intrinsic qualities of the stones, but

rather the conditions of their availability for use.

Because of bulk and comparatively low intrinsic value, the principal

commercial factors in the availability of the common rocks are

transportation and ease of quarrying, but these are by no means the only

factors determining availability. Their mineral and chemical

composition, their texture and structure, their durability, their

behavior under pressure and temperature changes, and other factors enter

in to important degrees. The weighting and integration of these factors,

for the purpose of reaching conclusions as to the availability of

particular rock materials, depend also on the purposes for which these

materials are to be used. The problem is anything but simple. The search

for a particular rock to meet a certain demand within certain limits of

cost is often a long and arduous one. On account of the abundance and

widespread distribution of common rocks and their variety of uses, there

is a good deal of popular misapprehension as to their availability. Many

building and manufacturing enterprises have met disastrous checks,

because of a tendency to assume availability of stone without making the

fullest technical investigation. Many quarrying ventures have come to

grief for the same reason. It is easy to assume that, because a granite

in a certain locality is profitably quarried and used, some other

granite in the same locality has equal chances. However, minor

differences in structure, texture, and composition, or in costs of

quarrying and transportation, may make all the difference between profit

and loss. Even though all these conditions are satisfactorily met,

builders and users are often so conservative that a new product finds

difficulty in breaking into the market. A well-established building or

ornamental stone, or a limestone used for flux, may hold the market for

years in the face of competition from equally good and cheaper supplies.

The very size of a quarry undertaking may determine its success or

failure.

GRANITE

The term granite, as used commercially, includes true granite and such

allied rocks as syenite and gneiss. In fact even quartzite is sometimes

called granite in commerce, as in the case of the Baraboo quartzites of

Wisconsin, but this is going too far. For statistical purposes, the

United States Geological Survey has also included small quantities of

diorite and gabbro. The principal uses of granite are, roughly in order

of importance, for monumental stone, building stone, crushed stone,

paving, curbing, riprap and rubble. Thirty states in the United States

produce granite, the leaders being Vermont, Massachusetts, North

Carolina, Maine, Wisconsin, Minnesota, and California.

BASALT AND RELATED TYPES

Basalt and related rocks are sometimes included under the name "trap

rock," which comprises,--besides typical basalt and diabase,--fine-grained

diorite, gabbro, and other basic rocks, which are less common in

occurrence and are similar in chemical and physical properties. The

principal use of these rocks is as crushed stone for road and ballast

purposes and for concrete. They are produced in some fifteen states, the

leaders being New Jersey, Pennsylvania, California, and Connecticut.

LIMESTONE, MARL, CHALK

In the United States limestone is used principally as crushed stone for

road material, railroad ballast, concrete, and cement, as fluxing stone

for metallurgical purposes, and in the manufacture of lime. Minor uses

are as building stones, paving blocks, curbing, flagging, rubble, and

riprap; in alkali works, sugar factories, paper mills, and glass works;

and for agricultural purposes. For the making of cement, in

metallurgical fluxes, and in most of the manufacturing and agricultural

uses, both limestone and lime (limestone with the CO_2 driven out by

heating) are used. Lime is also extensively used in the making of mortar

for building operations, in tanning leather, and in a great variety of

chemical industries. The total quantity of limestone used for all

purposes in the United States nearly equals that of iron ore. Nearly

every state in the union produces limestone, but the more important

producers are Pennsylvania (where a large amount is used for fluxing),

Ohio, Indiana, New York, Michigan, and Illinois.

Closely associated with limestone in commercial uses, as well as in

chemical composition, is calcareous marl, which is used extensively in

the manufacture of Portland cement.

Chalk is a soft amorphous substance of the same composition as

limestone. The main uses of chalk are as a filler in rubber, and as a

component of paint and putty. It is also used for polishing. The

principal producers of this commodity are England, Denmark, and France,

and the chief consumer is the United States. The United States depends

upon imports for its supply of chalk for the manufacture of whiting.

Before the war two-thirds came from England and a third from France.

During the war importation was confined to England, with a small tonnage

from Denmark. No deposits of domestic chalk have been exploited

commercially. A somewhat inferior whiting, but one capable of being

substituted for chalk in most cases, is manufactured from the waste fine

material of limestone and marble quarries.

MARBLE

Marble is limestone which has been coarsely recrystallized by

metamorphism. The marble of commerce includes a small quantity of

serpentine as quarried and sold in Massachusetts, California, Maryland,

Pennsylvania, and Vermont, and also a small amount of so-called onyx

marble or travertine obtained from caves and other deposits in Kentucky

and other states. The principal uses of marble are for building and

monumental stones. Of the twenty-two states producing marble, the

leaders are Vermont, Georgia, and Tennessee.

A small amount of marble of special beauty, adapted to ornamental

purposes, is imported from European countries, especially from Italy.

Marble imports from Italy constitute about two-thirds, both in tonnage

and value, of all stone imported into the United States.

SAND, SANDSTONE, QUARTZITE (AND QUARTZ)

Sand is composed mainly of particles of quartz or silica, though

sometimes feldspar and other minerals are present. Sandstones are

partially cemented sands. Quartzites are completely cemented sands. To

some extent these substances are used interchangeably for the same

purposes.

The principal uses of sand in order of commercial totals are for

building purposes--for mortar, concrete, sand-lime brick, etc.,--as

molding sand in foundries, as a constituent of glass, in grinding and

polishing, in paving, as engine sand, as fire or furnace sand, in the

manufacture of ferrosilicon (a steel alloy), and in filters. Reference

is made to sand as an abrasive and in the manufacture of steel in

Chapters XIII and IX. Almost every state produces some sand, but for

some of the more specialized uses, such as glass sand, molding sand, and

fire or furnace sand, the distribution is more or less limited. The

United States Geological Survey has collected information concerning the

distribution of various kinds of sand and gravel, and serves a very

useful function in furnishing data as to supplies of material for

particular purposes. Fine molding sands have been imported from France,

but during the war domestic sources in New York and Ohio were developed

sufficiently to meet any requirements.

The sandstone of commerce includes the quartzites of Minnesota, South

Dakota, and Wisconsin, and the fine-grained sandstones of New York,

Pennsylvania, and elsewhere, known to the trade as "bluestone." In

Kentucky most of the sandstone quarried is known locally as "freestone."

The principal uses of sandstone are for building stone, crushed stone,

and ganister (for silica brick and furnace-linings). Other uses are for

paving blocks, curbing, flagging, riprap, rubble, grindstones,

whetstones, and pulpstones (see also Chapter XIII). Sandstone is

sometimes crushed into sand and is used in the manufacture of glass and

as molding-sand. Most of the states of the union produce sandstone, the

principal producers being Pennsylvania, Ohio, and New York.

"SAND AND GRAVEL"

Where sand is coarse and impure and mixed with pebbles, it is Ordinarily

referred to as "sand and gravel." For sand and gravel the principal

uses are for railroad ballast, for road building, and for concrete. Sand

and gravel are produced in almost every state in the union, the largest

producers being Pennsylvania, Ohio, Illinois, New Jersey, and North

Carolina.

CLAY, SHALE, SLATE

Shale is consolidated clay, usually with a fine lamination due to

bedding. Slate is a more dense and crystalline rock, produced usually by

the anamorphism of clay or shale under pressure, and characterized by a

fine cleavage which is usually inclined to the sedimentary bedding.

Clays are used principally for building and paving brick and tile,

sewer-pipe, railroad ballast, road material, puddle, Portland cement,

and pottery. Clay is mined in almost every state. Ohio, Pennsylvania,

New Jersey, and Illinois have the largest production. There has been a

considerable importation of high-grade clays, principally from England,

for special purposes--such as the filling and coating of paper; the

manufacture of china, of porcelain for electrical purposes, and of

crucibles; and for use in ultramarine pigments, in sanitary ware, in

oilcloth, and as fillers in cotton bleacheries. War experience showed

the possibility of substitution of domestic clays for most of these

uses; but results were not in all cases satisfactory, and the United

States will doubtless continue to use imported clays for some of these

special purposes.

Shales, because of their thinly bedded character and softness, are of no

value as building stones, but are used in the manufacture of brick,

tile, pottery, and Portland cement.

Slates owe their commercial value primarily to their cleavage, which

gives well-defined planes of splitting. The principal uses are for

roofing and, in the form of so-called mill stock for sanitary,

structural, and electrical purposes. Small amounts are used for

tombstones, roads, slate granules for patent roofing, school slates,

blackboard material, billiard table material, etc. The color, fineness

of the cleavage, and size of the flakes are the principal features

determining the use of any particular slate. Ten states produce slate,

the principal production coming from Pennsylvania and Vermont.

THE FELDSPARS

Feldspars are minerals, not rocks, but mention of them is made here

because, with quartz, they make up such an overwhelming percentage of

earth materials. It is estimated that the feldspars make up 50 per cent

of all the igneous rocks and 16 per cent of the sedimentary rocks. As

the igneous rocks are so much more abundant than the sedimentary rocks,

the percentage of feldspars in the earth approaches the former rather

than the latter figure. In most rocks feldspar is in too small grains

and is too intimately associated with other minerals to be of commercial

importance; in only one type of rock, pegmatite, which is an igneous

rock of extremely coarse and irregular texture, are the feldspar

crystals sufficiently large and concentrated to be commercially

available.

Feldspar is used principally in the manufacture of pottery, china ware,

porcelain, enamel ware, and enamel brick and tile. In the body of these

products it is used to lower the fusing point of the other ingredients

and to form a firm bond between their particles. Its use in forming the

glaze of ceramic products is also due to its low melting point. A less

widespread use of feldspar is as an abrasive (Chapter XIII). One of the

varieties of feldspar carries about 15 per cent of potash, and because

of the abundance of the mineral there has been much experimental work to

ascertain the possibility of separating potash for fertilizer purposes;

but, because of cost, this source of potash is not likely for a long

time to compete with the potash salts already concentrated by nature.

Feldspar is mined in eleven states, but the important production comes

from North Carolina and Maine. The United States also imports some

feldspar from Canada.

HYDRAULIC CEMENT (including Portland, natural, and Puzzolan cements)

Cement is a manufactured product made from limestone (or marl) and clay

(or shale). Sometimes these two kinds of substances are so combined in

nature (as in certain clayey limestones) that they are available for

cement manufacture without artificial mixing. It is not our purpose in

this volume to discuss manufactured products; but the cement industry

involves such a simple transformation of raw materials, and is so

closely localized by the distribution of the raw materials, that a

mention of some of its outstanding features seems desirable.

Hydraulic cement is used almost exclusively as a structural material. It

is an essential ingredient of concrete. Originally used chiefly for the

bonding of brick and stone masonry and for foundation work, its uses

have grown rapidly, especially with the introduction of reinforced

concrete. It is being used in the construction of roads, and its latest

use is in ship construction.

With the exception of satisfactory fuels, the raw materials required for

the manufacture of cement are found quite generally throughout the

world. While practically all countries produce some cement, much of it

of natural grade, only the largest producers make enough for their own

requirements and as a result there is a large world movement of this

commodity. The world trade is chiefly in Portland cement.

Next to the United States, the producing countries having the largest

exportable surplus of cement in normal times are Germany and Great

Britain. France and Belgium were both large producers and exporters

before the war, but the war greatly reduced their capacity to produce

for the time being. Sweden, Denmark, Austria, Japan, and Switzerland all

produce less extensively but have considerable surplus available for

export. Italy and Spain have large productions, which are about

sufficient for their own requirements. Holland and Russia import large

amounts from the other European countries. The far eastern trade absorbs

the excess production of Japan. In South Africa and Australasia,

production nearly equals demand. In Canada, although the industry has

been growing very rapidly, the demand still exceeds production. In South

and Central America, Mexico and the West Indies, the demand is

considerable and will probably increase; production has thus far been

insufficient. Several modern mills are either recently completed or

under construction in these countries, and concessions have been granted

for several others. These new mills are largely financed by American

capital.

The United States is the largest single producer of cement in the world,

its annual production being about 45 per cent of the world's total.

Domestic consumption has always been nearly as great as the production,

and exports have usually not exceeded 4 per cent of the total shipments

from the mills. South and Central America offer fields for exportation

of cement from the United States.

GEOLOGIC FEATURES OF THE COMMON ROCKS

To describe the geologic features of the common rocks used in commerce

would require a full treatise on the subject of geology. These are the

bulk materials of the earth and in them we read the geologic history of

the earth. In preceding chapters a brief outline has been given of the

relative abundance of the common earth materials and of the processes

producing them. In comparison, the metalliferous deposits are the merest

incidents in the development of this great group of mineral resources.

In this section reference will be made only to a few of the rock

qualities and other geologic features which require first attention in

determining the availability of a common rock for commercial use. The

list is very fragmentary, for the reason that the uses are so many and

so varied that to describe all the geologic features which are important

from the standpoint of all uses would very soon bring the discussion far

beyond the confines of a book of this scope.[13]

BUILDING STONE

For building stones, the principal geologic features requiring attention

are structure, durability, beauty, and coloring.

The structures of a rock include jointing, sedimentary stratification,

and secondary cleavage. Nearly all rocks are jointed. The joints may be

open and conspicuous, or closed and almost imperceptible. The closed

joints or incipient joints cause planes of weakness, known variously as

rift, grain, etc., which largely determine the shapes of the blocks

which may be extracted from a quarry. Where properly distributed, they

may facilitate the quarrying of the stone. In other cases they may be

injurious, in that they limit the size of the blocks which can be

extracted and afford channels for weathering agents. Some rocks of

otherwise good qualities are so cut by joints that they are useless for

anything but crushed stone. The bedding planes or stratification of

sedimentary rocks exercise influences similar to joints, and like

joints may be useful or disadvantageous, depending on their spacing. The

secondary cleavage of some rocks, notably slates, enables them to be

split into flat slabs and thus makes them useful for certain purposes.

Proper methods of extraction and use of a rock may minimize the

disadvantageous effects of its structural features. The use of

channelling machines instead of explosives means less shattering of the

rock. By proper dressing of the surface the opening of small crevices

may be avoided. Stratified rocks set on bed, so that the bedding planes

are horizontal, last longer than if set on edge.

The durability of a rock may depend on its perviousness to water which

may enter along planes of bedding or incipient fracture planes, or along

the minute pore spaces between the mineral particles. The water may

cause disastrous chemical changes in the minerals and by its freezing

and thawing may cause splitting. For this reason, the less pervious

rocks have in general greater durability than the more pervious. Highly

pervious rocks used in a dry position or in a dry climate will last

longer than elsewhere.

Durability is determined also by the different coefficients of expansion

of the constituent minerals of the rock. Where the minerals are

heterogeneous in this regard, differential stresses are more likely to

be set up than where the minerals are homogeneous. Likewise a

coarse-textured rock is in general less durable than a fine-textured

one. Expansion and contraction of a stone under ordinary temperature

changes, and also under fire and freezing, must necessarily be known for

many kinds of construction.

Minerals resist weathering to different degrees, therefore the mineral

composition of a rock is another considerable factor in determining its

durability. Where pyrite is present in abundance it easily weathers out,

leaving iron-stained pits and releasing sulphuric acid which decomposes

the rock. Abundance of mica, especially where segregated along the

stratification planes, permits easy splitting of the rock under

weathering. Likewise the mica often weathers more quickly than the

surrounding minerals, giving a pitted appearance; in marbles and

limestones its irregular occurrence may spoil the appearance. Flint or

chert in abundance is deleterious to limestones and marbles, because,

being more resistant, it stands out in relief on the weathered surface,

interferes with smooth cutting and polishing, and often causes the rock

to split along the lines of the flint concretions. Abundance of

tremolite may also be disadvantageous to limestones and marbles, because

it weathers to a greenish-yellow clay and leaves a pitted surface.

The crushing strength of a rock has an obvious relation to its

structural uses. The rock must be strong enough for the specified load.

Most hard rocks ordinarily considered for building purposes are strong

enough for the loads to which subjected, and this factor is perhaps

ordinarily less important than the structural and mineral features

already mentioned.

It is often necessary to know the modulus of elasticity and other

mechanical constants of a rock, as in cases where it is to be combined

with metal or other masonry or to be subjected to exceptional shock.

The beauty and coloring of a rock are its esthetic rather than its

utilitarian features. They are particularly important in the

construction of buildings and monuments for public or ornamental

purposes.

CRUSHED STONE

The largest use of rock or stone is in the crushed form for road

building, railway embankments, and concrete, and the prospect is for

largely increased demands for such uses in the future. For the purpose

of road building, it is necessary to consider a stone's resistance to

abrasion, hardness, toughness, cementing value, absorption, and specific

gravity. Limestone cements well, but in other qualities it is not

desirable for heavy traffic. Shales are soft and clayey, and grind down

to a mass which is dry and powdery, and muddy in wet weather. Basalt and

related rocks resist abrasion, and cement well. Granites and other

coarse-grained igneous rocks do not cement well and are not resistant to

abrasion. Many sandstones are very hard and brittle and resist abrasion,

but do not cement.

The application of geology on a large scale to the study of sources and

qualities of crushed stone is now being required in connection with the

great state and national projects of highway building. This work is by

no means confined to a mere testing of the physical qualities of

road-building materials found along the proposed route, but includes a

careful study of their geologic occurrence, distribution, and probable

amounts. In certain of the northern states specialists in glacial

geology are preferred for this purpose.

STONE FOR METALLURGICAL PURPOSES

The use of limestone and other rock for metallurgical fluxes is

dependent very largely on chemical composition. Comparatively few

limestones are sufficiently pure for this purpose. For furnace linings,

the quartzite or ganister must be exceptionally pure. The field search

for rocks of the necessary composition has required geologic service.

CLAY

For a variety of uses to which clay is put, it is necessary to know its

degree of plasticity, tensile strength, shrinkage (both under air and

fire), fusibility, color, specific gravity, and chemical properties. The

testing of clay for its various possible uses is a highly specialized

job, usually beyond the range of a geologist, although certain

geologists have been leaders in this type of investigation. More

commonly within the range of a geologist are questions concerning

origin, field classification, distribution, quantities, and other

geologic conditions affecting quality and production.

Clay originates from the weathering of common rocks containing

silicates, by pretty well understood weathering processes (see Chapter

II). It may remain in place above the parent rock, or may be transported

and redeposited, either on land or under water, by the agencies of air,

water, and ice. The kind of parent rock, the climatic conditions and

nature of the weathering, and the degree of sorting during

transportation, all determine the composition and texture of the

resulting clay,--with the result that a classification on the basis of

origin may indicate the broad group characteristics which it is

desirable to know for commercial purposes. For instance, residual clays

from the weathering of granite may be broadly contrasted with residual

clays formed by the weathering of limestone, and both differ in group

characteristics from clays in glacial deposits. Classification according

to origin also may be useful in indicating general features of depth,

quantity, and distribution. However, a genetic classification of clays

is often not sufficient to indicate the precise characteristics which

it is necessary to know in determining their availability for narrow and

special technical requirements. Furthermore, clays suitable for certain

commercial requirements may be formed in several different ways, and

classification based on specific qualities may therefore not correspond

at all to geologic classification based on origin.

Geologists have been especially interested in the causes of plasticity

of clay and in its manner of hardening when dried. In general these

phenomena have been found to be due to content of colloidal substances

of a clayey nature, which serve not only to hold the substance together

during plastic flow but to bind it during drying. The part played by

colloids in the formation of clays, as well as of many other mineral

products, is now a question which is receiving intensive study.

The same processes which produce clay also produce, under special

conditions, iron ores, bauxites, the oxide zones of many sulphide ore

bodies, and soils, all of which are referred to on other pages.

LIMITATIONS OF GEOLOGIC FIELD IN COMMERCIAL INVESTIGATION OF COMMON ROCKS

In general the qualities of the earth materials which determine their

availability for use are only to a minor extent the qualities which the

geologist ordinarily considers for mapping and descriptive purposes. The

usual geological map and report on a district indicate the distribution

and general nature of the common rocks, and also the extent to which

they are being used as mineral resources. Seldom, however, is there

added a sufficiently precise description, for instance of a clay, to

enable the reader to determine which, if any, of the many different uses

the material might be put to. The variety of uses is so great, and the

technical requirements for different purposes are so varied and so

variable, that it is almost impossible to make a description which is

sufficiently comprehensive, and at the same time sufficiently exact, to

give all the information desired for economic purposes. If the geologist

is interested in disclosing the commercial possibilities in the raw

materials of an area, he may select some of the more promising features

and subject them to the technical analysis necessary to determine their

availability for special uses. In this phase of his work he may find it

necessary to enlist the cooperation of skilled technicians and

laboratories in the various special fields. The problem is simplified if

the geologist is hunting for a particular material for a specific

purpose, for then he fortifies himself with a knowledge of the

particular qualities needed and directs his field and laboratory study

accordingly.

Too often the geologist fails to recognize the complexity and

definiteness of the qualities required, and makes statements and

recommendations on the use of raw materials based on somewhat general

geologic observations. On the other hand, the engineer, or the

manufacturer, or the builder often goes wrong and spends money

needlessly, by failing to take into consideration general geologic

features which may be very helpful in determining the distribution,

amount, and general characters of the raw materials needed.

It is difficult to draw the line between the proper fields of the

geologist and those of the engineer, the metallurgist, and other

technicians. It is highly desirable that the specialist in any one of

these fields know at least of the existence of the other fields and

something of their general nature. Too often his actions indicate he is

not acutely conscious even of the existence of these related branches of

knowledge. The extent and detail to which the geologist will familiarize

himself with these other fields will of course vary with his training

and the circumstances of his work. Whatever his limit is, it should be

definitely recognized; his work should be thorough up to this limit and

his efforts should not be wasted in fields which he is not best

qualified to investigate.

These remarks apply rather generally to mineral resources, but they are

particularly pertinent in relation to the common rock materials which

the geologist is daily handling,--for he is likely to assume that he

knows all about them and that he is qualified to give professional

advice to industries using them. In connection with metallic resources,

the metallurgical and other technical requirements are likely to be more

definitely recognized and the lines more sharply drawn, with the result

that the geologist is perhaps not so likely to venture into problems

which he is not qualified to handle.

The limits to geologic work here discussed are not necessarily limits

separating scientific from non-scientific work. The study and

determination of the qualities of rocks necessary for commercial

purposes is fully as scientific as a study of the qualities commonly

considered in purely geologic work, and the results of technical

commercial investigations may be highly illuminating from a purely

geological standpoint. When a field of scientific endeavor has been

established by custom, any excursion beyond traditional limits is almost

sure to be regarded by conservatives in the field as non-scientific, and

to be lightly regarded. The writer is fully conscious of the existence

of limits and the necessity for their recognition; but he would explain

his caution in exceeding these limits on the ground of training and

effectiveness, rather than on fear that he is becoming tainted with

non-scientific matters the moment he steps beyond the boundaries of his

traditional field.

SOILS AS A MINERAL RESOURCE

Soils are not ordinarily listed as mineral resources; but as weathered

and altered rock of great economic value, they belong nearly at the head

of the list of mineral products.

ORIGIN OF SOILS

Soil originate from rocks, igneous, sedimentary, and "metamorphic" by

processes of weathering, and by the mixing of the altered mineral

products with decayed plant remains or _humus_. The humus averages

perhaps 3 or 4 per cent of the soil mass and sometimes constitutes as

much as 75 per cent. Not all weathered rock is soil in the agricultural

sense. For this purpose the term is mainly restricted to the upper few

inches or feet penetrated by plant roots.

The general process of soil formation constitutes one of the most

important phases of katamorphism--the destructive side of the

metamorphic cycle, described in Chapter II. Processes of katamorphism or

weathering, usually accompanied by the formation of soils, affect the

surface rocks over practically all the continental areas.

The weathering of a highly acid igneous rock with much quartz produces a

residual soil with much quartz. The weathering of a basic igneous rock

without quartz produces a clay soil without quartz, which may be high

in iron. Where disintegration has been important the soil contains an

abundance of the original silicates of the rock, and less of the altered

minerals.

The production of soil from sedimentary rocks involves the same

processes as alter igneous rocks; but, starting from rocks of different

composition, the result is of course different in some respects.

Sandstones by weathering yield only a sandy soil. Limestones lose their

calcium carbonate by solution, leaving only clay with fragments of

quartz or chert as impurities. A foot of soil may represent the

weathering of a hundred feet of limestone. Shales may weather into

products more nearly like those of the weathering of igneous rocks.

Silicates in the shales are broken down to form clay, which is mixed

with the iron oxide and quartz.

In some localities the soil may accumulate to a considerable depth,

allowing the processes of weathering to go to an extreme; in others the

processes may be interrupted by erosion, which sweeps off the weathered

products at intermediate stages of decomposition and may leave a very

thin and little decomposed soil.

Soils formed by weathering may remain in place as residual soils, or

they may be transported, sorted, and redeposited, either on land or

under water. It is estimated by the United States Bureau of Soils[14]

that upward of 90 per cent of the soils of the United States which have

been thus far mapped owe their occurrence and distribution to

transportation by moving water, air, and ice (glaciers), and that less

than 10 per cent have remained in place above their parent rock.

Glaciers may move the weathered rock products, or they may grind the

fresh rocks into a powder called _rock flour_, and thus form soils

having more nearly the chemical composition of the unaltered rocks.

Glacial soils are ordinarily rather poorly sorted, while wind and

water-borne soils are more likely to show a high degree of sorting.

The character of a transported soil is less closely related to the

parent rock than is that of a residual soil, because the processes of

sorting and mixture of materials from different sources intervene to

develop deposits of a nature quite different from residual soils; but

even transported soil may sometimes be traced to a known rock

parentage.

Where deposited under water, soil materials may be brought above the

water by physiographic changes, and exposed at the surface in condition

for immediate use. Or, they may become buried by other sediments and not

be exposed again until after they have been pretty well hardened and

cemented,--in which case they must again undergo the softening processes

of weathering before they become available for use. Where soils become

buried under other rocks and become hardened, they are classed as

sedimentary rocks and form a part of the geologic record. Many residual

and transported soils are to be recognized in the geologic column; in

fact a large number of the sedimentary rocks ordinarily dealt with in

stratigraphic geology are really transported soils.

The development of soils by weathering should not be regarded as a

special process of rock alteration, unrelated to processes producing

other mineral products. Exactly the same processes that produce soils

may yield important deposits of iron ore, bauxite, and clay, and they

cause also secondary enrichment of many metallic mineral deposits. For

instance the weathering of a syenite rock containing no quartz, under

certain conditions, as in Arkansas, results in great bauxite deposits

which are truly soils and are useful as such,--but which happen to be

more valuable because of their content of bauxite. The weathering of a

basic igneous rock, as in Cuba, may produce important residual iron ore

deposits, which are also used as soils. Weathering of ferruginous

limestone may produce residual iron and manganese ores in clay soils.

COMPOSITION OF SOILS AND PLANT GROWTH

The mineral ingredients in soils which are essential for plant growth

include water, potash, lime, magnesia, nitrates, sulphur, and phosphoric

acid--all of which are subordinate in amount to the common products of

weathering (pp. 20-22, 23-24). Of these constituents magnesia is almost

invariably present in sufficient quantity; while potash, nitrates, lime,

sulphur, and phosphoric acid, although often sufficiently abundant in

virgin soil, when extracted from the soils by plant growth are liable to

exhaustion under ordinary methods of cultivation, and may need to be

replenished by fertilizers (Chapter VII). Some soils may be so

excessively high in silica, iron, or other constituents, that the

remaining constituents are in too small amounts for successful plant

growth.

Even where soils originally have enough of all the necessary chemical

elements, one soil may support plant growth and another may not, for the

reason that the necessary constituents are soluble and hence available

to the plant roots in one case and are not soluble in the other. Plainly

the mineral combinations in which the various elements occur are

important factors in making them available for plant use. Similarly a

soil of a certain chemical and mineralogical composition may be fruitful

under one set of climatic conditions and a soil of like composition may

be barren at another locality--indicating that availability of

constituents is also determined by climatic and other conditions of

weathering. Even with the same chemical composition and the same

climatic conditions, there may be such differences in texture between

various soils as to make them widely different in yield.

The unit of soil classification is the _soil type_, which is a soil

having agricultural unity, as determined by texture, chemical character,

topography, and climate. The types commonly named are clay, clay loam,

silt loam, loam, fine sandy loam, sandy loam, fine sand, and sand. In

general the soil materials are so heterogeneous and so remote from

specific rock origin, that in such classification the geologic factor of

origin is not taken into account. More broadly, soils may be classified

into provinces on the basis of geography, similar physiographic

conditions, and similarity of parent rocks; for instance, the soils of

the Piedmont plateau province, of the arid southwest region, of the

glacial and loessal province, etc. In such classification the geologic

factors are more important. Soils within a province may be subdivided

into "soil series" on the basis of common types of sub-soils, relief,

drainage, and origin.

USE OF GEOLOGY IN SOIL STUDY

While the desirability of particular soils is related in a broad way to

the character of the parent rocks, and while by geologic knowledge

certain territories can be predicated in advance as being more favorable

than others to the development of good soils, so many other factors

enter into the question that the geologic factor may be a subordinate

one. A soil expert finds a knowledge of geology useful as a basis for a

broad study of his subject; but in following up its intricacies he gives

attention mainly to other factors, such as the availability of common

constituents for plant use, the existence and availability of minute

quantities of materials not ordinarily regarded as important by the

geologist, the climatic conditions, and the texture. As the geologic

factors are many of them comparatively simple, much of the expert work

on soils requires only elementary and empirical knowledge of geology.

The geologist, although he may understand fully the origin of soils and

may indicate certain broad features, must acquire a vast technique not

closely related to geology before he becomes effective in soil survey

work and diagnosis.

For these reasons the mapping and classification of soils, while often

started by geologists of state or federal surveys, have in their

technical development and application now passed largely into the hands

of soil experts in the special soil surveys affiliated with the U. S.

Department of Agriculture and with agricultural colleges.

FOOTNOTES:

[13] A good summary of this subject may be found in _Engineering

Geology_, by H. Ries and T. L. Watson, Wiley and Sons, 2d ed., 1915.

[14] Marbut, Curtis F., Soils of the United States: _Bull. 96, Bureau of

Soils_, 1913, p. 10.

CHAPTER VII

THE FERTILIZER GROUP OF MINERALS

GENERAL COMMENTS

Soils are weathered rock more or less mixed with organic material. The

weathering processes forming soils are in the field of geologic

investigation, but the study of soils in relation to agriculture

requires attention to texture and to several of their very minor

constituents which have little geologic significance. Soil study has

therefore become a highly specialized and technicalized subject,--for

which a geological background is essential, but which is usually beyond

the range of the geologist. To supply substances which are deficient in

soils, however, requires the mining, quarrying, or extraction of

important mineral resources, and in this part of the soil problem the

geologist is especially interested.

Soils may be originally deficient in nitrates, phosphates, or potash; or

the continued cropping of soils may take out these materials faster than

the natural processes of nature supply them. In some soils there are

sufficient phosphates and potash to supply all plant needs indefinitely;

but the weathering and alteration processes, through which these

materials are rendered soluble and available for plant life, in most

cases are unable to keep up with the depletion caused by cropping. A ton

of wheat takes out of the soil on an average 47 pounds of nitrogen, 18

pounds of phosphoric acid, 12 pounds of potash. On older soils in Europe

it has been found necessary to use on an average 200 pounds of mixed

mineral fertilizers annually per acre. On the newer soils of the United

States the average thus far used has been less than one-seventh of this

amount. The United States has thus far been using up the original

materials stored in the soil by nature, but these have not been

sufficient to yield anything like the crop output per acre of the more

highly fertilized soils of Europe.

In addition to the nitrates, phosphates, and potassium salts, important

amounts of lime and sulphuric acid, and some gypsum, are used in

connection with soils. Lime is derived from crushed limestone (pp.

82-83), and is used primarily to counteract acidity or sourness of the

soil; it is, therefore, only indirectly related to fertilizers.

Sulphuric acid is used to treat rock phosphates to make them more

soluble and available to plant life. It requires the mining of pyrite

and sulphur. Gypsum, under the name of "land-plaster," is applied to

soils which are deficient in the sulphur required for plant life;

increase in its use in the future seems probable. There are also

considerable amounts of inert mineral substances which are used as

fillers in fertilizers to give bulk to the product, but which have no

agricultural value. The proportions of the fertilizer substances used in

the United States are roughly summarized in Figure 4.

The United States possesses abundant supplies of two of the chief

mineral substances entering into commercial fertilizers,--phosphate rock

and the sulphur-bearing materials necessary to treat it. For potash the

United States is dependent on Europe, unless the domestic industry is

very greatly fostered under protective tariff. For the mineral nitrates

the United States has been dependent on Chile, and because of the

cheapness of the supply will doubtless continue to draw heavily from

this source. However, because of the domestic development of plants for

the fixation of nitrogen from the air, the recovery of nitrogen from

coal in the by-product processes, and the use of nitrogenous plants, the

United States is likely to require progressively less of the mineral

nitrates from Chile.

The fertilizer industry of the United States is yet in its infancy and

is likely to have a large growth. Furthermore much remains to be learned

about the mixing of fertilizers and the amounts and kinds of materials

to be used. The importance of sulphur as a plant food has been realized

comparatively recently. The use of fertilizers in the United States has

come partly through education and the activity of agricultural schools

and partly through advertising by fertilizer companies. The increased

use of potash has been due largely to the propaganda of the German sales

agents. An examination of a map showing distribution of the use of

fertilizers over the country indicates very clearly the erratic

distribution of the effects of these various activities. One locality

may use large amounts, while adjacent territory of similar physical

conditions uses little. The sudden withdrawal of fertilizers for a

period of three or four years during the war had very deleterious

effects in some localities, but was not so disastrous as expected in

others,--emphasizing the fact that the use of fertilizers has been

partly fortuitous and not nicely adjusted to specific needs.

[Illustration: FIG. 4. FERTILIZER SITUATION IN THE UNITED

STATES. SMITHSONIAN INSTITUTION--UNITED STATES NATIONAL MUSEUM]

NITRATES

ECONOMIC FEATURES

There are several sources of nitrogen for fertilizer purposes: mineral

nitrates, nitrogen taken from the air by certain plants with the aid of

bacteria and plowed into the soil, nitrogen taken directly from the air

by combining nitrogen and oxygen atoms in an electric arc, or by

combining nitrogen and hydrogen to form ammonia, nitrogen taken from the

air to make a compound of calcium, carbon, and nitrogen (cyanamid),

nitrogen saved from coal in the form of ammonia as a by-product of

coke-manufacture, and nitrogen from various organic wastes. Nitrogen in

the form of ammonia is also one of the potential products of oil-shales

(p. 150). While the principal use of nitrogenous materials is as

fertilizers, additional important quantities are used in ammonia for

refrigerating plants, and in the form of nitric acid in a large number

of chemical industries. During the war the use of nitrates was largely

diverted to explosives manufacture. The geologist is interested

principally in the mineral nitrates as a mineral resource, but the other

sources of nitrogen, particularly its recovery from coal, also touch his

field.

Almost the single source of mineral nitrates for the world at present is

Chile, where there are deposits of sodium nitrate or Chile saltpeter,

containing minor amounts of potassium nitrate. About two-thirds of the

Chilean material normally goes to Europe and about one-fourth to the

United States. The supply has been commercially controlled chiefly by

Great Britain and by Chilean companies backed by British and German

capital.

The dependence of the world on Chile became painfully apparent during

the war. Germany was the only nation which had developed other sources

of nitrogenous material to any great extent. The other nations were

dependent in a very large degree on the mineral nitrates, both for

fertilizer and munition purposes. Total demands far exceeded the total

output from Chile, requiring international agreement as to the division

of the output among the nations. The stream of several hundred ships

carrying nitrates from Chile was one of the vital war arteries. This

situation led to strenuous efforts in the belligerent countries toward

the development of other sources of nitrogen. The United States, under

governmental appropriation, began the building of extensive plants for

the fixation of nitrogen from the air, and the building of by-product

coke ovens in the place of the old wasteful beehive ovens was

accelerated. Germany before the war had already gone far in both of

these directions, not only within her own boundaries, but in the

building of fixation plants in Scandinavia and Switzerland. War

conditions required further development of these processes in Germany,

with the result that this country was soon entirely self-supporting in

this regard. One of the effects was the almost complete elimination in

Germany of anything but the by-product process of coking coal.

War-time development of the nitrogen industry in the United States for

munition purposes brought the domestic production almost up to the

pre-war requirements for fertilizers alone. With the increasing demand

for fertilizers and with the cheapness of the Chilean supply of natural

nitrates, it is likely that the United States will continue for a good

many years to import considerable amounts of Chilean nitrates. It may be

noted that, although this country normally consumes about one-fourth of

the Chilean product, American interests commercially control less than

one-twentieth of the output. Presumably, if for no other purpose than

future protection, effort will be made to develop the domestic industry

to a point where in a crisis the United States could be independent of

Chile. Particularly may an increase in the output of by-product ammonia

from coke manufacture be looked for (see also pp. 118-119), since

nitrogenous material thus produced need bear no fixed part of the cost

of production, and requires no protective tariff.

The reserves of Chilean nitrate are known to be sufficient for world

requirements for an indefinitely long future.

GEOLOGIC FEATURES

Mineral nitrates in general, and particularly those of soda and potash,

are readily soluble at ordinary temperatures. Mineral nitrate deposits

are therefore very rare, and are found only in arid regions or other

places where they are protected from rain and ground-water. The only

large deposits known are those of northern Chile and some extensions in

adjacent parts of Peru and Bolivia. These are located on high desert

plateaus, where there is almost a total absence of rain, and form

blankets of one to six feet in thickness near the surface. The most

important mineral, the sodium nitrate or Chile saltpeter, is mingled

with various other soluble salts, including common salt, borax minerals,

and potassium nitrate, and with loose clay, sand, and gravel. The

nitrate deposits occur largely around and just above slight basin-like

depressions in the desert which contain an abundance of common salt. The

highest grade material contains 40 to 50 per cent of sodium nitrate, and

material to be of shipping grade must run at least 12 to 15 per cent.

The origin of the nitrate beds is commonly believed to be similar to

that of beds of rock salt (pp. 295-298), borax, and other saline

residues. The source of the nitrogen was probably organic matter in the

soil, such as former deposits of bird guano, bones (which are actually

found in the same desert basin), and ancient vegetable matter. By the

action of nitrifying bacteria on this organic matter, nitrate salts are

believed to have formed which were leached out by surface and ground

waters, and probably carried in solution to enclosed bodies of water.

Here they became mingled with various other salts, and all were

precipitated out as the waters of the basins evaporated. Deliquescence

and later migration of the more soluble nitrates resulted in their

accumulation around the edges of the basins. The nitrate beds are thus

essentially a product of desiccation.

While the origin just set forth is rather generally accepted, several

other theories have been advanced. It has been suggested that the

deposits were not formed in water basins, but that ground water carrying

nitrates in solution has been and is rising to the surface,--where,

under the extremely arid conditions, it evaporates rapidly, leaving the

nitrates mixed with the surface clays. One group of writers accounts for

the deposits by the fixation of atmospheric nitrogen through electrical

phenomena. Still others note the frequent presence of nitrogen in

volcanic exhalations and the association of the Chilean nitrate beds

with surface volcanic rocks; they suggest that these rocks were the

source of the nitrogen, which under unusual climatic conditions was

leached out and then deposited by evaporation.

PHOSPHATES

ECONOMIC FEATURES

The principal use of natural phosphates is in the manufacture of

fertilizers. They are also used in the manufacture of phosphorus,

phosphoric acid, and other phosphorus compounds, for matches, for

certain metallurgical operations, and for gases used in military

operations.

The material mined is mainly a phosphate of lime (tricalcium phosphate).

To make it available for plant use, it is treated with sulphuric acid to

form a soluble superphosphate; hence the importance of sulphuric acid,

and its mineral sources pyrite and sulphur, in the fertilizer industry.

A small percentage of the phosphate is also ground up and applied

directly to the soil in the raw form. Other phosphatic materials are the

basic slag from phosphatic iron ores made into Thomas-process steel,

guano from the Pacific islands, and bone and refuse (tankage) from the

cattle raising and packing countries. These materials are used for the

same purposes as the natural phosphates.

The United States is the largest factor in the world's phosphate

industry, with reference both to production and reserves.

The largest and most available of the European sources are in Tunis and

Algeria, under French control, and in Egypt, under English control.

Belgium and northern France have been considerable producers of

phosphates, but, with the development of higher grade deposits in other

countries, their production has fallen to a very small fraction of the

world's total. There also has been very small and insignificant

production in Spain and Great Britain. Russia has large reserves which

are practically unmined.

While there is comparatively little phosphate rock in western Europe, a

considerable amount of the phosphate supply is obtained as a by-product

from Thomas slag, derived from phosphatic iron ores. These ores are

chiefly from Lorraine and Sweden, but English and Russian ores can be

similarly used.

Outside of Europe and the United States, there are smaller phosphate

supplies in Canada, the Dutch West Indies, Venezuela, Chile, South

Australia, New Zealand, and several islands of the Indian and South

Pacific Oceans. None of these has yet contributed largely to world

production, and their distance from the principal consuming countries

bordering the North Atlantic basin is so great that there is not likely

to be any great movement to this part of the world. On the other hand,

some of the South Sea islands have large reserves of exceptionally high

grade guano and bone phosphates, which will doubtless be used in

increasing amounts for export to Japan, New Zealand, and other nearby

countries. The most important of these islands are now controlled by

Great Britain, Japan, and France.

A striking feature of the situation is that the central European

countries, which have been large consumers of phosphate material, have

lost not only the Pacific island phosphates but the Lorraine phosphatic

iron ores, and are now almost completely dependent on British, French,

and United States phosphate.

In the United States, reserves of phosphate are very large. They are

mined principally in Florida, Tennessee, and South Carolina; but great

reserves, though of lower grade, are known in Arkansas, Montana, Idaho,

Wyoming, and Utah. There are possibilities for the development of local

phosphate industries in the west, in connection with the manufacture of

sulphuric acid from waste smelting gases at nearby mining centers. The

Anaconda Copper Mining Company has taken up the manufacture of

superphosphate as a means of using sulphuric acid made in relation to

its smelting operations. The United States is independent in phosphate

supplies and has a surplus for export. This country, England, and France

exercise control of the greater part of the world's supply of phosphatic

material. In competition for world trade, the Florida and Carolina

phosphates are favorably situated for export, but there is strong

competition in Europe from the immense fields in French North Africa,

which are about equally well situated.

GEOLOGIC FEATURES

Small amounts of phosphorus are common in igneous rocks, in the form of

the mineral apatite (calcium phosphate with calcium chloride or

fluoride). Apatite is especially abundant in some pegmatites. In a few

places, as in the Adirondacks where magnetic concentration of iron ores

leaves a residue containing much apatite, and in Canada and Spain where

veins of apatite have been mined, this material is used as a source of

phosphate fertilizer. The great bulk of the world's phosphate, however,

is obtained from other sources--sedimentary and residual beds described

below.

Phosphorus in the rocks is dissolved in one form or another by the

ground-waters; a part of it is taken up by land plants and animals for

the building of their tissues, and another part goes in solution to the

sea to be taken up by sea plants and animals. In places where the bones

and excrements of land animals or the shells and droppings of sea

animals accumulate, deposits of phosphatic material may be built up.

In certain places where great numbers of sea birds congregate, as on

desert coasts and oceanic islands, guano deposits have been formed. Some

of them, like the worked-out deposits of Peru and Chile, are in arid

climates and have been well preserved. Others, like those of the West

Indies and Oceania, are subjected to the action of occasional rains; and

to a large extent the phosphates have been leached out, carried down,

and reprecipitated, permeating and partially replacing the underlying

limestones. In this way deposits have been formed containing as high as

85 per cent calcium phosphate.

Even more important bodies of phosphates have been produced by the

accumulation of marine animal remains, probably with the aid of joint

chemical, bacterial, and mechanical precipitation. These processes have

formed the chief productive deposits of the world, including those of

the United States, northern Africa, and Russia, and also the phosphatic

iron ores of England and central Europe. The sedimentary features of

many phosphate rocks, particularly their oolitic textures, show a marked

similarity to the features of the Clinton type of iron ores (pp.

166-167).

The marine phosphate beds originally consist principally of calcium

phosphate and calcium carbonate in varying proportions. Depending on the

amount of secondary enrichment, they form two main types of deposits.

The extensive beds of the western United States (in the upper

Carboniferous) are hard, and very little enrichment by weathering has

taken place; they carry in their richer portions 70 to 80 per cent

calcium phosphate, and large sections range only from about 30 to 50 per

cent. In the southeastern deposits (Silurian and Devonian in Tennessee

and Tertiary in the Carolinas and Florida), there has been considerable

enrichment, the rock is softer, and the general grade ranges from 65 to

80 per cent. Both calcium carbonate and calcium phosphate are soluble in

ordinary ground waters, but the carbonate is the more soluble of the

two. Thus the carbonate has been dissolved out more rapidly, and in

addition descending waters carrying the phosphate have frequently

deposited it to pick up the carbonate. These enriching processes,

sometimes aided by mechanical concentration, have formed high-grade

deposits both in the originally phosphatic beds and in various

underlying strata. Concretionary and nodular textures are common. The

"pebble" deposits of Florida consist of the phosphatic materials broken

up and worked over by river waters and advancing shallow seas.

PYRITE

ECONOMIC FEATURES

The principal use of pyrite is in the manufacture of sulphuric acid.

Large quantities of acid are used in the manufacture of fertilizers from

phosphate rock, and during war times in the manufacture of munitions.

Sulphuric acid converts the phosphate rock into superphosphate, which is

soluble and available for plant use. Other uses of the acid are referred

to in connection with sulphur. Pyrite is also used in Europe for the

manufacture of paper from wood-pulp, but in the United States native

sulphur has thus far been exclusively used for this purpose. The residue

from the roasting of pyrite is a high-grade iron ore material frequently

very low in phosphorus, which is desirable in making up mixtures for

iron blast furnaces.

Most of the countries of Europe are producers of pyrite, and important

amounts are also produced in the United States and Canada. The European

production is marketed mainly on that continent, but considerable

amounts come to the United States from Spain.

Before the war domestic sources supplied a fourth to a third of the

domestic demand for pyrite. Imports came mainly from Spain and Portugal

to consuming centers on the Atlantic seaboard. The curtailment of

overseas imports of pyrite during the war increased domestic production

by about a third and resulted also in drawing more heavily on Canadian

supplies, but the total was not sufficient to meet the demand. The

demand was met by the increased use of sulphur from domestic deposits

(p. 109). At the close of the war supplies of pyrite had been

accumulated to such an extent that, with the prospect of reopening of

Spanish importation, pyrite production in the United States practically

ceased. War experience has demonstrated the possibility of substitution

of sulphur, which the United States has in large and cheaply mined

quantities. The future of the pyrite industry in the United States

therefore looks cloudy, except for supplies used locally, as in the

territory tributary to the Great Lakes, and except for small amounts

locally recovered as by-products in the mining of coal or from ores of

zinc, lead, and copper. Pyrite production in the past has been chiefly

in the Appalachian region, particularly in Virginia and New York, and in

California.

GEOLOGIC FEATURES

Pyrite, the yellow iron sulphide, is the commonest and most abundant of

the metallic sulphides. It is formed under a large variety of conditions

and associations. Marcasite and pyrrhotite, other iron sulphide

minerals, are frequently found with pyrite and are used for the same

purposes.

The great deposits of Rio Tinto, Spain, which produce about half of the

world's pyrite, were formed by replacement of slates by heated solutions

from nearby igneous rocks. The ores are in lenticular bodies, and

consist of almost massive pyrite with a small amount of quartz and

scattered grains and threads of chalcopyrite (copper-iron sulphide).

They carry about 50 per cent of sulphur, and the larger part carries

about 2 per cent of copper which is also recovered.

Similar occurrences of pyrite on a smaller scale are known in many

places. Pyrite is very commonly found in vein and replacement deposits

of gold, silver, copper, lead, and zinc. In the Mississippi valley it is

extracted as a by-product from the lead and zinc ores, and in the

Cordilleran region large quantities of by-product pyrite could easily be

produced if there were a local demand. The pyrite deposits of the

Appalachian region are chiefly lenses in schists; they are of uncertain

origin though some are believed to have been formed by replacement of

metamorphosed limestones and schists.

Under weathering conditions pyrite oxidizes, the sulphur forming

sulphuric acid,--an important agent in the secondary enrichment of

copper and other sulphides,--and the iron forming the minerals hematite

and limonite in the shape of a "gossan" or "iron-cap."

Pyrite is likewise frequently found in sediments, apparently being

formed mainly by the reducing action of organic matter on iron salts in

solution. In Illinois and adjacent states it is obtained as a by-product

of coal mining.

SULPHUR

ECONOMIC FEATURES

Sulphur is used for many of the same purposes as pyrite. Under pre-war

conditions, the largest use in the United States was in the manufacture

of paper pulp by the sulphite process. Minor uses were in agriculture as

a fungicide and insecticide, in vulcanizing rubber, and in the

manufacture of gunpowder. About 5 per cent of the sulphur of the United

States was used in the manufacture of sulphuric acid. During the war

this use was greatly increased because of the shortage of pyrite and the

large quantities of sulphuric acid necessary for the manufacture of

explosives. The replacement of pyrite by sulphur in the manufacture of

sulphuric acid has continued since the war, and in the future is likely

to continue to play an important part. Sulphuric acid is an essential

material for a great range of manufacturing processes. Some of its more

important applications are: in the manufacture of superphosphate

fertilizer from phosphate rock; in the refining of petroleum products;

in the iron, steel, and coke industries; in the manufacture of

nitroglycerin and other explosives; and in general metallurgical and

chemical practice.