domestic borax are not the playa deposits just described, but are masses
of colemanite in Tertiary clays and limestones with interbedded basaltic
flows. The principal deposits are in Death Valley and adjacent parts of
California. The colemanite occurs in irregular milky-white layers or
nodules, mingled with more or less gypsum. The deposits are believed to
be of the replacement type, rather than ones formed contemporaneously
with the sediments. Whether they are due to magmatic solutions carrying
boric acid from the associated flows, or to surface waters carrying
materials leached from other sediments, is not clear. The crude
colemanite as mined carries an average of about 25 per cent B_2O_3; it
is treated with soda in the manufacture of borax, or with sulphuric acid
in making boric acid.
Boron is present in minute quantities in sea water. When such water
evaporates, it becomes concentrated, along with the magnesium and
potassium salts, in the "mother liquor"; and upon complete evaporation,
it crystallizes out as boracite and other rarer minerals. Thus the
Stassfurt salts of Germany (p. 113) contain borates of this type in the
carnallite zone of the upper part of the deposits. This is the only
important case known of borate deposits of marine origin.
BROMINE
ECONOMIC FEATURES
Bromine finds a considerable use in chemistry as an oxidizing agent, in
separating gold from other metals, and in manufacturing disinfectants,
bromine salts, and aniline colors. The best known and most widely used
bromine salts are the silver bromide, used in photography, and the
potassium bromide, used in medicine to depress the nervous system.
During the war, large quantities of bromine were used in asphyxiating
and lachrymating gases.
The chief center of the bromine industry in Europe prior to 1914 was
Stassfurt, Germany. No other important commercial source in foreign
countries is known, though small quantities have been obtained from the
mother liquors of Chile saltpeter and from the seaweed, kelp, in various
countries. India has been mentioned as a possible large producer in the
future.
The United States is independent of foreign sources for bromine. The
entire domestic tonnage is produced from brines pumped in Michigan,
Ohio, West Virginia, and Pennsylvania. A large part of the output is not
actually marketed as bromine, but in the form of potassium and sodium
bromides and other salts. During the war considerable quantities of
bromine materials were exported to Great Britain, France, and Italy.
GEOLOGIC FEATURES
Bromine is very similar chemically to chlorine, and is found under much
the same conditions, though usually in smaller quantities. The natural
silver bromide (bromyrite) and the combined silver chloride and bromide
(embolite) are fairly common in the oxide zones of silver ores, but are
not commercial sources of bromine.
Bromine occurs in sea water in appreciable amounts, as well as in some
spring waters and many natural brines. When natural salt waters
evaporate, bromine is one of the last materials to be precipitated, and
the residual "mother liquors" or bitterns frequently show a considerable
concentration of the bromine. Where complete evaporation takes place, as
in the case of the Stassfurt salt deposits (p. 113), the bromine salts
are crystallized out in the final stages along with the salts of sodium,
magnesium, and potassium. The larger part of the world's bromine has
come from the mother liquor resulting from the solution and fractional
evaporation of these Stassfurt salts.
The bromine obtained from salt deposits in the eastern United States is
doubtless of a similar origin. It is produced as a by-product of the
salt industry, the natural or artificial brines being pumped from the
rocks (p. 295), and the bromides being extracted either from the mother
liquors or directly from the unconcentrated brines.
FULLER'S EARTH
ECONOMIC FEATURES
Fuller's earth is used chiefly for bleaching, clarifying, or filtering
mineral and vegetable oils, fats, and greases. The petroleum industry is
the largest consumer. Minor uses are in the manufacture of pigments for
printing wall papers, in detecting coloring matters in certain
food-products, and as a substitute for talcum powder.
Fuller's earths are in general rather widely distributed. The principal
producers are the United States, England, and the other large consuming
countries of Europe. The only important international trade in this
commodity consists of exports from the United States to various
countries for treating mineral oils, and exports from England for
treating vegetable oils.
There is a large surplus production in the United States of fuller's
earth of a grade suitable for refining mineral oils, but an inadequate
production of material for use in refining edible oils, at least by
methods and equipment now in most general use. However, the imports
needed from England are more than offset by our exports to Europe of
domestic earth particularly adapted to the petroleum industry.
Production in the United States comes almost entirely from the southern
states; Florida produces over three-fourths of the total and other
considerable producers are Texas, Georgia, California, and Arkansas.
Imports from England are normally equivalent to about a third of the
domestic production.
GEOLOGIC FEATURES
Fuller's earth is essentially a variety of clay having a high absorptive
power which makes it useful for decolorizing and purifying purposes.
Fuller's earths are in general higher in water content and have less
plasticity than most clays, but they vary widely in physical and
chemical properties. Chemical analyses are of little value in
determining whether a given clay will serve as fuller's earth, and an
actual test is the only trustworthy criterion.
Deposits of fuller's earth may occur under the same variety of
conditions as deposits of other clays. The deposits of Florida and
Georgia consist of beds in slightly consolidated flat-lying Tertiary
sediments, which are worked by open cuts. The Arkansas deposits are
residual clays derived from the weathering of basic igneous rocks, and
are worked through shafts.
GRAPHITE (PLUMBAGO)
ECONOMIC FEATURES
_Crystalline graphite_ is used principally in the manufacture of
crucibles for the melting of brass, bronze, crucible steel, and
aluminum. About 45 per cent of the quantity and 70 per cent of the value
of all the graphite consumed in the United States is employed in this
manner. Both _crystalline_ and _amorphous graphite_ are used in
lubricants, pencils, foundry facings, boiler mixtures, stove-polishes
and paint, electrodes, and fillers or adulterants for fertilizers. The
most important use of amorphous graphite is for foundry facings, this
application accounting for about 25 per cent of the total United States
consumption of graphite of all kinds. _Artificial graphite_ is not
suitable for crucibles or pencils but is adapted to meet other uses to
which natural graphite is put. It is particularly adapted to the
manufacture of electrodes.
The grade of graphite deposits varies widely, their utilization being
largely dependent on the size of the grains and the ease of
concentration. Some of the richest deposits, those of Madagascar,
contain 20 per cent or more of graphite. The United States deposits
contain only 3 to 10 per cent. The graphite situation is complicated by
the differences in the quality of different supplies. Crucibles require
coarsely crystalline graphite, but pencils, lubricants, and foundry
facings may use amorphous and finely crystalline material.
The largest production of high-grade crucible graphite has come from
Ceylon, under British control, and about two-thirds of the output has
come to the United States. The mines are now worked down to water-level
and costs are increasing.
In later years a rival supply has come from the French island of
Madagascar, where conditions are more favorable to cheap production, and
where reserves are very large. French, British, and Belgian interests
are concerned in the development of these deposits. The quality of
graphite is different from the Ceylon product; it has not found favor in
the United States but is apparently satisfactory to crucible makers in
Europe. Most of the output is exported to Great Britain and France, and
smaller amounts to Germany and Belgium.
Less satisfactory supplies of crystalline graphite are available in many
countries, including Bavaria, Canada, and Japan. Large deposits of
crystalline material have been reported in Greenland, Brazil, and
Roumania, but as yet have assumed no commercial importance.
Amorphous graphite is widely distributed, being produced in about twenty
countries,--chiefly in Austria, Italy, Korea, and Mexico. Certain
deposits have been found to be best for special uses, but most countries
could get along with nearby supplies.
A large part of the world's needs of crucible graphite will probably
continue to be met from Ceylon and Madagascar, while a large part of
the amorphous graphite will come from the four sources mentioned.
The United States has been largely dependent upon importations from
Ceylon for crucible graphite. Domestic supplies are large and capable of
further development, but for the most part the flake is of such quality
that it is not desired for crucible manufacture without large admixture
of the Ceylon material. Restrictions during the war required crucible
makers to use at least 20 per cent of domestic or Canadian graphite in
their mixtures, with 80 per cent of foreign graphite. This created a
demand for domestic graphite which caused an increased domestic output.
Most of the production in the United States comes from the Appalachians,
particularly from Alabama, New York, and Pennsylvania, and smaller
amounts are obtained from California, Montana, and Texas. One of the
permanently beneficial effects of the war was the improvement of
concentrating practice and the standardization of output, to enable the
domestic product to compete more effectively with the well-standardized
imported grades. Whether the domestic production will hold its own with
foreign competition under peace conditions remains to be seen. Domestic
reserves are large but of low grade.
The Madagascar graphite, in the shape and size of the flakes, is more
like the American domestic graphite than the Ceylon product. Small
amounts have been used in this country, but American consumers appear in
general to prefer the Ceylon graphite in spite of its greater cost. The
Madagascar product can be produced and supplied to eastern United States
markets much more cheaply than any other large supply; and, in view of
the possible exhaustion of the Ceylon deposits, it may be desirable for
American users to adapt crucible manufacture to the use of Madagascar
material as has already apparently been done in Europe.
Expansion of the American graphite industry during the war, and its
subsequent collapse, have resulted in agitation for a duty on imports of
foreign graphite.
Amorphous graphite is produced from some deposits in the United States
(Colorado, Nevada, and Rhode Island), but the high quality of Mexican
graphite, which is controlled by a company in the United States, makes
it likely that imports from this source will continue. Since the war the
Mexican material has practically replaced the Austrian graphite in
American markets. The output of Korea is divided between the United
States and England.
Artificial graphite, in amounts about equal to the domestic production
of amorphous graphite, is produced from anthracite or petroleum coke at
Niagara Falls.
GEOLOGIC FEATURES
The mineral graphite is a soft, steel-gray, crystalline form of carbon.
Ceylon graphite occurs in veins and lenses cutting gneisses and
limestones. Usually the veins consist almost entirely of graphite, but
sometimes other minerals occur in important amounts, especially pyrite
and quartz. The association of graphite with these minerals, and also
with feldspar, pyroxene, apatite, and other minerals, suggests that the
veins are of igneous origin, like some of the pegmatite veins in the
Adirondacks of New York. The graphite is mined from open pits and
shafts, and sorted by hand and mechanically. The product consists of
angular lumps or chips with a relatively small amount of surface in
proportion to their volume.
In Madagascar the graphite is mainly disseminated in a graphitic schist,
though to some extent it is present in the form of veins and in gneiss.
Most of the graphite is mined from a weathered zone near the surface,
and the material is therefore soft and easily concentrated. The product
is made up of flakes or scales, and in the making of crucibles requires
the use of larger amounts of clay binder than the Ceylon graphite.
The flake graphite of the United States, principally in the Appalachian
region, occurs in crystalline graphitic schists, resulting from the
anamorphism of sedimentary rocks containing organic matter. Certain beds
or zones of comparatively narrow width carry from 3 to 10 per cent of
disseminated graphite. The graphite is recovered by mechanical processes
of sorting. The graphite is believed to be of organic origin, the change
from organic carbon to graphite having been effected by heat and
pressure accompanying mountain-building stresses. Some of the graphite
also occurs in pegmatite intrusives and adjacent wall rocks. This
graphite is considered to be of inorganic origin, formed by the breaking
up of gaseous oxides of carbon in the original magma of the pegmatites.
The Montana graphite is similar in origin. This inorganic graphite in
pegmatite veins resembles Ceylon graphite, in breaking into large lumps
and chips, but supplies are very limited.
Amorphous graphite is formed in many places where coal and other
carbonaceous materials have undergone extreme metamorphism. It
represents simply a continuation in the processes by which high grade
coals are formed from plant matter (pp. 123-127). The Mexican deposits
are of this type, and occur in beds up to 24 feet in thickness
interbedded with metamorphosed sandstones.
In general, graphite is primarily concentrated both by igneous processes
in dikes, and by sedimentary processes in beds. In the latter case
anamorphism is necessary to recrystallize the carbon into the form of
graphite.
GYPSUM
ECONOMIC FEATURES
The principal use of gypsum is in structural materials. About two-thirds
of the gypsum produced in the United States is used in the manufacture
of various plasters--wall plaster, plaster of Paris, and Keene's cement
(for statuary and decorative purposes),--and about a fifth is used as a
retarder in Portland cement. Another important structural use is in the
manufacture of plaster boards, blocks, and tile for interior
construction. Gypsum is used as a fertilizer under the name of "land
plaster," and with the growing recognition of the lack of sulphur in
various soils an extension of its application is not unlikely. Minor
uses are in the polishing of plate glass, in the manufacture of dental
plaster, in white pigments, in steampipe coverings, and as a filler in
cotton goods.
The world's gypsum deposits are widely distributed. Of foreign
countries, France, Canada, and the United Kingdom are the principal
producers. Germany, Algeria, and India produce comparatively meager
amounts. The United States is the largest producer of gypsum in the
world. In spite of its large production, the United States normally
imports quantities equivalent to between one-fifteenth and one-tenth of
the domestic production, mainly in the crude form from Nova Scotia and
New Brunswick for consumption by the mills in the vicinity of New York.
This material is of a better grade than the eastern domestic supply,
and is cheaper than the western supply for eastern consumption. During
the war this importation was practically stopped because of governmental
requisition of the carrying barges for the coal-carrying trade, but with
the return of normal conditions it was resumed. There is no prospect of
importation of any considerable amount from any other sources. The
domestic supply is ample for all demands.
Production of gypsum in the United States comes from eighteen states.
Four-fifths of the total comes from New York, Iowa, Michigan, Ohio,
Texas, and Oklahoma. There are extensive deposits in some of the western
states, the known reserves in Wyoming alone being sufficient for the
entire world demands for many decades.
The United States exports a small amount of crude gypsum to Canada,
principally for use in Portland cement manufacture. This exportation is
due to geographic location. The United States is the largest
manufacturer of plaster boards, insulating materials, and tile, and
exports large quantities of these products to Cuba, Australia, Japan,
and South America.
GEOLOGIC FEATURES
Gypsum is a hydrated calcium sulphate. It is frequently associated with
minor quantities of anhydrite, which is calcium sulphate without water,
and under the proper natural conditions either of these materials may be
changed into the other.
Common impurities in gypsum deposits include clay and lime carbonate,
and also magnesia, silica, and iron oxide. In the material as extracted,
impurities may range from a trace to about 25 per cent. _Gypsite_, or
gypsum dirt, is an impure mixture of gypsum with clay or sand found in
Kansas and some of the western states; it is believed to have been
produced in the soil or in shallow lakes, by spring waters carrying
calcium sulphate which was leached from gypsum deposits or from other
rocks.
Gypsum deposits, like deposits of common salt, occur in beds which are
the result of evaporation of salt water. Calcium makes up a small
percentage of the dissolved material in the sea, and when sea waters are
about 37 per cent evaporated it begins to be precipitated as calcium
sulphate. Conditions for precipitation are especially favorable in arid
climates, in arms of the sea or in enclosed basins which may or may not
once have been connected with the sea. Simultaneously with the
deposition of gypsum, there may be occasional inwashings of clay and
sand, and with slight changes of conditions organic materials of a limey
nature may be deposited. Further evaporation of the waters may result in
the deposition of common salt. Thus gypsum beds are found interbedded
with shales, sandstones, and limestones, and frequently, but not always,
they are associated with salt beds. The nature of these processes is
further discussed under the heading of salt (pp. 295-298).
The anhydrite found in gypsum deposits is formed both by direct
precipitation from salt water and by subsequent alteration of the
gypsum. The latter process involves a reduction of volume, and
consequently a shrinkage and settling of the sediments. The hydration of
anhydrite to form gypsum, on the other hand, involves an increase of
volume and may result in the doming up and shattering of the overlying
sediments.
Gypsum is fairly soluble in ground-water, and sink-holes and solution
cavities are often developed in gypsum deposits. These may allow the
inwash of surface dirt and also may interfere with the mining.
All the important commercial gypsum deposits are believed to have been
formed by evaporation of salt water in the manner indicated. Small
quantities of gypsum are formed also when pyrite and other sulphides
oxidize to sulphuric acid and this acid acts on limestone. Thus gypsum
is found in the oxide zones of some ore bodies. These occurrences are of
no commercial significance.
MICA
ECONOMIC FEATURES
The principal use of sheet mica is for insulating purposes in the
manufacture of a large variety of electrical equipment. The highest
grades are employed particularly in making condensers for magnetos of
automobile and airplane engines and for radio equipment, and in the
manufacture of spark plugs for high tension gas engines. Sheet mica is
also used in considerable amounts for glazing, for heat insulation, and
as phonograph diaphragms. Ground mica is used in pipe and boiler
coverings, as an insulator, in patent roofing, and for lubricating and
decorative purposes.
India, Canada, and the United States are the important sheet
mica-producing countries, before the war accounting for 98 per cent of
the world's total. India has long dominated the sheet mica markets of
the world, and will probably continue to supply the standard of quality
for many years. The bulk of the Indian mica is consumed in the United
States, Great Britain, and Germany. The mica of India and the United
States is chiefly muscovite. Canada is the chief source of amber mica
(phlogopite), though other deposits of potential importance are known in
Ceylon and South Africa. Canadian mica is produced chiefly in Quebec and
Ontario, and is exported principally to the United States.
Important deposits of mica (principally muscovite) are also known in
Brazil, Argentina, and German East Africa. Large shipments were made
from the two former countries during the war, both to Europe and the
United States, and Brazil particularly should become of increasing
importance as a producer of mica. The deposits in German East Africa
were being quite extensively developed immediately before the war and
large shipments were made to Germany in 1913.
The United States is the largest consumer of sheet mica and mica
splittings, absorbing normally nearly one-half of the world's
production. Approximately three-fifths of this consumption is in the
form of mica splittings, most of which are made from muscovite in India
and part from amber mica in Canada. Due to the cheapness of labor in
India and the amenability of Indian mica to the splitting process, India
splittings should continue to dominate the market in this country. Amber
mica is a variety peculiarly adapted to certain electrical uses. There
are no known commercial deposits of this mica in the United States, but
American interests own the largest producing mines in Canada. Shipments
of Brazilian mica are not of such uniformly high quality as the Indian
material, but promise to become of increasing importance in American
markets.
Of the sheet mica consumed annually, the United States normally produces
about one-third. War conditions, although stimulating the production of
domestic mica very considerably, did not materially change the
situation in this country as regards the dependence of the United States
on foreign supplies for sheet mica.
About 70 per cent of the domestic mica comes from North Carolina and 25
per cent from New Hampshire. The deposits are small and irregular, and
mining operations are small and scattered. These conditions are largely
responsible for the heterogeneous nature of the American product. It is
hardly possible for any one mine to standardize and classify its
product, although progress was made in this direction during the war by
the organization of associations of mica producers. This lack of
standardization and classification is a serious handicap in competition
with the standard grades and sizes which are available in any desired
amounts from foreign sources.
For ground mica, the domestic production exceeds in tonnage the total
world production of sheet mica, and is adequate for all demands.
GEOLOGIC FEATURES
Mica is a common rock mineral, but is available for commerce only in
igneous dikes of a pegmatite nature, where the crystallization is so
coarse that the mica crystals are exceptionally large. Muscovite mica
occurs principally in the granitic pegmatite dikes. The phlogopite mica
of Canada occurs in pyroxenite dikes. The distribution of mica within
the dikes is very erratic, making predictions as to reserves hazardous.
The associated minerals, mainly quartz and feldspar, are ordinarily
present in amounts greater than the mica. Also, individual deposits are
likely to be small. For these reasons mining operations cannot be
organized on a large scale, but are ordinarily hand-to-mouth operations
near the surface. A large amount of hand labor is involved, and the
Indian deposits are favored by the cheapness of native labor. The output
of a district is from many small mines rather than from any single large
one.
Pegmatites which have been subjected to dynamic metamorphism are often
not available as a source of mica, because of the distortion of the mica
sheets.
The mining of a mica is facilitated by weathering, which softens the
associated feldspar, making it an easier task to take out the mica
blocks. On the other hand, iron staining by surface solutions during
weathering may make the mica unfit for electrical and certain other
uses.
Scrap or ground mica is obtained as a by-product of sheet mica and from
deposits where the crystals are not so well developed. Black mica
(biotite) and chlorite minerals, which are soft and flexible but not
elastic and are found extensively developed in certain schists, have
been used to a limited extent for the same purposes.
MONAZITE (THORIUM AND CERIUM ORES)
ECONOMIC FEATURES
The mineral monazite is the source of the thorium and cerium compounds
which, glowing intensely when heated, form the light-giving material of
incandescent gas mantles. Welsbach mantles consist of about 99 per cent
thorium oxide and 1 per cent cerium oxide. Cerium metal, alloyed with
iron and other metals, forms the spark-producing alloys used in various
forms of gas lighters and for lighting cigars, cigarettes, etc.
Mesothorium, a by-product of the manufacture of thorium nitrate for gas
mantles, is used as a substitute for radium in luminous paints and for
therapeutic purposes. The alloy ferrocerium is used to a small extent in
iron and steel.
The world's supply of monazite is obtained mainly from Brazilian and
Indian properties. Before the war German commercial interests controlled
most of the production, as well as the manufacture of the thorium
products. During the war German control was broken up.
The United States has a supply of domestic monazite of lower grade than
the imports, but is dependent under normal conditions on supplies from
Brazil and India. The American deposits are chiefly in North and South
Carolina, and have been worked only during periods of abnormally high
prices or of restriction of imports. Known reserves are small and the
deposits will probably never be important producers. During the war,
however, the United States became the largest manufacturer of thorium
nitrate and gas mantles and exported these products in considerable
quantity. An effort is now being made to secure protective legislation
against German thorium products.
GEOLOGIC FEATURES
Monazite is a mineral consisting of phosphates of cerium, lanthanum,
thorium, and other rare earths in varying proportions. The content of
thorium oxide varies from a trace up to 30 per cent, and commercial
monazite sands are usually mixed so as to bring the grade up to at least
5 per cent.
Yellowish-brown crystals of monazite have been found scattered through
granites, gneisses, and pegmatites, but in quantities ordinarily too
small to warrant mining. In general the mineral is recovered on a
commercial scale only from placers, where it has been concentrated along
with other dense, insoluble minerals such as zircon, garnet, ilmenite,
and sometimes gold. The Indian and Brazilian monazite is obtained
principally from the sands of ocean beaches, in the same localities from
which zircon is recovered (p. 189). The North and South Carolina
monazite has been obtained chiefly from stream beds, and to a slight
extent by mining and washing the rotted underlying rock, which is a
pegmatized gneiss. Monazite, together with a small amount of gold, is
also known in the stream gravels of the Boise Basin, Idaho, where a
large granitic batholith evidently carries the mineral sparsely
distributed throughout. These deposits have not been worked.
PRECIOUS STONES
ECONOMIC FEATURES
Precious stones range high in the world's annual production of mineral
values. A hundred or more minerals are used to some degree as precious
stones; but those most prized, representing upwards of 90 per cent of
the total production value, are diamond, pearl, ruby, sapphire, and
emerald. In total value the diamonds have an overwhelming dominance.
Over a ton of diamonds is mined annually.
Diamonds come mainly from South Africa, which produces over 99 per cent
of the total. Pearls come chiefly from the Indian and Pacific oceans.
Burma is the principal source of fine rubies. Siam is the principal
producer of sapphires. Colombia is the principal source of fine
emeralds.
The United States produces small amounts of sapphires (in Montana) and
pearls (from fresh-water molluscs). Diamonds, rubies, and emeralds are
practically absent on a commercial scale. Of other precious and
semi-precious gem stones produced in the United States, the principal
ones are quartz, tourmaline, and turquoise.
On the other hand, the United States absorbs by purchase over half of
the world's production of precious stones. It is estimated roughly that
there are now in the United States nearly one billion dollars' worth of
diamonds, or over one-half of the world's accumulated stock, and
probably the proportions for the other stones are not far different.
Value attaches to a precious stone because of its qualities of beauty,
coupled with endurance and rarity, or because of some combination of
these features which has caught the popular fancy. No one of these
qualities is sufficient to make a stone highly prized; neither does the
possession of all of them insure value. Some beautiful and enduring
stones are so rare that they are known only to collectors and have no
standard market value. Others fail to catch the popular fancy for
reasons not obvious to the layman. While the intrinsic qualities go far
in determining the desirability of a stone, it is clear that whim and
chance have been no small factors in determining the demand or lack of
demand for some stones. As in other minerals, value has both its
intrinsic and extrinsic elements.
For the leading precious stones above named, the values are more nearly
standard throughout the world than for any other minerals, with the
exception of gold and possibly platinum. Highly prized everywhere and
easily transported, the price levels show comparatively little variation
over the world when allowance is made for exchange and taxes. The
valuation of precious stones is a highly specialized art, involving the
appraisal not only of intrinsic qualities, but of the appeal which the
stone will make to the buying public. In marking a sale price for some
exceptional stone not commonly handled in the trade, experts in
different parts of the world often reach an almost uncanny uniformity of
opinion.
It is estimated that the world stock of precious stones approximates
three billion dollars, or a third of the world's monetary gold reserve.
Because of small bulk and standard value, this wealth may be easily
secreted, carried, and exchanged. When the economic fabric of
civilization is disturbed by war or other conditions, precious stones
become a medium of transfer and exchange of wealth of no inconsiderable
importance.
The beauty of a stone may arise from its color or lack of color, from
its translucency or opaqueness, from its high refraction of light, and
from the manner of cutting and polishing to bring out these qualities.
Hardness and durability are desirable qualities. The diamond is the
hardest known mineral and the sapphire, ruby, and emerald rank high in
this regard. On the other hand the pearl is soft and fragile and yet
highly prized.
GEOLOGIC FEATURES
The principal precious stones above named are of simple composition.
Diamond is made of carbon; the pearl is calcium carbonate; ruby and
sapphire are aluminum oxide--varieties of the mineral corundum; the
emerald is silica and alumina, with a minor amount of beryllia. Minute
percentages of chromite, iron, manganese, and other substances are often
responsible for the colors in these stones. Carbon also constitutes
graphite and is the principal element in coal. Lime carbonate is the
principal constituent of limestone and marble. Alumina is the principal
constituent of bauxite, the ore of aluminum, and of the natural
abrasives, emery and corundum. Silica, the substance of common quartz,
also constitutes gem quartz, amethyst, opal, agate, onyx, etc.
Most of the world's diamonds come from the Kimberley and Transvaal
fields of South Africa, where they are found in a much decomposed
volcanic rock called "blue ground." This is a rock of dull, greasy
appearance consisting largely of serpentine. It was originally
peridotite, occurring in necks or plugs of old volcanoes penetrating
carbonaceous sediments. When the rock is mined and spread at the
surface, it decomposes in the course of six months or a year, allowing
it to be washed and mechanically sorted for its diamond content. The
amount of ground treated in one of the large mines is about equal to
that handled in operating the huge porphyry copper deposit of Bingham,
Utah; the annual production of diamonds from the same mine could be
carried in a large suit-case.
The diamonds were clearly formed at high temperatures and pressures
within the igneous rocks. It has been suggested that the igneous magma
may have secured the carbon by the melting of carbonaceous sediments
through which it penetrated, but proof of this is difficult to obtain.
Artificial diamonds of small size have been made in the electric furnace
under high-pressure conditions not unlike those assumed to have been
present in nature.
Weathering and transportation of rocks containing diamonds have resulted
in the development of diamond-bearing placers. The South African
diamonds were first found in stream placers, leading to a search for
their source and its ultimate discovery under a blanket of soil which
completely covered the parent rock. The proportion of diamonds now mined
from placers is very small.
The diamonds of Brazil come from placer deposits. This is the principal
source of the black diamond so largely used in diamond-drilling.
The United States produces no diamonds on a commercial scale. Small
diamonds have been found in peridotite masses in Pike County, Arkansas,
but these are of very little commercial value. A few diamonds have been
found in the glacial drift of Wisconsin and adjacent states, indicating
a possible diamond-bearing source somewhere to the north which has not
yet been located (p. 317).
Pearls are concretions of lime carbonate of organic origin, and are
found in the shells of certain species of molluscs. Their color or
luster is given by organic material or by the interior shell surface
against which the pearl is formed. The principal supply comes from the
Indian and Pacific Oceans, but some are found in the fresh water mussels
of North America, in the Caribbean, and on the western coast of Mexico
and Central America.
From the beginning of history the principal source of rubies has been
upper Burma, where the stones are found in limestone or marble near the
contact with igneous rocks, associated with high-temperature minerals.
The weathering of the rock has developed placers from which most of the
rubies are recovered. Siam is also an important producer. In the United
States rubies have been found in pegmatites in North Carolina, but these
gems are of little commercial importance.
Sapphires are of the same composition as rubies and are found in much
the same localities. Most of the sapphires of the best quality come from
Siam, where they are found in sandy clay of placer origin. In the United
States sapphires are recovered from alluvial deposits along the Missouri
River near Helena, Montana, where they are supposed to have been derived
from dikes of andesite rocks. In Fergus County, Montana, they are mined
from decomposed dikes of lamprophyre (a basic igneous rock). In North
Carolina sapphire has been found in pegmatite dikes.
The principal source of fine emeralds is in the Andes in Colombia. Their
occurrence here is in calcite veins in a bituminous limestone, but
little seems to be known of their origin. The only other emerald
locality of commercial importance is in the Ural Mountains of Siberia.
Emeralds have been found in pegmatite dikes in North Carolina and New
England, but the production is insignificant.
Tourmaline is a complex hydrous silicate of aluminum and boron, with
varying amounts of magnesium, iron, and alkalies. It is a rather common
mineral in silicated zones in limestones near igneous contacts, but gem
tourmalines are found principally in pegmatite dikes. They have a wide
variety of colors, the red and green gems being the most prized. Maine,
California, and Connecticut are the principal American producers.
Turquoise is a hydrated copper-aluminum phosphate. It is found in
veinlets near the surface in altered granites and other igneous rocks.
It is usually associated with kaolin and frequently with quartz, and is
believed to have been formed by surface alterations. In the United
States it is produced chiefly in Nevada, Arizona, and Colorado.
In general the principal gem minerals, except pearl and turquoise, occur
as original constituents in igneous intrusives, usually of a pegmatite
or peridotite nature. Sapphire, ruby, emerald, and tourmaline result
also from contact metamorphism of sediments in the vicinity of igneous
rocks. Weathering softens the primary rocks, making it possible to
separate the gem stones from the matrix. When eroded and transported the
gems are concentrated in placers.
SALT
ECONOMIC FEATURES
The principal uses of salt are in the preserving and seasoning of foods
and in chemical industries. Chemical industries require salt for the
manufacture of many sodium compounds, and also as a source of
hydrochloric acid and chlorine. A minor use of salt is in the making of
glazes and enamel on pottery and hardware.
Because of the wide distribution of salt in continental deposits and
because of the availability of ocean and salt-lake brines as other
sources, most countries of the world either possess domestic supplies of
salt adequate for the bulk of their needs, or are able to obtain
supplies from nearby foreign countries. Certain sea salts preferred by
fish packers and other users are, however, shipped to distant points.
About a fifth of all the salt consumed in the world annually is produced
in the United States, and other large producers are Great Britain,
Germany, Russia, China, India, and France.
The United States produces almost its entire consumption of salt, which
is increasing at a very rapid rate. Salt is produced in fourteen states,
but over 85 per cent of the total output comes from Michigan, New York,
Ohio, and Kansas. Reserves are practically inexhaustible.
Exports and imports of salt form a very minor part of the United States
industry, each being equivalent to less than 5 per cent of the domestic
production. A large part of the imported material is coarse
solar-evaporated sea salt, which is believed by fish and pork packers to
be almost essential to their industry. Imports of this salt come from
Spain, Italy, Portugal, and the British and Dutch West Indies; during
the war, on account of ship shortage, they were confined chiefly to the
West Indies. A considerable tonnage of specially prepared kiln-dried
salt, desired by butter-makers, is imported from Liverpool, England.
There are also some small imports from Canada, probably because of
geographic location. Exports of domestic salt go chiefly to Canada,
Cuba, and New Zealand, with smaller amounts to practically all parts of
the world.
Salt is recovered from salt beds in two ways. About a fourth of the
salt produced in the United States is mined through shafts in the same
manner as coal, the lumps of salt being broken and sized just as coal is
prepared for the market. The larger part of the United States
production, however, is derived by pumping water down to the beds to
dissolve the salt, and pumping the resulting brine to the surface where
it is then evaporated. A considerable amount of salt, also, is recovered
from natural brines--which represent the solution of rock salt by
ground-waters--and from the waters of salt lakes and the ocean.
GEOLOGIC FEATURES
Common salt constitutes the mineral halite, the composition of which is
sodium chloride. It is rarely found perfectly pure in nature, but is
commonly mixed with other saline materials, such as gypsum and
anhydrite, and occasionally with salts of potassium and magnesium. The
general grade of rock-salt deposits, where not admixed with clay, is
perhaps 96 to 99 per cent of sodium chloride.
The ultimate source of salt deposits is the sodium and chlorine of
igneous rocks. In the weathering of these rocks the soda, being one of
the more soluble materials, is leached out and carried off by
ground-waters, and in the end a large part of it reaches the sea. The
chlorine follows a similar course; however, the amount of chlorine in
ordinary igneous rocks is so extremely small that, in order to explain
the amount of chlorine present in the sea, it has been thought necessary
to appeal to volcanic emanations or to some similar agency. Ocean water
contains about 3.5 per cent by weight of dissolved matter, over
three-fourths of which consists of the constituents of common salt.
Chief among the other dissolved materials are magnesium, calcium,
potassium, and SO_4 (the sulphuric acid radical).
When sea water evaporates it becomes saturated with various salts,
according to the amounts of these salts present and their relative
solubilities. In a general way, after 37 per cent of the water has
evaporated gypsum begins to separate out, and after 93 per cent has
evaporated common salt begins to be deposited. After a large part of the
common salt has been precipitated, the residual liquid, called a
"bittern" or "mother liquid," contains chiefly a concentration of the
salts of magnesium and potassium. Still further evaporation will result
in their deposition, mainly as complex salts like those found in the
Stassfurt deposit (p. 113).
The actual processes of concentration and precipitation in sea water or
other salt waters are much more complex than is indicated by the above
simple outline. The solubility of each of the various salts present, and
consequently the rate at which each will crystallize out as evaporation
proceeds, depends upon the kinds and concentrations of all the other
salts in the solution. Temperature, pressure, mass-action, and the
crystallization of double salts are all factors which influence the
nature and rate of the processes and add to their complexity. During a
large part of the general process, several different salts may be
crystallizing out simultaneously. It is evident that gypsum may be
precipitated in some quantity, and that external conditions may then
change, so that evaporation ceases or so that the waters are freshened,
before any common salt is crystallized out. This fact may explain in
part why gypsum beds are more widely distributed than beds of common
salt. At the same time the much greater amount of sodium chloride than
of calcium sulphate in sea water may explain the greater thickness of
many individual salt beds.
The evaporation of salt waters, either from the ocean or from other
bodies of water, is believed to have been responsible for nearly all of
the important deposits of common salt. This process has been going on
from Cambrian time down through all the intervening geologic ages, and
can be observed to be actually operative today in various localities.
The beds of salt so formed are found interstratified with shales,
sandstones, and limestones, and are frequently associated with gypsum.
On a broad scale, they are always lens-shaped, though they vary greatly
in extent and thickness.
The necessary conditions for the formation of extensive salt beds
include arid climate and bodies of water which are essentially
enclosed--either as lakes, as lagoons, or as arms of the sea with
restricted outlets,--where evaporation exceeds the contributions of
fresh water from rivers, and where circulation from the sea is
insufficient to dilute the water and keep it at the same composition as
the sea water. Under such conditions the dissolved salts in the enclosed
body become concentrated, and precipitation may occur. A change of
conditions so that mud or sand is washed in or so that calcareous
materials are deposited, followed by a recurrence of salt-precipitation,
results in the interstratification of salt beds with shales, sandstones,
and limestones.
For the formation of very thick beds of salt, and especially of thick
beds of fairly pure composition, however, this simple explanation of
conditions is insufficient. The deposits of Michigan and New York occur
in beds as much as 21 feet in thickness, with a considerable number of
separate beds in a section a few hundred feet thick. Beneath the potash
salt deposits of Stassfurt, beds of common salt 300 to 500 feet in
thickness are found, and beds even thicker are known in other
localities. When we come to investigate the volume of salts deposited
from a given volume of sea water, we find it to be so small that for the
formation of 500 feet of salt over a given area, an equivalent area of
water 25,000 feet deep would be required. It has therefore been one of
the puzzling problems of geology to determine the exact physical
conditions under which deposition of these beds took place.
One of the most prominent theories, the "bar" theory, suggests that
deposition may have taken place in a bay separated from the sea by a
bar. Sea water is supposed to have been able to flow in over the bar or
through a narrow channel, so that evaporation in the bay was about
balanced by inflow of sea water. Thus the salts of a very large quantity
of sea water may have accumulated in a small bay. As the process went
on, the salts would become progressively more concentrated, and would be
precipitated in great thickness. A final complete separation of the
basin from the sea, for instance by the relative elevation of the land,
might result in complete desiccation, and deposition of
potassium-magnesium salts such as those found at Stassfurt (p. 113).
Another suggestion to explain the thickness of some salt beds is that
the salts in a very large basin of water may, as the water evaporated
and the basin shrank, have been deposited in great thickness in a few
small depressions of the basin.
Other writers believe that certain thick salt deposits were formed in
desert basins (with no necessary connection with the sea), through the
extensive leaching of small quantities of salt from previous sediments,
and its transportation by water to desert lakes, where it was
precipitated as the lakes evaporated. Over a long period of time large
amounts of salt could accumulate in the lakes, and thick deposits could
result. Such hypotheses also explain those cases where common salt beds
are unaccompanied by gypsum, since land streams can easily be conceived
to have been carrying sodium chloride without appreciable calcium
sulphate; in ocean waters, on the other hand, so far as known both
calcium sulphate and sodium chloride are always present, and gypsum
would be expected to accompany the common salt.
A partial explanation of some great thicknesses found in salt beds is
that these beds, especially when soaked with water, are highly plastic
and incompetent under pressure. In the deformation of the enclosing
rocks, the salt beds will flow somewhat like viscous liquids, and will
become thinned on the limbs of the folds and correspondingly thickened
on the crests and troughs.
The salt deposits of the Gulf Coast of Texas and Louisiana should be
referred to because of their exceptional features. They occur in low
domes in Tertiary and more recent sands, limestones, and clays. Vertical
thicknesses of a few thousand feet of salt have been found, but the
structure is known only from drilling. In some of these domes are also
found petroleum, gypsum, and sulphur (p. 110). No igneous rocks are
known in the vicinity. It has been thought by some that the deposits
were formed by hot waters ascending along fissures from underlying
igneous rocks, and the upbowing of the rocks has been variously
explained as due to the expanding force of growing crystals, to
hydrostatic pressure of the solutions, and to laccolithic intrusions. On
the other hand, the uniform association of other salt and gypsum
deposits with sedimentary rocks, and the absence of igneous rocks,
suggest that these deposits may have had essentially a sedimentary
origin, and that they have been modified by subsequent deformation and
alteration. The origin is still uncertain.
Other mineral deposits formed under much the same conditions as salt are
gypsum, potash, borax, nitrates, and minerals of bromine; and in a study
of the origin of salt deposits these minerals should also be
considered.
TALC AND SOAPSTONE
ECONOMIC FEATURES
Soapstone is a rock composed mainly of the mineral talc. Popularly the
terms _talc_ and _soapstone_ are often used synonymously. The softness,
greasy feel, ease of shaping, and resistance to heat and acids of this
material make it useful for many purposes. Soapstone is cut into slabs
for laundry tubs, laboratory table tops, and other structural purposes.
Finer grades are cut into slate pencils and acetylene burners. Ground
talc or soapstone is used as a filler for paper, paint, and rubber
goods, and in electrical insulation. Fine grades are used for toilet
powder.
Pyrophyllite (hydrated aluminum silicate) resembles talc in some of its
properties and is used in much the same way. Fine English clays (p. 85)
are sometimes used interchangeably with talc as paper filler.
The United States produces nearly two-thirds of the world's talc. The
other large producers are France, Italy, Austria, and Canada (Ontario).
The United States is independent of foreign markets for the bulk of its
talc consumption, but some carefully prepared talc of high quality is
imported from Canada, Italy, and France. Italy is our chief source of
talc for pharmaceutical purposes, though recently these needs have been
largely supplied by high-grade talc from California. In the United
States, Vermont and New York are the leading producers of talc and
Virginia of soapstone slabs. Reserves are large.
GEOLOGIC FEATURES
Talc is hydrated magnesium silicate, as is also serpentine, a mineral
with which talc is closely associated. Both are common alteration
products of magnesian silicate minerals such as olivine, pyroxene, and
amphibole. Talc is also derived from the recrystallization of magnesian
carbonates.
Talc deposits consist of lenses and bands in metamorphic limestones,
schists, and gneisses of ancient age. The talc itself is usually
schistose like the wall rocks, and is largely a product of mechanical
mashing. In some cases, also, talc results from the alteration of
igneous rocks without mashing--as in the case of the large talc and
soapstone deposits of Virginia, which are the result of rather complete
alteration of basic igneous rocks such as peridotites and pyroxenites.
Talc is known to result from the weathering of magnesian silicates under
surface conditions, but the common occurrence of the principal deposits,
in highly crystalline rocks which have undergone extensive deep-seated
metamorphism, is an indication that processes other than weathering have
been effective. It has been suggested that hot ascending solutions have
been responsible for the work, but without much proof. A more plausible
explanation for many deposits is that the talc results from the dynamic
metamorphism or shearing of impure magnesian carbonates (as in highly
magnesian limestones), the process resulting in elimination of the
carbon dioxide and recrystallization of the residue. Certain talc
deposits, such as those of Ontario, show clearly traces of the original
bedding planes of limestone crossing the cleavage of the talc, and the
rock bears all the evidence of having formed in the same manner as a
common slate. Talc and slate are almost the only mineral products which
owe their value principally to dynamic metamorphism.
CHAPTER XIV
EXPLORATION AND DEVELOPMENT
THE GENERAL RELATIONS OF THE GEOLOGIST TO EXPLORATION AND DEVELOPMENT
The economic geologist is more vitally concerned with exploration and
development than with any other phase of his work. This comes closest to
being his special field. Here is a fascinating element of adventure and
chance. Here is the opportunity to converge all his knowledge of geology
and economics to a practical end. The outcome is likely to be definite
one way or the other, thus giving a quantitative measure of the accuracy
of scientific thinking which puts a keen edge on his efforts. It is not
enough merely to present plausible generalizations; scientific
conclusions are followed swiftly either by proof or disproof. With this
check always in mind, the scientist feels the necessity for the most
rigid verification of his data, methods, and principles.
The general success of the application of geology to exploration and
development is indicated by the rapid increase in demand for such
service in recent years, and by the large part it plays in nearly all
systematic and large-scale operations. The argument is sometimes made
that many mineral deposits have been found without geologic assistance,
and that therefore the geologist is superfluous. The answer to this
argument is that there are often hundreds of "practical" explorers in
the field to one geologist, and that in proportion to numbers the story
is quite a different one. The very fact that many large mining
organizations, as a result of their experience, now leave these matters
of exploration and development largely in the hands of geologists, is a
tribute to the usefulness of the science. Also, it is to be remembered
that not all applications of geology are made by geologists. It is hard
to find a prospector or explorer who has not absorbed empirically some
of the elements of geology, and locally this may be enough. Very often
men who take pride in the title of "practical prospectors" are the ones
with the largest stock of self-made geological theories.
During a prospecting boom it is not uncommon for speculators and
promoters to attempt to discount geologic considerations where these run
counter to their plans. The catching phrase "bet against the geologist"
has a broad appeal to an instinctive preference for the practical as
opposed to the theoretical. If the public would stop to note the
character of the support behind the geologist, including as it does the
larger and more successful operators, it would not be so ready to accept
this implication.
Another aspect of this question might be mentioned. There is scarcely an
oil field or mining camp in the world without a cherished tradition to
the effect that, prior to discovery, the mineral possibilities had been
reported on unfavorably by the geologists,--again implying that success
has been due to the hard common sense of the horny-handed prospector.
These traditions persist in the face of favorable geological reports
published before discovery; they are natural expressions of the
instinctive distrust of any knowledge which is beyond the field of
empirical experience. In many cases the discoveries were made long
before geologists appeared on the scene. In others, possibly one or two
geologic reports were unfavorable, while many were favorable. In the
aggregate, there can be no question that, in proportion to the scale of
its use, geological advice has had more than its proportion of success.
Even under the most favorable conditions, the chances against the
success of an individual drill hole or underground development are
likely to be greater than the chances for it. The geologist may not
change this major balance; but if he can reduce the adverse chances by
only a few per cent, his employment is justified on purely commercial
grounds.
The above comments refer to sound geological work by competent
scientists. The geologic profession, like many others, is handicapped by
numbers of ill-trained men and by many who have assumed the title of
geologist without any real claim whatever,--who may do much to discredit
the profession. The very newness of the field makes it difficult to draw
a sharp line between qualified and unqualified men. With the further
development of the profession this condition is likely to be improved
(see pp. 427-428).
So new is the large-scale application of geology to exploration and
development, and so diverse are the scientific methods of approach, that
it is difficult to lay out a specific course for a student which will
prepare him for all the opportunities he may have later. In the writer's
experience, both in teaching and practice, the only safe course for the
student is to prepare broadly on purely scientific lines. With this
background he will be able later to adapt himself to most of the special
conditions met in field practice.
PARTLY EXPLORED VERSUS VIRGIN TERRITORIES
In selecting an area to work, the geologic explorer will naturally
consider various factors mentioned in succeeding paragraphs; but the
natural first impulse is to start for some place where no one else has
been, and to keep away from the older principal mining camps,--on the
assumption that such grounds have been thoroughly explored and that
their geological conditions bearing on exploration are fully understood.
It is safe to say that very few mineral districts are thoroughly
understood and explored. Numerous important discoveries of recent years
have been in the extensions of old mines and old districts; and when one
considers the scale of even the most extensive mine openings in
comparison with the vast body of rock available for exploration, it is
clear that this will continue to be the situation far into the future.
It is the writer's belief that the economic geologist stands at least as
good a chance of success in exploration in the older districts as he
does in new fields. Nature is exceedingly erratic and economical in
providing places favorable for mineral production; in a producing
district the geologic conditions have been proved to be right, and the
explorer starts here with this general pragmatic advantage. The explorer
here has another great advantage, that much essential information has
been gathered which can be built into his plan of operations. He can
start, scientifically and practically, where the other man left off. One
of the best-known economic geologists has maintained that the more
previous work done, the better, because it furnished him more tools to
work with. There is no such thing as "skimming the cream" from a
geologic problem; there is no end in sight in the search for more
knowledge.
This attitude toward the problem of exploration has also proved
advantageous on the business or financial side. A successful backer of
mineral enterprises once remarked that his best prospecting was done
from the rear platform of a private car,--meaning that this mode of
transportation had carried him to the center of important mining
activities, where the chances for large financial success showed a
better percentage than in more general and miscellaneous exploration.
THE USE OF ALL AVAILABLE INFORMATION
Effective scientific exploration requires the use of all available
information applying to the specific area. This might seem to be too
obvious to require mention, yet observance of the methods of explorers
seems to call for warning against the rather common tendency to go into
a field unprepared with a thorough knowledge of preceding work. It is
easy to forget or overlook some investigation made many years
previously; or to assume that such work is out of date, and of no
special consequence in the application of new thought and method which
is the basis of the faith and confidence of each new geologic explorer.
A study of the reports on an old camp shows how often the younger
generations have ignored the results of the older. Many of the same
elementary truths are rediscovered by successive generations, after
large efforts which could have been saved by means of proper care and
investigation of the previous literature and mapping.
In outlying parts of the world, the existing information bearing on
exploration may be at a minimum. In many of the older mining camps and
throughout most civilized countries, however, careful investigation will
usually disclose a considerable range of useful information bearing on
the territory to be explored. In the United States the natural course to
be pursued is to hunt carefully through the reports of the U. S.
Geological Survey, the Bureau of Mines, various state surveys,
universities, and private organizations (so far as these reports are
available), and through the technical journals and the reports of
technical societies, for something bearing on the district to be
explored. Even if no specific report or map is to be found, it is
usually possible to locate general maps or accounts which are likely to
be of use.
COOPERATION IN EXPLORATION
Competition in exploration often develops an atmosphere of suspicion and
furtiveness which is highly unfavorable to cooperative efforts.
Individuals and companies may handicap themselves greatly by a desire to
play a lone hand, and by failure to take advantage of an exchange of
information. This action may be based, particularly on the part of
strong mining companies, on the assumption that they know all that is
necessary about the problem, and that an outsider has nothing to
contribute. Financial and other conditions may require this attitude;
but in large part it is a result of temperament, as clearly indicated by
the difference in methods followed by different groups and in different
mining districts. From the scientific point of view this attitude can
hardly be justified, in view of the extremely narrow limits of human
knowledge as compared with the scientific field to be explored. The sum
total of knowledge from all sources is only a small fraction of that
necessary for the most effective results. The mutual exchange of
information and discussion is usually justified on the basis of
self-interest alone, to say nothing of the larger interest to the
mineral district, to the country, or to science.
National and state survey organizations exercise considerable effort to
secure records of drilling. In some cases they have the legal power to
command this information, particularly in relation to appraisals for
taxation and "blue sky" laws. In a larger number of cases drill records
are secured through voluntary cooperation with explorers. A considerable
number of records are nevertheless not filed with public agencies and
some of these are permanently lost. Even where the records are turned in
to a public organization, they are in most cases not directly available
to explorers.
Public registration of all drilling records is a highly desirable
procedure in the interests of the development of the mineral industry as
a whole. A vast amount of unnecessary duplication can thus be avoided.
The record of a drill hole, even though barren, may be of vast
significance in the interpretation of future developments and should be
recorded as carefully as an abstract of land title. The property right
of the explorer in such information can be and usually is protected by
withholding the record from public inspection until sufficient time has
elapsed to give him full opportunity to use the information to his own
best advantage.
The opportunities for cooperation with specialists of public
organizations are almost unlimited. These organizations are likely to
have an accumulation of data and experience extending through long
periods and over large areas, which the private explorer ordinarily
cannot hope to duplicate. With proper restrictions this information may
be available for public use. A good illustration of current cooperative
effort of this kind is in the deep exploration for oil in the Trenton
limestone of Illinois. Outcrops and other specific indications are not
sufficient to localize this drilling; but the information along broad
geologic and structural lines which has been collected previously by the
Illinois Survey is sufficient so that, with a comparatively small amount
of shallow drilling, the locus of the more favorable structural
conditions may be determined. In this case the Survey is directing the
initial exploration, which is financed by private capital.
ECONOMIC FACTORS IN EXPLORATION
The approach to the problem of exploration is very often determined by
local requirements and conditions; but if one were to come at the
problem from a distance and to keep matters in broad perspective, the
first step would be a consideration of what might be called the
_economic factors_. Let us suppose that the geologist is free to choose
his field of exploration. An obvious preliminary step is to eliminate
from consideration mineral commodities which are not in steady or large
demand and are much at the mercy of market conditions, or which are
otherwise not well situated commercially. The underlying factors are
many and complex. They include the present nature and future
possibilities of foreign competition, the domestic competition, the
grades necessary to meet competition, the cost of transportation, the
cost of mining under local conditions--including considerations of labor
and climatic and topographic conditions,--the probability of increase or
decrease in demand for the product, the possible changes in
metallurgical or concentrating practice (such as those which made
possible the mining of low-grade porphyry copper ores), the size of
already available reserves, and the mining laws in relation to
ownership and regulation. Most of these factors are discussed at some
length on other pages. After looking into the economic conditions
limiting the chromite, nickel, or tin developments in the United States,
the explorer might hesitate to proceed in these directions,--for he
would find that past experience shows little promise of quantities and
grades equivalent to those available in other countries, and that there
is little likelihood of tariffs or other artificial measures to improve
the domestic situation. Before and during the war, commercial conditions
might have shown the desirability of hunting for pyrite, but more recent
developments in the situation cast some doubt on this procedure. To go
ahead blindly in such a case, on the assumption that the pyrite market
would in some fashion readjust itself, would not be reasoned
exploration. Again, in considering exploration for copper, account
should be taken in this country of the already large reserves developed
far in advance of probable demand, which require that any new
discoveries be very favorably situated for competition. In oil, on the
other hand, a very brief survey of the economic factors of the situation
indicates the desirability of exploration. The comparative shortage of
lead supplies at the present time suggests another favorable field for
exploration.
In short, before actual field exploration is begun, intelligent
consideration of the economic factors may go far toward narrowing the
field and toward converging efforts along profitable lines. Looked at
broadly, this result is usually accomplished by the natural working of
general laws of supply and demand; but there are many individual cases
of misdirected effort, under the spell of provincial conditions, which
might easily be avoided by a broader approach to the problem.
GEOLOGIC FACTORS IN EXPLORATION
Coming to the geological aspects of exploration, the procedure in its
early stages is again one of elimination. Oil and coal, for instance,
are found in certain sediments of certain ages, and one would not look
for them in an area of granite. For every mineral resource there are
broad geologic conditions of this sort, particularly the genetic,
structural, and metamorphic conditions, which make it possible to
eliminate vast areas from consideration and to concentrate on relatively
small areas.
After the elimination of unfavorable areas, there comes the hunt for
positively favorable geologic conditions--for a definite kind of
sediment or igneous rock, for a definite structure, for the right kind
of mineralogic and metamorphic conditions, or for the right combination
of these and other geologic elements. The geologic considerations used
in exploration for the various mineral deposits are so many and so
diverse, and they require so much adjustment and interpretation in their
local application, that one would be rash indeed to attempt anything in
the nature of an exhaustive discussion. It is hardly practicable to do
more than to outline, for illustrative purposes, a few of the geologic
factors most commonly used in exploration.
MINERAL PROVINCES AND EPOCHS
Mineral deposits may be similar in their mineralogic and geologic
characters and relations over a considerable area. They may give
evidence of having developed under the same general conditions of
origin; perhaps they may even be of the same geologic age. The
gold-silver deposits of Goldfield, of Tonopah, the Comstock Lode of
Virginia City, and many other deposits through the Great Basin area of
the southwestern United States and Mexico have group characteristics
which have led geologists to refer to this area as a "metallogenic" or
"metallographic" province. The gold-silver ores on the west slope of the
Sierra Nevadas, for nearly the entire length of California, likewise
constitute a metallogenic province. The Lake Superior copper ores on the
south shore of Lake Superior, the silver ores on the north shore,
miscellaneous small deposits of copper, silver, and gold ores to the
east of Lake Superior, the nickel ores of Sudbury, and the
silver-nickel-cobalt ores of the Cobalt district are all characterized
by similar groups of minerals (though in highly differing proportions),
by similar geologic associations, by similar age, and probably by
similar conditions of origin. This area is a metallogenic province. The
lead and zinc ores of the Mississippi Valley constitute another such
province. The oil pools of the principal fields are characterized by
common geologic conditions over great areas (p. 149), which may likewise
be considered as forming mineral provinces; for them the term
"petroliferous provinces" has been used. The list might be extended
indefinitely. Knowledge of such group distributions of minerals is a
valuable asset to the explorer, in that it tends to localize and direct
search for certain classes of ores in certain provinces; also, within a
province, it tells the explorer what is to be normally expected as
regards kinds and occurrences of mineral deposits. In searching for
minerals of sedimentary origin, the explorer will use stratigraphic
methods in following definite sedimentary horizons. In searching for
ores related to igneous intrusions he will naturally hunt for the
intrusions, and then follow the periphery of the intrusions for
evidences of mineralization, taking into account possible features of
zonal arrangement of minerals about the intrusives (see pp. 42-44), and
the preference of the ores for certain easily replaced horizons like
limestones, or for certain planes or zones of fracturing.
Just as minerals may be grouped by provinces, they may be grouped by
geologic ages. Such groupings are especially useful in the case of
minerals which are closely related to certain stratigraphic horizons,
such as coal, oil, and iron. The greater number of the productive coal
deposits of the United States are of Carboniferous age, and the
distribution of sediments of this age is pretty well understood from
general geologic mapping. The Clinton iron ores all follow one general
horizon in the lower-middle Paleozoic. The Lake Superior iron ores are
pre-Cambrian, and over three-fourths of them occur at one horizon in the
pre-Cambrian. Gold deposits of the United States were formed mainly in
the pre-Cambrian, the early Cretaceous, and the Tertiary. Copper
deposits of the United States were formed chiefly in pre-Cambrian,
Cretaceous, and Tertiary time. While there are many exceptions and
modifications to general classifications of this sort, they seem to
express essential geologic facts which can be made very useful in
localizing exploration.
CLASSIFICATION OF MINERAL LANDS
In recent years there has been considerable development of the practice
of classifying mineral lands in given areas for purposes of exploration
and valuation, or for purposes of formulation and administration of
government laws. This has been done both by private interests and by the
government. These classifications take into account all of the geologic
and economic factors ascertainable. The classes of mineral land
designated vary with the mineral, the district, and the purpose for
which the classification is made.
Common procedure for commercial exploration purposes is to divide the
lands of a given territory into three groups--(1) lands which are
definitely promising for mineral exploration, (2) lands of doubtful
possibilities, and (3) lands in which the mineral possibilities are so
slight that they may be excluded from practical consideration. Each of
these classes may be subdivided for special purposes. Another commonly
used classification is, (1) proved mineral lands, (2) probable mineral
lands, usually adjacent to producing mines, (3) possible mineral lands,
and (4) commercially unpromising mineral lands.
The classification of the public mineral lands by government agencies is
fully discussed by George Otis Smith and others in a bulletin of the
United States Geological Survey.[37] The purposes, methods, and results
of this classification should be familiar to every explorer. Nowhere
else is there available such a vast body of information of practical
value. Quoting from this report:
A study of the land laws shows the absolute necessity of some
form of segregation of the lands into classes as a
prerequisite to their disposition. Agricultural entry may not
be made on lands containing valuable minerals, nor coal entry
on lands containing gold, silver, or copper; lands included in
desert entries or selected under the Carey Act must be desert
lands; enlarged-homestead lands must not be susceptible of
successful irrigation; placer claims must not be taken for
their timber value or their control of watercourses; and lands
included in building-stone, petroleum, or salt placers must be
more valuable for those minerals than for any other purpose.
So through the whole scheme of American land laws runs the
necessity for determining the use for which each tract is best
fitted.
For this purpose the Geological Survey has made extensive classification
of coal lands, oil and gas lands, phosphate lands, lands bearing potash
and related salines, metalliferous mineral lands, miscellaneous
non-metalliferous mineral lands, and water resources. The scope of the
work may be indicated by the factors considered. For instance coal is
investigated in relation to its character and heat-giving qualities
(whence comes its value), quantity, thickness, depth, and other
conditions that effect the cost of its extraction. Metalliferous mineral
lands are considered in relation to general geology, country rock,
intrusions and metamorphism, structure, outcrops and float of lodes,
prospects and mines, samples, and history of the region.
Classifications of this kind have often proved useful to large holders
of land as a basis for intelligent handling of problems of sale,
taxation, and the granting of rights to explorers. Because of the lack
of this elementary information, there has been in some quarters timidity
about dealing with large holdings, for fear of parting with possible
future mineral wealth,--with the result that such tracts are carried at
large expense and practically removed from the field of exploration. To
the same cause may be attributed some of the long delays on the part of
the government in opening lands for mineral entry or in issuing patents
on land grants.
OUTCROPS OF MINERAL DEPOSITS
Many mineral deposits have been found because they outcrop at the
surface; the discoveries may have been by accident or they may have been
aided by consideration of geologic factors. There are still vast
unexplored areas in which mineral deposits are likely to be found
standing out at the surface. For much of the world, however, the surface
has been so thoroughly examined that the easy surface discoveries have
been made, and the future is likely to see a larger application of
scientific methods to ground where the outcrops do not tell an obvious
story. Mineral deposits may fail to outcrop because of covering by
weathered rock or soil, by glacial deposits, or by younger formations
(surface igneous flows or sediments), or the outcrop of a deposit may be
so altered by weathering as to give little clue to the uninitiated as to
what is beneath. Mineral deposits formed in older geologic periods have
in most cases been deeply covered by later sediments and igneous rocks.
Such deposits are in reach of exploration from the surface only in
places where erosion has partly or wholly removed the later covering. An
illustration of this condition is furnished in the Great Basin district
of Nevada, where ore bodies have been covered by later lava flows. The
ore-bearing districts are merely islands exposed by erosion in a vast
sea of lava and surface sediments. Beyond reasonable doubt many more
deposits are so covered than are exposed, and it is no exaggeration to
say that by far the greater part of the mineral wealth of the earth may
never be found. Where a mineral-bearing horizon is exposed by erosion at
the surface, underground operations may follow this horizon a long way
below the capping rocks; but, after all, such operations are
geographically small as compared with the vast areas over which the
covering rocks give no clue as to what is beneath. One of the principal
problems of economic geology for the future is to develop means for
exploration in territories of this sort. A beginning has been made in
various districts by the use of reconnaissance drilling, combined with
interpretation of all the geologic and structural features. The
discovery of one of the largest nickel deposits in the Sudbury district
of Canada was made by reconnaissance drilling to ascertain the general
geologic features, in an area so deeply covered as to give little
suggestion as to the proper location for attack.
SOME ILLUSTRATIVE CASES
The use of outcrops in oil exploration has been noted on other pages
(pp. 146-147).
Outcrops of coal seams may be found in folded or deeply eroded areas.
For the most part, however, and especially in areas of flat-lying rocks,
the presence of coal is inferred from stratigraphic evidence and from
the general nature of the geologic section--which has been determined by
outcrops of associated rocks or by information available at some distant
point. The structural mapping of coal beds on the basis of outcrops and
drill holes has been referred to (pp. 126-127).
Iron ores are very resistant to solution. Where hard and compact they
tend to form conspicuous outcrops, and where soft they may be pretty
well covered by clay and soil. In glaciated areas, like the Lake
Superior region, outcrops of iron ore are much less numerous because of
the drift covering. Certain of the harder iron ores of the Marquette,
Gogebic and Menominee districts of Michigan and of the Vermilion
district of Minnesota project in places through the glacial drift, and
these ores were the first and most easily found. Much the greater
number of iron ore deposits of Lake Superior, including the great soft
deposits of the Mesabi range of Minnesota, fail to outcrop. On the other
hand the _iron formation_, or mother rock of the ore, is hard and
resistant and outcrops are numerous. The hematite ores of Brazil have
many features in common with the Lake Superior ores in age and
occurrence, but they have not been covered with glacial deposits.
Outcrops of the iron ore are large and conspicuous, and the surface in
this territory gives one some idea of what the Lake Superior region may
have looked like before the glaciers came along. Certain of the soft
iron ores of the lateritic type, as in Cuba, outcrop over great areas
where their topographic situation is such that erosion has not swept
them off. On erosion slopes they are seldom found. The Clinton iron ores
of the southeastern United States outcrop freely.
Some of the lead and zinc deposits of the Mississippi Valley outcrop at
the grass roots as varying mixtures of iron oxide, galena, chert, and
clay, though they seldom project above the general surface. The old lead
ranges of Wisconsin and Illinois, found at the surface a century ago by
the early explorers and traders, have served as starting points for
deeper exploration which has located the zinc deposits. Erosion channels
have freely exposed these ore bodies, and in the Wisconsin-Illinois
deposits most of the ores thus far found are confined to the vicinity of
these channels. The greater number of the lead and zinc deposits of the
Mississippi Valley, however, are covered with weathered material or with
outliers of overlying sediments, with the result that underground
exploration is necessary to locate them.
Sulphide deposits in general, including those carrying gold, silver,
copper, lead, zinc, and other metals, have many common features of
outcrop. The iron sulphide commonly present in these ore bodies is
oxidized to limonite at the surface, with the result that prospectors
look for iron-stained rocks. These iron-stained rocks are variously
called the "gossan," the "iron capping," the "colorado," or the
"eiserner Hut" (iron hat). The gossan is likely to resist erosion and to
be conspicuous at the surface,--though this depends largely on the
relative resistance of the wall rocks, and on whether the gangue is a
hard material like quartz, or some material which weathers more rapidly
like limestone or igneous rock. The gossan does not often carry much
value, though it may show traces of minerals which suggest what may be
found below. Gold, silver, and lead are not easily leached out of the
surface outcrops. Copper and zinc are much more readily leached, and in
the outcrop may disclose their existence only by traces of staining. It
happens not infrequently, therefore, that copper and zinc deposits are
found through the downward exploitation of oxidized gold, silver, and
lead ores. The veins at Butte were first worked for silver, and the ore
bodies at Bingham, Utah, and Jerome, Arizona, were first mined for gold.
Exceptionally, copper ore in enriched, oxidized form outcrops, as at
Bisbee, Arizona.
It is not always true that valuable sulphide deposits have an
iron-stained outcrop, for in some of them iron sulphide or pyrite is so
scarce that the surface outcrops may be light-colored clayey and
siliceous rocks.
Silver is often represented in the outcrop by silver chloride or
cerargyrite, which may be easily identified. The prospecting for such
surface ores is sometimes called "chloriding."
The presence in the outcrop of dark manganese oxides associated with
vein quartz sometimes indicates the presence below of copper and zinc
and other minerals, as at Butte.
Extensive alterations of the country rock in the way of silicification
and sericitization, and the presence of minerals like garnet,
tourmaline, diopside, and others, known to be commonly deposited by the
same hot solutions which make many ore deposits, may furnish a clue for
exploration below. These characteristics of the country rock, however,
are likely to be masked at the outcrop by later weathering, which
superposes a kaolinic or clayey alteration.
TOPOGRAPHY AND CLIMATE AS AIDS IN SEARCHING FOR MINERAL OUTCROPS
The topographic expression of a mineral deposit depends upon its
hardness and resistance to erosion as compared with the adjacent rocks.
If more resistant it will stand out at the surface; if less resistant,
it will form a depression. The conditions determining resistance are
exceedingly variable, and no broad generalization can be made; but
within a local province a given group of mineral deposits may
characteristically form depressions or ridges, and thus topographic
criteria may be very useful in exploration. Even with such limitations,
the variations of the topographic factor may be so great as to require
much care in its use. Sulphide ores in quartzites are likely to develop
depressions under erosion. In limestones they are more likely to stand
out in relief, because of the softer character of the limestone, though
this does not always work out. Crystalline magnetite and hematite are
more resistant to erosion than almost any other type of rock, and stand
out at the surface with proportional frequency.
Climatic conditions may determine the locus of search for certain
surface minerals. Bauxite and lateritic iron ores, for instance, are
known to favor tropical climates. In exploration for these minerals, the
climatic factor must be applied in connection with the topographic
considerations already mentioned, and both, in turn, in connection with
the character of the country rock as determined by general geologic
surveys. A combination of climatic, topographic, and other physiographic
conditions may be used also in exploration for certain types of residual
clays.
SIZE AND DEPTH OF ORE BODIES AS DETERMINED FROM OUTCROP
Where the ore body is harder than the surrounding rock, it stands out in
conspicuous outcrops and is likely to show a narrowing below. Where it
is softer than the surrounding rocks, and outcrops in a topographic
depression, it is perhaps more likely to show widening below. These
features are due to the general facts that, where the ore body is hard
and resistant, the downward progress of erosion is likely to be arrested
where the adjacent rocks occupy the larger part of the surface, that is,
where the ore body is narrower. This principle is often vaguely
recognized in the assumption that an exceptionally large outcrop of an
ore vein may be "too good to last." Again, such a generalization must be
applied to a specific case with much caution.
Attempts to forecast the depth of veins from their extent at the surface
meet with only partial success. In a very general way great persistence
horizontally suggests persistence in depth, on the ground that the
section exposed on the surface is as likely to be a section of average
dimensions as one along vertical lines.
Faith is the first article of the prospector's creed, and it is hard to
shake his conviction that every ore outcrop must widen and improve
below. As expressed by the French-Canadian prospector in the Cobalt
district, the "vein calcite can't go up, she must go down." While the
scientist may have grounds to doubt this reasoning, he is not often in a
position to offer definite negative evidence.
THE USE OF PLACERS IN TRACING MINERAL OUTCROPS
Outcrops of ore-bearing rock may occasionally be located by tracing a
placer deposit back to its source, or by following up ore fragments in
the "wash" on mountain sides to the place of origin, or by noting ore
fragments in glacial deposits. The presence of an ore mineral in a
placer naturally raises the question as to whence it came. If it is a
recent placer, it may be comparatively easy to follow up the stream
channels to the head-water territory which is delivering the main mass
of sediment, and there to locate a vein in place. The problem is
complicated by multiplicity of tributaries and by large size of the
drainage areas. In such cases careful panning and testing of the gravels
at frequent intervals may show which of several tributaries are
contributing most of the values, and thus may further localize the area
of search. Many important mining districts, including Butte, Bisbee, the
Mother Lode region of southern California, the diamond fields of Africa,
and others, have been found by tracing up placers in this manner. In the
case of an older placer deposit, where the topography and drainage have
been much altered since its formation, or where the deposit has been
covered by later sediments, the problem is of course much more
difficult.
Much less than a commercially valuable placer deposit in unconsolidated
surface rocks may start a search for the mother lode. A single fragment
of ore in the "wash" naturally directs attention up the slope, and the
repetition of fragments in a certain direction may lead unerringly to
the source. The fragments may not even in themselves carry value, but
may consist of detrital material from the leached outcrop--such as iron
or manganese oxides, which, because of their red or black color, stand
out conspicuously in the rock debris.
In the Lake Superior region large angular fragments of iron ore or iron
formation in the glacial drift immediately raise question as to source.
If the fragments are rounded and small, they usually indicate a very
distant source. The general direction of glacial movement is known in
most places, and by tracing up the fragments in this direction the
outcrop may be found; or the chain of fragments may be traced to a point
where they stop, which point may serve to locate the parent bedrock
carrying the ore body, even though it does not outcrop.
An interesting suggestion was made some years ago with reference to the
diamonds found sporadically in the terminal moraines in Wisconsin and
other mid-west states. The diamonds are of such size and quality as to
indicate surely the existence of a real diamond field somewhere to the
north. The locations of these diamond finds were platted on a glacial
map, and lines were projected in a general northerly direction along the
known lines of the glacial movement. It was found that these lines
converged at a point near Hudson's Bay. The data were too meager and the
base line too short for this long projection, and the indicated source
of the diamonds can be regarded as the merest speculation. However, with
the finding of additional diamonds in the drift, as seems very likely,
the refinement of this method might conceivably bring results in time.
THE USE OF MAGNETIC SURVEYS IN TRACING MINERAL LEDGES
Magnetic surveys are often useful in tracing iron-bearing rocks beneath
the surface, in the discovery of outcrops of such rocks, and in working
out their lines of connection. This method is in general use for the
crystalline iron ores in the Lake Superior region, Canada, the
Adirondacks, and elsewhere in the glaciated portions of the United
States. It is not so useful for the brown ores and the Clinton ores of
the southeastern United States, which are only slightly magnetic and can
be commonly located by other methods.
Where the ore is strongly magnetic, and is associated with other rocks
which are non-magnetic, the nature of the magnetic field determined by a
surface survey with vertical and horizontal needles may tell something
about the shape and size of the ore body. Commonly, however, magnetic
ores are associated with leaner magnetic rocks,--with the result that
the magnetic survey, unless it happens to lead to an outcrop of ore,
indicates only the general area through which underground exploration
might be warranted. In the hematitic iron ores of Lake Superior,
magnetism is less pronounced than in the magnetites; and in the soft
hydrous hematites, like those of the Mesabi district, it may cause only
slight disturbance of the magnetic needle. This disturbance is usually
sufficient to locate the position of the iron-bearing formation, though
not the position of the ore.
Where the iron formation has been highly metamorphosed, and rendered
resistant to weathering and erosion so that it will not concentrate into
ore, it is likely to have higher magnetic attraction than the richer
ores. For this reason an area of strong magnetic attraction is
ordinarily regarded as not particularly favorable to the finding of
important hematite deposits. However, this attraction may be very useful
in tracing out the formation to a place where it is less metamorphic,
less resistant to erosion, less likely to outcrop, and yet more
promising for the discovery of iron ore. For instance, on the east end
of the Mesabi and on the east and west ends of the Gogebic district,
magnetic surveys trace the iron formation with great ease to points
where the attraction is low and the conditions for exploration more
favorable.
The magnetic needle has also been used in the search for nickel ore in
the Sudbury district of Ontario, but without great success, because of
the variety of rocks other than nickel which are more or less magnetic,
and because of the slight magnetic properties of the nickel ore itself.
In a large-scale exploration of this type, conducted some years ago, a
favorable magnetic belt was discovered, and a pit was sunk to water
level but not to bedrock. Years later, the extension of this pit by only
a few feet disclosed one of the great ore bodies of the district.
Experimental work on the use of the magnetic needle on copper deposits
has yielded some interesting and suggestive results, but this
investigation is still under way and the results have not been
published.
THE USE OF ELECTRICAL CONDUCTIVITY AND OTHER QUALITIES OF ROCKS IN
EXPLORATION
In addition to magnetism, rocks and ores have other properties
susceptible to observations made at a distance, such as electrical
conductivity, transparency to X-rays, specific induction, elasticity,
and density. All these qualities have been of interest to geologists in
some connection or another, but none of them have yet been used
effectively in exploration for mineral resources. The only one of these
properties that has thus far seemed to promise practical results is
electrical conductivity. The results yet obtained are slight, and this
kind of investigation has rested under something of a cloud, due to
extravagant claims of inventors. Nevertheless, there has been a
considerable amount of scientific work by physicists, geologists, and
engineers, supplemented by special war-time investigations of rock and
earth conductivity in connection with ground telephones and the tapping
of enemy conversations, which seems to indicate a distinct possibility
of practical results in the future,--perhaps not so much in locating
specific ore bodies as in locating general types of formation and
structures,--which may serve to supplement other methods of search.[38]
The transmission and reflection of sound waves in rocks have also been
more or less investigated with reference to their possible military use.
It seems not impossible that these phenomena may be of some geologic aid
in the future, but experimental work is yet in a very early stage.
THE USE OF STRUCTURE AND METAMORPHISM IN EXPLORATION
The necessity for careful use of structural data in exploration scarcely
requires discussion. References have been made to structural features in
connection with coal, oil, iron ore, and other minerals. This phase of
study can scarcely be too intensively followed. The tracing of a folded
or faulted vein, in a particularly complex system of veins, requires
application of all of the methods and principles of structural geology.
Similarly, the importance of applying the principles of metamorphism,
embodied in the _metamorphic cycle_ (pp. 27-28) is almost self-evident.
Certain kinds of metamorphism are suggestive of the nature of the
mineral deposits with which they are associated. One would not look for
minerals known to be caused mainly by surficial processes in rocks which
have been altered mainly by deep-seated processes. The presence of
metamorphism indicating high temperatures and pressures to some extent
limits the kinds of minerals which one may expect to find. On the other
hand, minerals known to be primarily formed at great depths, providing
they are resistant to surface weathering, may be found in deposits which
are the result of surficial alterations or katamorphic processes; that
is, they may become concentrated as residual materials in weathered
zones or as placers.
DRILLING IN EXPLORATION
In the absence of distinctive outcrops, as well as when outcrops are
found, drilling is a widely used method of underground exploration in
advance of the sinking of shafts or the driving of tunnels. Drilling is
more useful in the locating and proving of mineral deposits of large
bulk, like deposits of coal, iron, and oil, than mineral deposits of
small bulk and high value, like gold and silver deposits. However, it is
not always used in the exploration of the first class of deposits and is
not always eliminated in the exploration of the second class. With the
development of better mechanical devices, better methods of controlling
and ascertaining the direction of the drill hole, and more skillful
interpretation of drill samples, the use of drilling is rapidly
extending into mineral fields where it was formerly thought not
applicable.
The geologist takes an active part in drilling operations by locating
the drill holes, by determining the angle of the holes, by identifying
and interpreting the samples, by studying bedding, cleavage, and other
structures as shown in the samples, and determining the attitude of
these structures in the ground, by determining when the horizon is
reached which is most promising for mineral, and by determining when the
hole shall be stopped. With a given set of surface conditions, the
problem of locating and directing a drill hole to secure the maximum
possible results for the amount expended requires the careful
consideration of many geologic factors,--and, what is more important,
their arrangement in proper perspective and relationship. Faulty
reasoning from any one of the principal factors, or over-emphasis on any
one of them, or failure to develop an accurate three-dimensional
conception of the underground structural conditions, may lead to failure
or extra expense. Success or failure is swiftly and definitely
determined. The geologist is usually employed by the company financing
the drilling; but in recognition of the importance of his work, some of
the large contracting drill companies now employ their own geologists.
The technique of the geologic interpretation and direction of drilling
has become rather complicated and formidable, and has resulted in the
introduction of special college courses in these subjects.
The desirability of public registration of drilling records is discussed
on another page (pp. 305-306).
QUANTITATIVE ASPECTS OF GEOLOGIC EXPLORATION
In recent years there has been a tendency to reduce the geologic factors
in exploration to some kind of a quantitative basis. While these factors
may be very variable and very complex, their net effect frequently may
be expressed in terms of quantitative averages. In various mines and
mining districts where operations are of wide extent, local quantitative
factors have been worked out which are useful in predicting results from
proposed explorations in undeveloped portions. Figures of this sort may
be useful and practical guides in planning any given exploration, its
cost, and its probable outcome.
Quantitative methods are illustrated in the general account of Lake
Superior iron ore exploration in a later section.
Curves of production from oil wells and from oil districts have been
found to have certain characteristic features in common which are often
used in predicting the future output and life of a given well, property,
or district. Where associated with coal, the percentage of fixed carbon
in the coal may be a guide to the presence and nature of the oil (see
Chapter VIII).
The geological staff of the Netherlands East Indies estimated the tin
reserves of one of these islands by the use of a factor or coefficient,
based on the experience of another island.
In the Cobalt district of Canada a factor for future discoveries and
output, based on past experience, was similarly developed.
Hoover[39] made a statistical study of several hundred metal mines in
various parts of the world, and found that not 6 per cent of the mines
that yielded profits ever made them from ore mined below 2,000 feet; and
that of the mines that paid dividends, 80 per cent did not yield profit
below 1,500 feet, and most of them died above 500 feet.
Attempt has been made by a Swedish geologist to estimate the iron ore
resources of continents by the use of an iron coefficient. This
coefficient was obtained by dividing the known iron ore resources of the
comparatively well-investigated portions of the world by the number of
square miles in which they occurred, and was then multiplied into the
area of the continents whose resources were to be determined.
The application of quantitative methods of this kind has not yet become
very general, nor is it possible to use them in some cases; where
applied many of them have been very crude and others have been partly
disproved by experience. With increasing knowledge and experience, such
methods are becoming more accurate and useful, and are likely to have
wider use in the future.
ORIGIN OF MINERAL DEPOSITS AS A FACTOR IN EXPLORATION
In exploration, the geologist is keen to ascertain the origin of the
mineral deposit. This is often a source of wonder to the layman or
"practical" man, and the geologist may be charged with having let his
fondness for theory run away with him. A widespread fatalistic
conception is expressed in the Cornishman's dictum on ore, "Where it is,
there it is." Yet an understanding of the origin of any particular ore,
the "why" of it, is coming to be recognized as the most effective means
of reaching sound practical conclusions. By ascertaining the approximate
origin of the ore, it may be possible at once to infer a whole group of
practical considerations based on experience with ores of like origin in
other localities. The origin of the ore is the geologist's primary
interest, and it is this which gives him his most effective and
distinctive tool in exploration. Many other phases of exploration work
may be picked up empirically by any one familiar with the local
conditions; but when the man without sound geologic training attempts to
go into this particular field, his lack of background and perspective
often leads to fantastic hypotheses which may vitiate the inferences on
which he plans his exploration.
The scientific investigator, while not accepting the fanciful theories
of the local observer, will make a mistake if he fails to recognize the
residuum of solid fact on which they are built. Many practical explorers
are shrewd observers of empirical facts, even though their explanations
may show a lack of comprehension of the processes involved. Any
assumption of superiority, intolerance, or lack of sympathy, on the part
of the geologist, toward the inadequate explanations and descriptions
given him by the practical man, is likely to indicate a weakness or
limitation in his own mental processes. The geologist's business is to
sift out the fact from the inference, and not to throw over the whole
structure because some of the inferences are faulty.
LAKE SUPERIOR IRON ORE EXPLORATION AS AN ILLUSTRATION
To illustrate the application of some of the methods of exploration of
the kinds described in this chapter, the writer selects an example from
his own experience in the Lake Superior iron fields.[40]
In this region, consideration of the economic aspects of the problem may
eliminate from the best explorable field certain Canadian portions which
are far from water transportation, because the conditions in these
sections would prevent the use of anything but an exceptionally large
and rich deposit. Economic conditions determine in advance also that it
is not worth while looking for ores of certain grades, either because
they are not usable on account of deleterious constituents or low
content of iron, or because these particular grades have already been
developed in excess of requirements. Having determined what ore is
desired, whether Bessemer or non-Bessemer, whether open-hearth or
foundry, further elimination of area is possible on the basis of past
experience.
Coming to the geologic phases of the problem, the first step is to
eliminate great areas of rock which are known never to contain iron ore,
like the granite areas and the quartzite and limestone areas. Within the
remaining areas, by examination of the surface outcrops and with the aid
of magnetic surveys, iron formations are found which are the mother rock
of the ores. In Michigan, it has been possible to use certain percentage
expectations in the areal location of iron formations within certain
series of rocks extending over wide areas. Such percentage coefficients
have been useful, not only in exploration, but also in the valuation of
lands which are so covered with drift that no one knows whether they
carry an iron formation or not.
Examination of the iron formations results in elimination of large parts
of them, because their metamorphic condition is not favorable to ore
concentration. In the remaining areas more intensive methods are
followed. It is scarcely possible to summarize briefly all of the
structural and stratigraphic methods used in locating the ore bodies.
These have often been described in print.[41] Comparatively recent
advances in this phase of exploration work have been in the more
detailed application of stratigraphic methods to the iron formation. The
group characteristics of the iron formation are fairly uniform and
distinctive as compared with all other rocks; yet within the iron
formation there are so many different kinds of layers represented that
it is possible to use these variations with great effectiveness, in
correlating favorable horizons for ore deposition, in interpreting drill
records, and in other ways. Another method of approach, employed chiefly
on the Mesabi Range, relates to the slumping of the ore layers which
results from the leaching of silica during the concentration of the ore.
This slumping can be measured quantitatively, and has been used to much
advantage in exploration, in correlation of ore horizons, in preparation
of sections and ore estimates, etc.
Early geologic explorations in the Lake Superior country were based on
the assumption that the ores were concentrated by waters working down
from the present erosion surface; but recognition of the fact that the
waters which did the work were related to a far older and different
erosion surface, under conditions which allowed of a far deeper
penetration, has modified exploration plans for certain of the districts
like the Marquette and Gogebic.
Notwithstanding the complexity of the geologic factors involved, their
net result has been to concentrate iron ores in a surprisingly uniform
ratio to the mass of the formation in different parts of the
region,--with the result that on an average it may be predicted for any
district, in an exploration of sufficient magnitude, how much ore is
likely to be cut in either vertical or horizontal dimension. Thirteen
per cent of the productive area of the Mesabi iron formation is iron
ore. For the remainder of the Lake Superior region five or six per cent
is the factor. These figures mean that, if a person could explore a
broad enough area of iron formation, any miscellaneous group of drill
holes or underground openings would tend to yield these percentage
results. Such percentages are amply sufficient to pay a large profit on
the exploration. The question may be raised why the application of
geology is required, if such average results can be secured from
miscellaneous undirected work. The answer is that seldom is it possible
to conduct an exploration on a sufficiently large scale to be sure of
approximating this average, and that geologic study has made it possible
in many cases to secure a better percentage result. If the geologist is
able to raise the percentage ever so little, the expenditure is amply
justified. He is not expected to have 100 per cent success; but he is
expected to better the average returns, and in this on the whole he has
not failed.
Applying this method specifically to the Gogebic Range, it appears that
up to January 1, 1918, exploration and development had covered 3,650
acres of iron formation, measured along the dip in the plane of the
footwall, within the limits of the area in which the formation is in
such condition as to allow concentration of the ore. The total area of
the footwall to a depth of 3,000 feet is approximately 9,650 acres. The
range, therefore, was 38 per cent developed to this depth. In the
developed area, 160,000,000 tons of ore had been found, or approximately
one ton per square foot of footwall area, or 43,800 per acre of
footwall explored. The total area of ore measured on the footwall was
785 acres. The ratio of ore area to total explored area, measured in the
plane of the footwall, was 21-1/2 per cent. This may be taken in a rough
way to indicate the average exploring possibilities in new ground, where
local conditions to the contrary do not exist. This means that over the
whole range about one drill hole or cross-cut in five will strike ore on
an average. Or, looked at in another way, about 200 feet of drifting in
every 1,000 on the footwall will be in ore. Applying this factor to the
unexplored area, amounting to 6,000 acres, the range had an expectation
on January 1, 1918, to a depth of 3,000 feet, over and above ores
already discovered, of approximately 262,800,000 tons. This was
sufficient to extend the life of the range by about forty-four years.
Knowing the average cost of development of ore per foot in the past, and
knowing the annual output and its rate of acceleration, it is possible
to figure with some accuracy how much expenditure should be planned for
annually in the future in order to maintain a safe margin of reserves
against output.
Such quantitative considerations in the Lake Superior region serve not
only to guide the general conduct of the exploration and development
work, but in some cases as a basis for valuation both for commercial and
taxation purposes.
DEVELOPMENT AND EXPLOITATION OF MINERAL DEPOSITS
The search for new ore bodies is closely related to the development,
extension, and mining of ore bodies already found. In this field the
geologist finds wide application of his science. Here he may not be so
much concerned with the economic factors or with the broader methods of
geologic elimination; his study is more likely to be based mainly on the
local geologic conditions.
Some of the larger and more successful mining companies, perhaps the
greater number of them these days, have geologists whose business it is
to follow closely the underground operations, with a view to advising on
the conduct of the development work. This requires the most precise and
intensive study. For instance, the Anaconda Copper Mining Company has a
staff of several geologists, who follow the underground work in the
utmost detail and whose approval must be obtained by the operating
department in the formulation of any development plan. The complexity
and fault relations of the veins in this company's mines are such that
the application of these methods has abundantly justified itself on the
cost sheet.
Too often mining companies leave the planning and execution of the
underground development work to the local management, commonly to the
underground mining captain, without geologic consultation. This
procedure does not eliminate the economic geologist; for when the
development fails at any point, or new and unexpected conditions are
met, the geologist is likely to be called in. In such cases the practice
of a geologist is like that of the ordinary medical practitioner; he is
called in only when his patients are in trouble. The use of adequate
geologic advice in the planning stages is about as little advanced in
some localities as the practice of preventive medicine.
The work of the economic geologist may not be ended by the finding and
development of the ore; for the moment this is accomplished, he should
again consider the economic phases of the problem--the grade of his ore,
its probable amount, and other features, in relation to the general
economic setting. In his enthusiasm for physical results, he may be
carried into expenditures not justified by the economic factors in the
problem. Some one else may and usually does look out for the economic
elements, but the prudent geologist will at least see to it that someone
is on the job.
FOOTNOTES:
[37] Smith, George Otis, and others, The classification of the public
lands: _Bull. 537, U.S. Geol. Survey_, 1913.
[38] Schlumberger, C., _Study of underground electrical prospecting_:
Translated from the French by Sherwin F. Kelly, Paris, 1920.
Bergstrom, Gunnar, and Bergholm, Carl, "Teknisk Tidskrift, Kemi och
Bergvetenskap," 1918, Book 12.
[39] Hoover, Herbert C., _Principles of mining_: McGraw-Hill Book Co.,
New York, 1909, p. 32.
[40] Leith, C. K., Use of geology in iron ore exploration: _Econ.
Geol._, vol. 7, 1912, pp. 662-675.
[41] Van Hise, C. R., and Leith, C. K., Geology of the Lake Superior
region: _Mon. 52, U.S. Geol. Survey_, 1911.
CHAPTER XV
VALUATION AND TAXATION OF MINERAL RESOURCES
POPULAR CONCEPTION OF MINERAL VALUATION
The total returns from mining may not in the aggregate be far above the
expenditure for exploration, development, and extraction; yet the total
mineral wealth of the United States, on the basis of earning power and
aside from the industries based on it, cannot be far from sixty billions
of dollars, and this wealth has virtually come into existence since the
1849 gold rush to California. The mining industry supports a large
population. These facts are the solid basis for the widespread popular
interest in mineral investment--and mineral speculation. But there are
other reasons for this interest,--the gambler's chance for quick
returns, the "lure of gold," the possibility of "getting something for
nothing," the mushroom nature of certain branches of the industry, the
element of mystery related to nature's secrets, and the conception of
minerals as bonanzas with ready-made value, merely awaiting discovery
and requiring no effort to make them valuable. In the United States a
factor contributing to the popular interest is the large freedom allowed
by the laws to discover and acquire minerals on the public domain.
Perhaps no other field of industry comes so near being common ground for
all classes of people. The mineral industry is a field in which it is
easy to capitalize not only honest and skillful endeavor, but hopes,
guesses, and greed. It is not to be wondered at, therefore, that in the
popular mind the valuation of a mineral resource is little more than a
guess, and sometimes not even an honest one.
Nevertheless, the mineral industry has become second only to agriculture
in its capital value and in its earning capacity. In this industry it is
hardly possible to arrive at valuations as securely based as in many
other industries, but the elements of hazard are not so hopeless of
measurement as might be supposed. The great mineral and financial
organizations do not depend on mere guesses, but use well-tried methods.
If the general investor were to give more attention to these methods he
would doubtless save himself money, and the mineral industry would be
rid of a great incumbrance of parasites who live on the credulity of the
public. To anyone familiar with the mineral field, it is often
surprising to see the rashness with which a conservative business man,
who would not think of entering another industrial field without close
study of all the factors in the situation, will invest in minerals
without using ordinary methods of analysis of values.
In the following account of valuation of minerals in the ground, and the
closely related subject, taxation of such minerals, the attempt is made
to state some of the principles briefly and simply with a view to making
them intelligible to the layman. Values beyond the mine are concerned
with so many factors of a non-geologic nature that they are not here
discussed.
VALUATION AND TAXATION OF MINES
INTRINSIC AND EXTRINSIC FACTORS IN VALUATION
It is essential to recognize at the outset that the value of a mineral
deposit, like the value of any other commercial material, comprises two
main elements; an intrinsic element based on the qualities of the
material itself, and an extrinsic element based on its availability and
the nature of the demands for it. The two elements may not be sharply
separated, and neither exists without the other. A mineral deposit in
easy reach of a populous community, which has sufficiently advanced
methods and requirements to use it, may have high value; an exactly
similar deposit, if far removed from points of consumption, handicapped
by transportation, or available only to people without developed methods
for its use, may have little or no value. Intrinsically the deposits are
alike; but extrinsically they are far different, and their values are
correspondingly unlike. Even two adjacent properties, differently
managed and controlled, and with different relations to markets, may
have somewhat different values depending on the use made of them. The
value of a deposit may vary from year to year with changes in demand for
its output, or with changes in metallurgical and other processes which
make its use possible. Minerals of small bulk and high value, as for
instance gold, platinum, and diamonds, have a nearly standard value
related to their intrinsic properties, because they can be transported
so easily to any part of the world. On the other hand, materials of
large bulk and low unit value, such as coal, iron ore, and clay, may
have highly varying values independently of their physical
characteristics, because of their relative immobility. But the values
even of gold and precious stones represent a combination of intrinsic
qualities and of demand. A diamond is made of carbon but is more
valuable than coal or graphite because it appeals to the esthetic taste.
It is only because man introduces an element of demand that the diamond
takes on value. In short, man is the multiplier and the mineral
substance is the multiplicand in the product known as value.
Recognition of the two elements of value is vital to a clear
understanding of the methods and problems of valuation of minerals. It
is too often assumed that the physical properties constitute the sole
factor.
Looked at in a large way, the returns from the mineral industry are
commensurate with the effort put into discovery and development of
mineral resources, even though the returns to lucky individuals have
been excessive. In respect to the importance of the human energy
element, the mining of minerals is not unlike the cropping of soils.
Some interesting economic studies have been made of mining districts to
ascertain whether the total return has been equal to the total
investments by both successful and unsuccessful participants. The
results show that, even in some of the most successful districts, there
is not a large "social surplus,"--that is, a surplus of receipts over
total expenditures. It is difficult to generalize from such studies with
any degree of accuracy; but it seems likely that if we could measure the
vast amount of fruitless effort which has been expended in
non-productive territories, the result would tend to bear out the
general conclusion that the social surplus for the mineral industry as a
whole is a modest one, if it exists at all. Of course, it is to be
remembered that the total benefits from mineral resources are not to be
measured in terms of gain to the producers,--but that their measurement
must take into account the satisfying of all the complex demands of
modern civilization.
VALUES OF MINERAL DEPOSITS NOT OFTEN ESTABLISHED BY MARKET TRANSFERS
While minerals as extracted and used may have standard market values,
mineral deposits in the ground are not bought and sold on the open
market with sufficient frequency to establish standard market values. A
sale may establish a criterion of value for the particular deposit, but
not for the class of deposits,--for no two mineral deposits are exactly
alike. Stock quotations may establish a certain kind of market value,
but these are often vitiated by extraneous considerations. For these
reasons the valuation of a mineral deposit is in each case a special
problem.
THE AD VALOREM METHOD OF VALUATION