fragments of elytra, scales of the wings of butterflies, etc., which are
brought together in those places by the millions of cubic meters. The
nitrification, which takes place in these deposits, has been found to
extend its products to a distance of several kilometers through the
soil. In some places the quantity of the nitrate of lime is so great in
the soils that they are converted into a plastic paste by this
deliquescent salt.
The theory of Müntz and Marcano in regard to the nitrates of soils,
especially in the neighborhood of caves, is probably a correct one, but
there are many objections to accepting it to explain the great deposits
of nitrate of soda which occur in many parts of Chile. Another point,
which must be considered also, is this: That the processes of
nitrification can not now be considered as going on with the same vigor
as formerly. Some moisture is necessary to nitrification, inasmuch as
the nitrifying ferment does not act in perfectly dry soil, and in many
localities in Chile where the nitrates are found it is too dry to
suppose that any active nitrification could now take place.
The existence of these nitrate deposits has long been known.[13] The old
Indian laws originally prohibited the collection of the salt, but
nevertheless it was secretly collected and sold. Up to the year 1821,
soda saltpeter was not known in Europe except as a laboratory product.
About this time the naturalist, Mariano de Rivero, found on the Pacific
coast, in the Province of Tarapacá, immense new deposits of the salt.
Later the salt was found in equal abundance in the Territory of
Antofogasta and further to the south in the desert of Atacama, which
forms the Department of Taltal.
At the present time the collection and export of saltpeter from Chile is
a business of great importance. The largest export which has ever taken
place in one year was in 1890, when the amount exported was 927,290,430
kilograms; of this quantity 642,506,985 kilograms were sent to England
and 86,124,870 kilograms to the United States. Since that time the
imports of this salt into the United States have largely increased.
According to Pissis[14] these deposits are of very ancient origin. This
geologist is of the opinion that the nitrate deposits are the result of
the decomposition of feldspathic rocks; the bases thus produced
gradually becoming united with the nitric acid provided from the air.
According to the theory of Nöllner[15] the deposits are of more modern
origin and due to the decomposition of marine vegetation. Continuous
solution of soils, gives rise to the formation of great lakes of
saturated water, in which occurs the development of much marine
vegetation. On the evaporation of this water, due to geologic isolation,
the decomposition of nitrogenous organic matter causes generation of
nitric acid, which, coming in contact with the calcareous rocks, attacks
them, forming nitrate of calcium, which, in presence of sulfate of
sodium, gives rise to a double decomposition into nitrate of sodium and
sulfate of calcium.
The fact that iodin is found in greater or less quantity in Chile
saltpeter is one of the chief supports of this hypothesis of marine
origin, inasmuch as iodin is always found in sea and not in terrestrial
plants. Further than this, it must be taken into consideration that
these deposits of nitrate of soda contain neither shells nor fossils,
nor do they contain any phosphate of lime. The theory, therefore, that
they were due to animal origin is scarcely tenable.
=13. Boron= occurs chiefly in volcanic regions, but is much more widely
distributed in the soil than formerly believed. It is a regular
constituent of the ash of many plants,[16] and is, therefore, thought to
be a true plant food. It is one of the least abundant of the elements,
not occurring in sufficient quantity to find a place in the table
showing their relative abundance, which is to follow. Boracic acid is
used to some extent as a preservative.
=14. Fluorin= does not occur free in nature, but it exists chiefly in
combination with calcium, forming fluorspar, and traces of it are found
in sea water. It occurs in bone, teeth, blood, and the milk of mammals.
It is the only element that does not combine with oxygen, and it can be
isolated only with the greatest difficulty. Only very small traces of it
are found ordinarily and it is usually not considered in the chemical
analysis of soils. Fluorin is found, however, in considerable quantities
in certain phosphate deposits.
=15. Aluminum= is, probably, next to oxygen and silicon, the most
abundant element of the earth’s crust, of which it is estimated to form
about one-twelfth. It has never been found, in nature, in the free
state, but commonly occurs in combination with silicon and oxygen, in
which form it is an abundant constituent of feldspar, mica, kaolin,
clay, slate, and many other rocks and minerals.
By the weathering of feldspar, mica, and other minerals containing
aluminum, kaolin or true clay is formed, which is of the greatest
importance in the constitution of the soil. The compounds of aluminum
are not so important as plant food as they are as the constituents of
the soil, forming a large part of its bulk, and modifying in the most
profound degree its physical properties. It is the custom of some
authors to use the word clay to designate the fine particles of soil
which have in general the same relations to moisture and tilth as the
particles of weathered feldspar, etc. In a strict chemical sense,
however, the term clay is applied only to the hydrated silicate of
alumina formed as indicated above. The fertility of a soil is largely
dependent on the quantity of clay which it contains, its relations to
moisture and amenability to culture being chiefly conditioned by its
clay content. The determination of the percentage of clay in soils is an
operation of the highest utility in forming an opinion of the value of a
soil on analytical data alone.
=16. Calcium= is one of the commonest and most important elements of the
earth’s crust, of which it has been estimated to compose about
one-sixteenth. It does not occur free in nature, but its most common
form is in combination with carbon dioxid, forming the mineral calcite,
marble, and the very abundant limestone rocks. In this form it is
slightly soluble in water containing carbon dioxid, and hence lime has
become a universal component of all soils and is very generally found in
natural waters, in which it furnishes the chief ingredient necessary for
the formation of the shells and skeletons of the various tribes of
mollusca and corals. In combination with sulfuric acid calcium forms the
rock gypsum. Lime is not only a necessary plant food, but influences in
a marked degree the physical condition of the soil and the progress of
nitrification. Many stiff clay soils are rendered porous and pulverulent
by an application of lime, and thus made far more productive. On account
of its great abundance and low price, it has not commanded the degree of
attention from farmers and agricultural chemists which its merits
deserve. It forms an essential ingredient of plants and animals, in the
latter being collected chiefly in the bones, while in plants it is
rather uniformly distributed throughout all the tissues.
=17. Magnesium= occurs chiefly in combination with carbon dioxid or with
lime and carbon dioxid in the mineral dolomite. It is intimately
associated with calcium and a trace of it is nearly always found where
lime occurs in any considerable quantity. The bitter taste of sea water
and some mineral waters is due to the presence of salts of magnesia. In
combination with silica it forms an essential part of such rocks as
serpentine, soapstone, and talc. Magnesia is not of much importance as a
plant food nor as a fertilizing material.
=18. Potassium= combined with silica is an important element in many
mineral silicates as, for instance, orthoclase. Granitic rocks usually
contain considerable quantities of potassium, and on their decomposition
this becomes available for plant food. In the form of chlorid, potassium
is found in small quantities in sea water, and as a nitrate it forms the
valuable salt known as niter or saltpeter. Potassium, as is the case
with phosphorus, is universally distributed in soils, and forms one of
the great essential elements of plant food. Under the form of kainite
and other minerals large quantities of potassium are used for
fertilizing and for the manufacture of pure salts for commercial and
pharmaceutical purposes. The ordinary potassium salts are very soluble
and for this reason they can not accumulate in large quantities in soils
exposed to heavy rain-fall. In the form of carbonate, potassium forms
one of the chief ingredients of hard wood ashes, and in this form of
combination is especially valuable for fertilizing purposes. Potash
salts, being extremely soluble, are likely to be held longest in
solution. Some of them, are recovered in animal and vegetable life, but
the great mass of potash carried into the sea still remains unaccounted
for. The recovery of the waste of potash is chiefly secured by the
isolation of sea waters containing large quantities of this salt and
their subsequent evaporation. Such isolation of sea waters takes place
by means of geologic changes in the level of the land and sea. In the
raising of an area above the water level there is almost certain to be
an enclosure, of greater or less extent, of the sea water in the form of
a lake. This enclosure may be complete or only partial, the enclosed
water area being still in communication with the main body of the sea by
means of small estuaries. If this body of water be exposed to rapid
evaporation, as was doubtless the case in past geologic ages, there will
be a continual influx of additional sea water through these estuaries to
take the place of that evaporated. The waters may thus become more and
more charged with saline constituents. Finally a point is reached in the
evaporation when the less soluble of the saline constituents begin to be
deposited. In this way the various formations of mineral matter,
produced by the drying up of enclosed waters, take place.
The most extensive potash deposits known are those in the neighborhood
of Stassfurt, in Germany. The following description probably represents
the method of formation of these deposits:[17]
“The Stassfurt salt and potash deposits had their origin, thousands of
years ago, in a sea or ocean, the waters of which gradually receded,
leaving near the coast, lakes which still retained communication with
the great ocean by means of small channels. In that part of Europe the
climate was then tropical, and the waters of these lakes rapidly
evaporated but were constantly replenished through these small channels
connecting them with the main body. Decade after decade this continued,
until by evaporation and crystallization, the various salts present in
the sea water were deposited in solid form. The less soluble material,
such as sulfate of lime or ‘anhydrite,’ solidified first and formed the
lowest stratum. Then came common rock salt with a slowly thickening
layer which ultimately reached 3000 feet, and is estimated to have been
13,000 years in formation. This rock salt deposit is interspersed with
lamellar deposits of ‘anhydrite,’ which gradually diminish towards the
top and are finally replaced by the mineral ‘polyhalite,’ which is
composed of sulfate of lime, sulfate of potash, and sulfate of magnesia.
The situation in which this polyhalite predominates is called the
‘polyhalite region’ and after it comes the ‘kieserite region,’ in which,
between the rock salt strata, kieserite (sulfate of magnesia) is
imbedded. Above the kieserite lies the ‘potash region,’ consisting
mainly of deposits of carnallite, a mineral compound of muriate of
potash and chlorid of magnesia. The carnallite deposit is from 50 to 130
feet thick and yields the most important of the crude potash salts and
that from which are manufactured most of the concentrated articles,
including muriate of potash.”
“Overlying this region is a layer of impervious clay which acts as a
water-tight roof to protect and preserve the very soluble potash and
magnesia salts, which, had it not been for the very protection of this
overlying stratum, would have been long ages ago washed away and lost by
the action of the water percolating from above. Above this clay roof is
a stratum, of varying thickness of anhydrite, and still above this a
second salt deposit, probably formed under more recent climatic and
atmospheric influences or possibly by chemical changes in dissolving and
subsequent precipitation. This salt deposit contains ninety-eight per
cent (often more) of pure salt, a degree of purity rarely elsewhere
found. Finally, above this are strata of gypsum, tenacious clay, sand,
and limestone, which crop out at the surface.”
“The perpendicular distance from the lowest to the upper surface of the
Stassfurt salt deposits is about 5000 feet (a little less than a mile),
while the horizontal extent of the bed is from the Harz Mountains to the
Elbe River in one direction, and from the city of Madgeburg to the town
of Bernburg in the other.”
According to Fuchs and DeLauny[18] the saline formation near Stassfurt
is situated at the bottom of a vast triassic deposit surrounding
Madgeburg. The quantity of sea water which was evaporated to produce
saline deposits of more than 500 meters in thickness must have been
enormous and the rate of evaporation great. It appears that a
temperature of 100° would have been quite necessary, acting for a long
time, to produce this result.
These authors therefore admit that all the theories so far advanced to
explain the magnitude of these deposits are attended with certain
difficulties. What, for instance, could have caused a temperature of
100°? The most reasonable source of this high temperature must be sought
for in the violent chemical action produced by the double decompositions
of such vast quantities of salts of different kinds. There may also have
been at the bottom of this basin some subterranean heat such as is found
in certain localities where boric acid is deposited.
Whatever be the explanation of the source of the heat it will be
admitted that at the end of the permian period there was thrown up to
the northeast of the present saline deposits a ridge extending from
Helgoland to Westphalia. This dam established throughout the whole of
North Germany saline lagoons in which evaporation was at once
established, and these lagoons were constantly fed from the sea.
There was then deposited by evaporation, first of all a layer of gypsum
and afterwards rock salt, covering with few exceptions the whole of the
area of North Germany.
But around Stassfurt there occurred at this time geologic displacements,
the saline basin was permanently closed and then by continued
evaporation the more deliquescent salts, such as polyhalite, kieserite,
and carnallite, were deposited.
These theories account with sufficient ease for the deposition of the
saline masses, but do not explain why in those days the sea water was so
rich in potash and why potash is not found in other localities where
vast quantities of gypsum and common salt have been deposited. It may be
that the rocks composing the shores of these lagoons were exceptionally
rich in potash and that this salt was, therefore, in a certain degree, a
local contribution to the products of concentration.
=19. Sodium= is never found free in nature, but its most common form is
in combination with chlorin as common salt, an important ingredient of
sea water. Combined with silica sodium is an important element in many
silicates. Sodium, although closely related to potassium chemically,
cannot in any case be substituted therefor in plant nutrition. In
combination with nitrogen it forms soda or Chile saltpeter which is a
valuable fertilizer on account of its content of nitric acid.
=20. Iron= is the most abundant of the heavy metals, and occurs in
nature both free and combined with other elements. In the free state it
is found only to a limited extent in basaltic rocks and meteorites, but
in combination with oxygen it is one of the most widely diffused of
metals, and forms the coloring matter of a large number of rocks and
minerals. In this form, too, it exists as the valuable ores of iron
known as magnetite and hematite. In combination with sulfur it forms the
mineral pyrite, FeS₂. The yellow and red colors of soils are due chiefly
to iron oxids. It is an important plant food, although not taken up in
any great quantity by the tissues of plants.
=21. Manganese=, next to iron, is the most abundant of the heavy metals.
It occurs in nature only in combination with oxygen, in which form it is
associated in minute quantities with iron in igneous rocks or in the
forms known mineralogically as pyrolusite, psilomelane and wad. As the
peroxid of manganese it occurs in concretionary forms scattered
abundantly over the bottom of the deep sea. It is found in the ash of
some plants but is not believed to be an essential to plant growth.
=22. Barium= occurs in nature combined with sulfuric acid, forming the
mineral barite, or heavy spar, or with carbon dioxid forming the mineral
witherite. It is of small importance from an agricultural standpoint.
=23. Relative Abundance of the More Important Chemical Elements.=—It
will be of interest to the agricultural analyst to know as nearly as
possible the relative abundance of the more important chemical elements.
This subject has been carefully studied by Prof. F. W. Clarke in a paper
read before the Philosophical Society of Washington.[19] The materials
considered in these calculations are the atmosphere, the water, and the
solid crust of the earth to the depth of ten miles below the sea level.
Of these materials the relative quantities of the three constituents
named are as follows:
Per cent.
Atmosphere 0.03
Water 7.08
Solid crust of the earth to the depth of ten miles 92.89
According to these calculations the relative abundance of the important
elements composing the atmosphere, the water of the ocean and the solid
crust of the earth to the depth given is as follows:
Solid crust, Ocean, seven per Mean, including
ninety-three per cent. air.
cent.
Oxygen 47.29 per cent. 85.79 per cent. 49.98 per cent.
Silicon 27.21 „ „ „ „ 25.30 „ „
Aluminum 7.81 „ „ „ „ 7.26 „ „
Iron 5.46 „ „ „ „ 5.08 „ „
Calcium 3.77 „ „ 0.05 „ „ 3.51 „ „
Magnesium 2.68 „ „ 0.14 „ „ 2.50 „ „
Sodium 2.36 „ „ 1.14 „ „ 2.28 „ „
Potassium 2.40 „ „ 0.04 „ „ 2.23 „ „
Hydrogen 0.21 „ „ 10.67 „ „ 0.94 „ „
Titanium 0.33 „ „ „ „ 0.30 „ „
Carbon 0.22 „ „ 0.002 „ „ 0.21 „ „
Chlorin 0.01 „ „ 2.07 } „ „ 0.15 „ „
Bromin „ „ 0.008} „ „ „ „
Phosphorus 0.10 „ „ „ „ 0.09 „ „
Manganese 0.08 „ „ „ „ 0.07 „ „
Sulfur 0.03+ „ „ 0.09 „ „ 0.04+ „ „
Barium 0.03 „ „ „ „ 0.03 „ „
Nitrogen „ „ „ „ 0.02 „ „
Chromium 0.01 „ „ „ „ 0.01 „ „
—————— ——————— ——————
100.00 „ „ 100.000 „ „ 100.00 „ „
=24. Fluorin= is not mentioned in this table but it is stated that its
probable percentage is 0.02 to 0.03 making it thus slightly more
abundant than nitrogen.
One of the chief points of interest in connection with this table is
that the nitrogen which is regarded by most persons as one of the most
abundant of the elements is almost the least abundant of those
mentioned.
THE MINERALS OCCURRING IN ROCKS.
=25. The Soil=, as before stated, being comprised almost exclusively of
decayed rocks, its characteristics would naturally be determined by the
character of the minerals contained in the rocks.
A rock may be composed of a single mineral or an aggregation of several
minerals.
According to the authority of the National Museum[20] it may occur,
either in the form of stratified beds, eruptive masses, sheets or dikes,
or as veins and other chemical deposits of comparatively little
importance as regards size and extent. The mineral composition of rocks
is greatly simplified by the wide range of conditions under which the
commonest minerals can be formed. Thus quartz, feldspar, mica, the
minerals of the hornblende, or pyroxene group, can be formed from a mass
cooling from a state of fusion; they may be crystallized from solution,
or be formed from volatilized products. They are therefore the commonest
of minerals and are rarely excluded from rocks of any class, since there
is no process of rock formation which determines their absence.
Most of the common minerals, like the feldspars, micas, hornblendes,
pyroxenes, and the alkaline carbonates possess the capacity of adapting
themselves to a very considerable range of compositions. In the
feldspars, for example, lime, soda, or potash may replace one another
almost indefinitely, and it is now commonly assumed that true species do
not exist, but all are but isomorphous admixtures passing into one
another by all gradations, and the names albite, oligoclase, anorthite,
etc., are to be used only as indicating convenient stopping and starting
points in the series. Hornblende or pyroxene, further, may be pure
silicate of lime and magnesia, or iron and manganese may partially
replace these substances. Lime carbonate may be pure, or magnesia may
replace the lime in any proportion.
These illustrations are sufficient to show the reason for the great
simplicity of rock masses as regards their chief mineral constituents.
Whatever may be the conditions of the origin of a rock mass, the
probabilities are that it will be formed essentially of one or more of a
half a dozen minerals in some of their varieties.
But however great the adaptability of these few minerals may be they
are, nevertheless, subject to very definite laws of chemical
equivalence. There are elements which they cannot take into their
composition, and there are circumstances which retard their formation
while other minerals may be crystallizing. In a mass of rock of more or
less accidental composition formed under these widely varying conditions
it may, therefore, be expected that other minerals will form, in
considerable numbers, but minute quantities. It is customary to speak of
those minerals which form the chief ingredients of any rock, and which
may be regarded as characteristic of any particular variety, as the
essential constituents, while those which occur in but small quantities,
and whose presence or absence does not fundamentally affect its
character, are called accessory constituents. The accessory mineral
which predominates, and which is, as a rule, present in such quantities
as to be recognizable by the unaided eye, is the characterizing
accessory. Thus a biotite granite is a stone composed of the essential
minerals quartz and potash feldspar, but in which the accessory mineral
biotite occurs in such quantities as to give a definite character to the
rock.
=26. Classification Of Minerals.=—The minerals of rocks may also be
conveniently divided into two groups, according as they are products of
the first consolidation of the mass or of subsequent changes. This is
the system here adopted. We thus have:
(1) The original or primary constituents, those which formed upon its
first consolidation. All the essential constituents are original, but on
the other hand all the original constituents are not essential. Thus, in
granite, quartz and orthoclase are both original and essential, while
beryl and zircon or apatite, though original, are not essential.
(2) The secondary constituents are those which result from changes in a
rock subsequent to its first consolidation, changes which are due in
great part to the chemical action of percolating water. Such are the
calcite, chalcedony, quartz, and zeolite deposits which form in the
druses and amygdaloidal cavities, of traps and other rocks.
Below is given a list of the more common, original and secondary
minerals occurring in rocks. It will be observed that the same mineral
may, in certain cases, occur in both original and secondary forms. The
tables following were prepared by Dr. George P. Merrill.