soluble in ordinary meteoric waters, the rocks have undergone extensive
decomposition, their lime being removed, while their less soluble
constituents or impurities remain to form soil. A single ton of residual
soil represents not infrequently a loss of 100 tons of original rock
matter. As this mass of lime carbonate is removed by solution the
residual soil settles, and as the limestone rocks are more soluble than
the adjacent rock formations limestone formations usually form valley
lands with ridges on either side. Caves are frequently found in such
formations. Furthermore, as the lime is almost all in the form of the
easily soluble lime carbonate it can be very completely removed and the
fertile “limestone soils” are often very deficient in lime and respond
readily to an application of burnt lime, which, not infrequently, is
quarried from the same field. From an agricultural standpoint this group
is of very great interest and importance.
The third subdivision of this group, namely, that of vegetable origin,
includes peat, lignite, coals, etc. Rocks of this group are made up of
more or less fragmental remains of plants. In many of them, as the peats
and lignites, the traces of plant structure are still apparent. In
others, as the anthracite coals, these structures have been wholly
obliterated by metamorphisms.
Plants when decomposing on the surface of the ground give off their
carbon to the atmosphere in the shape of carbon dioxid gas leaving only
the strictly inorganic or mineral matter behind. When, however, they are
protected from the oxidizing influence of the air, by water or by other
plant growth, decomposition is greatly retarded, and a large portion of
the carbonaceous and volatile matters is retained, and by this means
together with pressure from the overlying mass, the material becomes
slowly converted into coal. When this process goes on near the surface
of the earth, and without much pressure, peat or muck is the product.
The fourth subdivision of this group, the phosphatic, forms a class of
rocks limited in extent but of the greatest economic importance. Guano,
coprolites, and phosphatic rocks (the phosphorites) come under this
head.
=34. Aeolian Rocks.=—This class of rocks is of less importance than the
others, either geologically or agriculturally. It is formed from
materials drifted by the winds and this material has various degrees of
compactness. Usually the components of these drifts form rocks or
deposits of a friable texture and of a fragmental nature. The very
extensive deposits of loess in China, forming their most fertile lands,
are admitted now to have been formed in this way, but it is now
generally admitted that similar deposits in this country are of
subaqueous origin.
Chief among these rocks, are the volcanic ashes which are often carried
to a long distance by the wind before they are deposited and
consolidated into rock masses. Many loose soils may be carried to great
distances by the wind, the deposits forming new aggregates. This is
particularly the case in arid regions.
=35. Metamorphic Rocks.=—This class of rocks includes all sedimentary or
eruptive rocks, which, after their deposition and agglomeration, have
been changed in their nature through the action of heat, pressure, or by
chemical means. Sometimes these changes are so complete that no
indication of the character of the original rocks remains. At other
times the changes may be found in all the stages of progress, so that
they can be traced from the original fragmental or irruptive to the
completely metamorphosed deposit. This is especially true of rocks
containing large quantities of lime. In those containing silica, the
changes are less readily traced.
[Illustration:
FIG. 8.
MICROSTRUCTURE OF CRYSTALLINE LIMESTONE.
(West Rutland, Vermont.)
]
The metamorphic rocks may be divided into two subclasses, namely,
stratified or bedded, and foliated or schistose.
The rocks of the first class are represented by the crystalline
limestones and dolomites. The microstructure of a crystalline limestone
is shown in Fig. 8.[25] When lime and magnesia occur together in
combination with carbon dioxid, the substance is known as dolomite. The
chemical nature of these rocks and their soil-forming properties are the
same as those of the ordinary, non-metamorphosed limestones and
dolomites to which reference has already been made. The subject need
not, therefore, be further dwelt upon here.
[Illustration:
FIG. 9.
MICROSTRUCTURE OF GNEISS.
(West Andover, Massachusetts.)
At _a a_ are shown plagioclase crystals broken and rounded by the
sheering force producing the foliation.
]
The second variety of metamorphic rocks is represented by the gneisses
and crystalline schists. Gneiss has essentially the same composition as
granite and can frequently hardly be distinguished from it, except by a
microscopic study of its sections, and even thus it is sometimes
difficult to determine. Frequently a number of new minerals is formed in
the metamorphic changes. The microstructure of a gneiss is shown in Fig.
9.[26] The schists include an extremely variable class of rocks, of
which quartz is the prevailing constituent, and which, as a rule, are
deficient in potash and other important ingredients.
=36. Rocks Formed Through Igneous Agencies, or Eruptive Rocks.=—This
group includes all those rocks, which, having been at some time in a
state of igneous fusion, have been solidified into their present form by
a process of cooling. It may be stated, as a general principle, that the
greater the pressure under which a rock solidifies and the slower and
more gradual the cooling the more perfect will be found the crystalline
structure. Hence, it follows that the older and more deep-seated rocks
which are forced up in the form of dikes, bosses, or intrusive sheets,
into the overlying masses, and which have become exposed only through
erosion and removal of the overlying rocks, are the more highly
crystalline.
The eruptive rocks are divided into two main groups, _viz._:
(a) Intrusive or plutonic rocks, and
(b) Effusive or volcanic rocks.
Among the more important of the first division of the plutonic form,
from an agricultural point of view, are the granites. The essential
constituents of granite are quartz, potash feldspar, and plagioclase.
One or more minerals of the mica, amphibole or pyroxene groups are also
commonly present, and in microscopic proportions apatite and particles
of magnetic iron. The more valuable constituents, from an agricultural
standpoint, are the minerals potash feldspar, and apatite, which furnish
by their decomposition the essential potash and phosphoric acid.
In addition to the granites, which have already been mentioned, the
group includes the syenites, the nepheline syenites, the diorites, the
gabbros, the diabases, the theralites, the peridotites, and the
pyroxenites.
The second group, the effusive or volcanic rocks, includes those igneous
rocks, which, like the first group, have been forced up through the
overlying rocks, but which were brought to the surface, flowing out as
lavas. These, therefore, represent, in many cases, only the upper or
surface portions of the first group, differing from them structurally,
because they have cooled under little pressure more rapidly, and hence
are not so distinctly crystalline. These comprise the following groups:
(a) Quartz porphyries. (b) Liparites. (c) Quartz-free porphyries.
(d) Trachytes. (e) Phonolites. (f) Porphyrites. (g) Andesites.
(h) Melaphyrs and augite porphyrites. (i) Basalts. (j) Tephrites
and Basanites. (k) Picrite porphyrites. (l) Limburgites and
augitites. (m) Leucite rocks. (n) Nepheline rocks. (o) Melilite
rocks.
It is, in most cases, impossible to state which of the above classes is
of most importance from an agricultural standpoint, since, in the
process of soil formation, both chemical and physical processes are
involved, whereby the character of the resultant soil is so modified as
to but remotely resemble its parent rock. In most soils, the prevailing
constituent is but the least soluble one of the rock mass from which it
was derived. Thus a limestone soil may contain upwards of ninety per
cent of silica and alumina, while the original limestone itself may not
have carried more than one or two per cent of these substances. Of
course, if a rock mass contains none of the constituents essential to
plant growth, its resultant soil must by itself alone be quite barren.
It does not absolutely follow, however, that those rocks containing the
highest percentages of valuable constituents will yield the most fertile
soils, since much depends on the manner in which they have been formed,
the amount of leaching, etc., they may have undergone. Nevertheless, the
study of the composition of the rocks in their relation to soils, is an
extremely interesting and by no means unimportant one.
A comparative table of rock compositions is here given. It will be
observed that there is a considerable range of variation even among
rocks of the same class, a fact due to the varying abundance of their
mineral constituents. The figures given are not those of actual analyses
on any one particular rock, but are selected from a number of
comparatively typical cases; and, it is thought, fairly well represent
the composition of the class of rocks indicated.
COMPOSITION OF ROCKS.—THE FIGURES INDICATE PARTS PER HUNDRED.
──────────────────────┬────────┬────────────┬────────────┬────────
│ SiO₂. │ Al₂O₃. │ Fe₂O₃. │ MgO.
│ │ │ FeO. │
──────────────────────┼────────┼────────────┼────────────┼────────
Granite │ │ │ │
Quartz poryhyries} │ 63–78 │ 10–15 │ 2–3 │0.3–0.5
Liparite │ │ │ │
│ │ │ │
Syenite │ │ │ │
Orthoclase porphyries}│ 55–73 │ 12–16 │ 5–7 │ 2–6
Trachyte │ │ │ │
│ │ │ │
Nepheline syenites} │ 54–56 │ 16–22 │ 4–6 │0.4–0.88
Phonolites │ │ │ │
│ │ │ │
Diorites │ │ │ │
Porphyrites} │ 52–65 │ 16–20 │ 7–10 │ 5–7
Andesites │ │ │ │
│ │ │ │
Gabbros │ │ │ │
Norites} │ 48–55 │ 12–20 │ 8–15 │ 2–7
Melaphyrs │ │ │ │
│ │ │ │
Theralites │ │ │ │
Tephrites} │ 43–47 │ 15–23 │ 9–18 │ 1–6
Basanites │ │ │ │
│ │ │ │
Peridotites │ │ │ │
Picrite porphyrites} │ 23–43 │ 1–10 │ 10–15 │ 15–45
Limburgites │ │ │ │
│ │ │ │
Pyroxenites} │ 50–55 │ 0.5–4 │ 4–10 │ 20–25
Augitites │ │ │ │
│ │ │ │
Leucite rocks │ 48–50 │ 15–20 │ 7–10 │ 1–2
│ │ │ │
Nepheline rocks │ 40–45 │ 8–20 │ 10–20 │ 1–13
──────────────────────┴────────┴────────────┴────────────┴────────
──────────────────────┬─────┬────────┬───────┬───────────
│CaO. │ Na₂O. │ K₂O. │P₂O₅.
│ │ │ │
──────────────────────┼─────┼────────┼───────┼───────────
Granite │ │ │ │
Quartz poryhyries} │ 1–2 │ 2–3 │ 4–5 │ 0.05–0.15
Liparite │ │ │ │
│ │ │ │
Syenite │ │ │ │
Orthoclase porphyries}│ 3–5 │ 2–6 │ 4–7 │ trace.
Trachyte │ │ │ │
│ │ │ │
Nepheline syenites} │ 2–4 │ 3–7 │ 4–6 │ 0.15
Phonolites │ │ │ │
│ │ │ │
Diorites │ │ │ │
Porphyrites} │ 5–7 │ 2–4 │ 1–2 │ 0.1–0.3
Andesites │ │ │ │
│ │ │ │
Gabbros │ │ │ │
Norites} │6–10 │ 2–4 │ 0.5–2 │ 0.1–0.33
Melaphyrs │ │ │ │
│ │ │ │
Theralites │ │ │ │
Tephrites} │6–10 │ 5–7 │ 2–4 │ trace.
Basanites │ │ │ │
│ │ │ │
Peridotites │ │ │ │
Picrite porphyrites} │ 1–4 │ 0–4 │trace. │ 0.0
Limburgites │ │ │ │
│ │ │ │
Pyroxenites} │8–15 │ │ │
Augitites │ │ │ │
│ │ │ │
Leucite rocks │5–10 │ 3–5 │ 5–7 │ 0.5–2
│ │ │ │
Nepheline rocks │4–10 │ 4–8 │ 1–3 │ 0.2
──────────────────────┴─────┴────────┴───────┴───────────
=37. Origin of Soils.=—The soils in which crops grow and which form the
subject of the analytical processes to be hereinafter described have
been formed under the combined influences of rock decay and plant and
organic growth. The mineral matters of soils have had their origin in
the decay of rocks, while the humic and other organic constituents have
been derived from living bodies. It is not the object of this treatise
to discuss in detail the processes of soil formation, but only to give
such general outlines as may bear particularly on the proper conceptions
of the principles of soil investigation.
=38. The Decay of Rocks.=—The origin and composition of rocks are fully
set forth in works on geology and mineralogy. Only a brief summary of
those points of interest to agriculture has been given in the preceding
pages. The soil analyst should be acquainted with these principles, but
for practical purposes he has only to understand the chief factors
active in securing the decay of rocks and in preparing the débris for
plant growth.
The following outline is based on the generally accepted theories
respecting the formation of soils.[7]
The forces ordinarily concerned in the decay of rocks are:
1. Changes of temperature, including the ordinary daily and
monthly changes, and the conditions of freezing and thawing.
2. Moving water or ice.
3. Chemical action of water and air.
4. Influence of vegetable and animal life:
(a) Shades the rock or soil surface.
(b) Penetrates the rock or soil material with its roots, thus
admitting air.
(c) Solvent action of roots.
(d) Chemical action of decaying organic matter.
5. Earth worms.
6. Bacteria.
=39. The Action of Freezing and Thawing.=—In those parts of a rock
stratum exposed near the surface of the earth the processes of freezing
and thawing have perhaps had considerable influence in rock decay. The
expansive force of freezing water is well known. Ice occupies a larger
volume than the water from which it was formed. The force with which
this expansion takes place is almost irresistible. The phenomenon of
bursted water pipes which have been exposed to a freezing temperature is
not an uncommon one. While the increase in volume is not large, yet it
is entirely sufficient to produce serious results. The way in which
freezing affects exposed rock is easily understood. The effect is
unnoticeable if the rock be dry. If, on the other hand, it be saturated
with water, the disintegrating effect of a freeze must be of
considerable magnitude. This effect becomes more pronounced if the
intervals of freezing and thawing be of short duration. The whole
affected portion of the rock may thus become thoroughly decayed. But
even in the most favorable conditions this form of disintegration must
be confined to a thin superficial area. Even in very cold climates the
frost only penetrates a few feet below the surface, and therefore the
action of ice cannot in any way be connected with those changes at great
depths, to which attention has already been called. Nevertheless,
certain building stones seem very sensitive to this sort of weathering,
and the crumbling of the stone in the Houses of Parliament is due
chiefly to this cause.
On the whole it appears that the action of ice in producing rock decay
has been somewhat overrated, although its power must not by any means be
denied. But on the other hand a freeze extending over a long time tends
to preserve the rocks, and it therefore appears that the entire absence
of frost would promote the process of rock decay.
At best it must be admitted that frost has affected the earth’s crust
only to an insignificant depth, but its influence in modifying the
arable part of the soil is of the utmost importance to agriculture.
=40. The Action of Glaciers.=—The action of ice in soil formation is not
confined alone to the processes just described. At a period not very
remote geologically, a great part of our Northern States was covered
with a vast field of moving ice. These seas of ice crept down upon us
from more northern latitudes and swept before them every vestige of
animal and vegetable life. In their movement they leveled and destroyed
the crests of hills and filled the valleys with the débris. They crushed
and comminuted the strata of rocks which opposed their flow and carried
huge boulders of granite hundreds of miles from their homes. On melting
they left vast moraines of rocks and pebbles which will mark for all
time the termini of these empires of ice. When the ice of these vast
glaciers finally melted the surface which they had leveled presented the
appearance of an extended plain. No estimate can be made of the enormous
quantities of rock material which were ground to finest powder by these
glaciers. This rock powder forms to-day no inconsiderable part of those
fertile soils which are composed of glacial drift. The rich materials of
these soils probably bear a more intimate relation to the rocks from
which they were formed than of any other kinds of soil in the world. The
rocks were literally ground into a fine powder, and this powder was
intimately mixed with the soils which had already been formed _in situ_.
The melting ice also left exposed to disintegrating forces large
surfaces of unprotected rocks in which decay would go on much more
rapidly than when covered with the débris which protected them before
the advent of the ice. The area of glacial action extended over nearly
all of New England and over the whole area of the northern tier of
States. It extended southward almost to the Ohio river, and in some
places crossed it. The effect of the ice age in producing and modifying
our soil must never be forgotten in a study of soil genesis. It is not a
part of our purpose here to study the causes which produced the age of
ice. Even a brief reference to some of the more probable ones might be
entirely out of place. Before the glacial period it is certain that a
tropical climate extended almost, if not quite, to the North Pole. The
fossil remains of tropical plants and animals which have been found in
high northern latitudes are abundant proofs of this fact.
In the opinion of Sterry Hunt,[27] rock decay has taken place largely in
preglacial and pretertiary times. The decay of crystalline rocks is a
process of great antiquity. It is also a universal phenomenon. The fact
that the rocks of the southern part of this country seem to be covered
with a deeper débris than those further north is probably due to the
mechanical translation of the eroded particles towards the south. The
decay and softening of the material were processes necessarily preceding
the erosion by aqueous and glacial action.
It is possible that a climate may have existed in the earlier geologic
ages more favorable to the decay of rocks than that of the present time.
=41. Progress of Decay as Affected by Latitude.=—Extensive
investigations carried on along the Atlantic side of the country show
wide differences in the rate of decay in the same kind of rocks in
different latitudes. In general, the progress of decay is more marked
toward the south. The same fact is observed in the great interior
valleys of the country; at least, everywhere except in the arid and
semi-arid regions. Wherever there is a deficiency of water the processes
of decay have been arrested. Where the rock strata have been displaced
from a horizontal position the progress of decay has been more rapid.
This is easily understood. The percolation of water is more easy as the
displacement approaches a vertical position.
A most remarkable example of this is seen in the rocks of North
Carolina.[28] A kind of rock known as trap is found in layers called
dikes in the Newark system of rocks in that State. These dikes have been
so completely displaced from the horizontal position they at first
occupied as to have an almost vertical dip. The edges thus exposed vary
from a few feet to nearly 100 feet in thickness. The trap rock in those
localities is composed almost exclusively of the mineral dolerite, which
is so hard and elastic in a fresh state as to ring like a piece of metal
when struck with a hammer. In building a railroad through this region
these dikes were in some places uncovered to a depth of forty feet and
more. At this depth they were found completely decomposed and with no
indications of having reached the lower limit of disintegration. The
original hard bluish dolerite has been transformed into a yellowish
clay-like mass that can be molded in the fingers and cut like putty.
Similar geologic formations in New Jersey and further North do not
exhibit anything like so great a degree of decomposition, thus
illustrating in a marked degree the fact that freezing weather for a
part of the year is a protection against rock decay. The ice of winter
at least protects the rocks from surface infiltration, although it can
not stop the subterranean solution which must go on continuously.
Other things being equal, therefore, it appears that as the region of
winter frost is passed the decay of the rocks has been more rapid than
in the North, because the chief disintegrating forces act more
constantly.
=42. The Solvent Action of Water.=—The water of springs and wells is not
pure. It contains in solution mineral matters and often a trace of
organic matter. The organic matter comes from contact with vegetable
matter and other organic materials near the surface of the earth. The
mineral matter is derived from the solvent action of the water and its
contents on the soil and rocks.
The expressions “hard” and “soft” applied to water indicate that it has
much or little mineral matter in suspension, as the case may be. When
surface and spring waters are collected into streams and rivers they
still contain in solution the greater part of the mineral matters which
they at first carried.
When well or spring waters have more than forty grains of mineral matter
per gallon they are not suitable for drinking waters. Mineral waters, so
called, are those which carry large quantities of mineral matter, or
which contain certain comparatively rare mineral substances which are
valued for their medicinal effects.
The analysis of spring, well, or river waters will always give some
indication of the character of the rocks over which they have
passed.[29] The vast quantities of mineral matters carried into oceans
and seas are gradually deposited as the water is evaporated. If,
however, these matters be very soluble, such as common salt, sulfate of
magnesia, etc., they become concentrated, as is seen with common salt in
sea waters. In small bodies of waters, such as inland seas, which have
no outlet, this concentration may proceed to a much greater extent than
in the ocean. As an instance of this, it may be noted that the waters of
the Dead Sea and Great Salt Lake are impregnated to a far greater degree
with soluble salts than the water of the ocean. The solvent action of
water on rocks is greatly increased by the traces of organic or carbonic
acids which it may contain. When surface water comes in contact with
vegetable matter it may become partially charged with the organic acids
which the growing vegetable may contain or decaying vegetable matter
produce. Such acids coming in contact with limestone under pressure will
set free carbon dioxid. Water charged with carbon dioxid acts vigorously
on limestone and other mineral aggregates. If such solutions penetrate
deeply below the surface of the earth their activity as solvents may be
greatly increased by the higher temperature to which they are subjected.
Hence, all these forces combine to disintegrate the rocks, and through
such agencies vast deposits of original and secondary rocks have been
completely decomposed.
The gradual passing of the firm rock into an arable soil is beautifully
shown in Fig. 10, a print from a negative taken by Mr. Geo. P. Merrill,
near Washington, D. C.
[Illustration:
FIGURE 10.
View on the Broad Branch of Rock Creek, Washington, D. C.
The fresh but badly decomposed granitic rock is shown passing upward
into material more and more decomposed until it becomes sufficiently
pulverulent and soluble to support plant life. The roots showing in
the upper part of the picture formerly penetrated the decomposed
rock, but have been exposed through quarrying operations. Photograph
by George P. Merrill, 1891.
]