or the shallowness of the soil, seems to be the principal cause
why the _Beech_ grows better on many calcareous mountains than the
_Oak_, which, on the other hand, finds a fitter station on mountains
in which sandstone predominates, where the soil is usually deeper.
It would seem to be for a similar reason that the _Beech_ grows in
many rocky districts, for example, on the Hartz Mountains, at pretty
considerable heights, especially on the sides of valleys which run to
the south, while these places do not admit the _Oak_, which is found
in the middle provinces of Sweden and Norway; while the Beech, on the
other hand, grows only in the southern parts. From the deficiency of
soil, the Upper Hartz can produce neither the _Pinus pinea_, nor _P.
sylvestris_; the horizontal roots, however, of the _Abies_, or Norway
Spruce, are content with the small portion of earth which covers the
greywacke and slate, although they cannot find sufficient hold to
protect its lofty trunks from being thrown down by the tempest. In
some parts of the Forest of Thuringia, where the covering of loose
earth is deeper than in the Hartz, the _Pinus picea_, or pitch pine,
grows luxuriantly. The common fir, _Pinus sylvestris_, which attains
a great height in proper soil, on the contrary, is stunted and
distorted on rocky mountains, where the roots soon come in contact
with the rock. It there loses the character of a tree, and assumes
that of a shrub, as in place of a single upright stem, several
branches shoot out, and these, not unfrequently, are creeping or bent.
The different conditions of rocks, especially their structure
and their state of cohesion, are of some importance in producing
these effects; for the surface of rocks must be detrimental or
impervious to the roots of plants, in proportion to the compactness
of their structure, and the cohesion of their parts. Schistose
rocks, for example, afford a more easy passage to roots, than
granular crystalline ones; pure quartz resists the roots of plants
in the highest degree; sandstone much less; and pure limestone, on
account of its comparatively small number of fissures, is much less
favourable to vegetation than marl, chalk, or slightly cohering
calcareous rocks, the masses of which are usually split in all
directions.
The direction and inclination of the strata have also some influence
in this matter; for, in proportion as the principal fissures of
the strata are, from their direction or inclination, more readily
presented to the roots of vegetables, the less prejudicial will
their surface be to vegetation. Horizontal strata, therefore, are
the least favourable to vegetation, perpendicular ones the most. In
the inclination of strata intermediate in some degree between these
positions, the roots of vegetables will find a greater obstacle on
the side of a hill in which the surface of a stratum is opposed to
them, than on the other, in which the principal fissures of the
strata are open. The effects of this circumstance may frequently be
observed in mountainous tracts having two principal inclinations, the
state of vegetation, and especially the growth of wood, being more
prosperous on the one of these declivities than on the other.
The surface of the solid strata of the earth may also have an
_indirect influence_ upon the cultivation of vegetables. The various
_inclinations of this surface_ deserve first to be considered, being
of the greatest effect with regard to fixing the fertile soil. The
horizontal position of a rocky surface is in the highest degree
favourable to the stability of vegetable earth; and the greater its
angle of inclination, the greater is the danger of its losing the
soil upon it. In a highly inclined plane, the imperfect support
of the centre of gravity is the sole cause of the loss of earth;
in a less inclined plane the diminution of soil is usually caused
by water, which produces this effect in a greater or less degree,
according to the difference of inclination. In both these modes, by
which a removal of soil is produced, the effect may be modified by
a difference in the condition of the loose earth, as not only its
stability as to situation, but also its resistance to the power of
water, vary according to the size, figure, and cohesion of the parts,
as well as their adhesion to the surface of the rock. Sandy loose
soils, for example, are more liable to transposition than marly or
loamy ones; and these, again, are more easily moved than such as are
clayey and adhesive.
Whatever be the nature of the soil, a small degree of inclination
in the solid rock is sufficient to favour its denudation by the
removal of the former; and the inclinations of the surfaces of rocks
having a covering of earth and vegetation, are in reality much less
considerable than we usually suppose them to be, judging merely by
the eye. The celebrated Humboldt has published observations on this
subject. According to his measurements, a slope of even fifteen
degrees appears steep, and a declivity of thirty-seven degrees is so
abrupt, that if it be covered with a dense sward, it can scarcely be
climbed. The inclination of the pastures of the Alps seldom exceeds
an angle of ten or fifteen degrees, and a slope of twenty degrees
is pretty steep. At an inclination of forty degrees, the surface
of the rock is sometimes covered with earth bearing a sward, but
at a greater inclination the rocks are usually destitute of soil
and vegetation. In the Upper Hartz, the most common inclination of
the declivities of the mountains is twenty-five degrees; nor does
it usually exceed thirty-three, at which inclination the _beech_
and _spruce_ grow. The greatest declivities at which ground can be
advantageously cultivated have an inclination of thirty degrees.
The roots of vegetables, especially of grasses, shrubs, and trees,
are of much importance in supporting the earth upon the declivities
of rocks. Care must therefore be taken that the declivities of
mountains which are covered with turf or wood, be not altogether
deprived of these coverings, as sometimes happens in consequence of
loosening the turf for agricultural purposes, or of incautiously
extirpating the wood. In Norway, near Roraas, there occur mountains,
destitute of all vegetation, that had formerly been covered with
woods, but where now, from the deficiency of soil, no seeds could
take root. The same is the case in many parts of the Alps, where,
from the irregular long-continued removal of the timber, the sides
of mountains which were formerly covered with thick woods, now show
nothing but naked rocks. For this reason, in mountainous countries
with very steep declivities, the breeding of cattle and planting of
woods are often more advantageous than agriculture. In France the
greatest inclination of the public roads is limited by law to an
angle of four degrees and forty-six minutes: a similar restriction
with regard to agriculture might not be without benefit in certain
mountainous countries.
The inclinations of the surface of the solid crust of the earth vary
much, according to the different qualities of the rocks; some having
a tendency to form abrupt precipices, others, again, to produce
gentle declivities. For this reason, mountains consisting of quartz
or porphyry, for example, very frequently present surfaces destitute
of vegetation; while, on the other hand, those of granite, slate or
sandstone, are more frequently adapted for agriculture and planting.
In the northern parts of Scotland, quartz rocks, destitute of all
vegetation, rise in the midst of mountains covered with gramineous
plants, and sometimes wood. In the most fertile part of the south of
Norway porphyritic mountains rise from a calcareous and schistose
base, with lofty, rugged, and bare cliffs. In the southern parts of
the Tyrol the rocky sterility of the abrupt and lofty porphyritic
mountains presents a striking contrast to the fertility of the
neighbouring limestone mountains, which are covered with vines,
walnuts and chesnuts.
The _surface_ of the solid strata of the earth has also an indirect
influence upon the cultivation of plants, in so far as the water
which the vegetable mould acquires from the atmosphere, is retained
in the soil, or is drawn off by the subjacent rock. Different rocks
produce very different effects in this respect, depending as well
upon their constitution as their structure. The component parts of
rocks imbibe water in different modes and degrees; and different
sorts of rocks not only attract water with different celerity, but
also imbibe different quantities of it. The latter difference depends
chiefly upon the various substances of which rocks are composed,
partly, also, upon their porosity. Siliceous rocks attract water in
the lowest degree, argillaceous ones in the highest, and calcareous
rocks appear to have an intermediate action in this respect.
Compact and granular crystalline rocks attract water in a smaller
degree, and more slowly; friable or crumbled rocks imbibe it in
greater quantity, and with more celerity than those which are not
disintegrated. The condition of rocks with regard to the attraction
of water, affects, in a different manner, the humidity of soil;
for, by this attraction, moisture may as well be abstracted from,
as imparted to, the loose earth or soil by which rocks are covered.
Part of the moisture which vegetable earth or soil derives from the
atmosphere passes into the subjacent mass of rock, but this may again
be compensated by evaporation; on which account the soil of such
rocks as have but a small attraction for water usually dries up more
readily than soils whose solid substratum attracts and retains the
moisture in a greater degree.
It is probable that the structure of rocks has also a greater, and
not less, diversified influence upon the humidity of productive
soil. Solid rocks, which are not traversed by numerous perpendicular
fissures penetrating to a considerable depth, allow the water
to remain in the soil; but columnar and schistose rocks, with
perpendicular fissures, and strata declined from the horizontal
position, draw off the water from the soil covering their surface,
into lower places, where it often re-appears under the form of
springs. In these circumstances, we find a partial explanation of the
great difference between the humidity of soil covering a surface of
solid granite, and that lying upon limestone, which is intersected
by numerous fissures. Granitic mountains are often furnished with
marshes, whereas, on the other hand, the dryness of the soil upon
calcareous mountains is generally excessive[411], the cause of
which phenomenon is, in a great measure, to be attributed to the
circumstances above mentioned. Columella observes, that silex having
a moderate covering of earth, preserves to the latter its humidity;
and Palladius repeats the remark. In districts which consist of
quartzose rocks, not less than of granitic ones, the surface is often
covered with marshes. Porphyritic rocks, on the contrary, which have
a remarkable segregation of parts, as well as columnar basalt, let
off the water to lower places. Springs are very frequently found
at the bottom of basaltic mountains; for the atmospheric waters
penetrate by the perpendicular fissures to the strata on which the
basalt rests, and appear at the place where the two rocks meet.
The effect of different rocks upon the preservation and diminution
of the moisture of fertile soil, influences vegetation in various
degrees. The retentive power of the surface of rocks is of the
greatest importance, where the soil consists chiefly of sand, through
which the water percolates, and passes off entirely, unless it meets
with a stratum of such a nature, as to obstruct its passage, or comes
upon a surface of solid rock. The cause of the sterility of sandy
plains is not merely their sandy nature, but also the great depth
of the mass or rock capable of retaining the water. The same sand,
when covering mountains consisting of sandstone, has a much less
degree of sterility than in those plains, because the surface of the
subjacent rock impedes the progress of the water, and consequently
retains it in the soil[412]. It has been sufficiently proved by
experiments, that plants can grow in pure sand, when furnished
with the necessary quantity of water. A subjacent rocky surface
has an entirely different effect upon soil which is very retentive
of moisture, upon a clay soil for example, as, in that case, the
humidity is increased to a prejudicial degree. In land of this
nature, a substratum of rock having the property of drawing off the
water would be useful.
The different conditions of rocks with regard to caloric, may have
some indirect influence upon the vigour of plants. Heat, whether
imparted to the vegetable soil by the sun’s rays, or generated by
various chemical processes in the earth itself, penetrates to the
surface of the subjacent rocks, and is more or less drawn from it in
a longer or shorter time. Columella observes, that rocks in the upper
part of the soil are prejudicial to vines and trees, but in the lower
part cool them. The heat of soil will be more or less drawn from it,
according to the greater or less conducting power of the subjacent
rock. Compact crystalline rocks are probably better conductors of
caloric than those which are of looser texture; siliceous rocks than
argillaceous and calcareous ones. The influence of the subjacent rock
must be greater in this respect, in proportion to the thinness of the
superincumbent soil. The effect of the abduction of caloric is more
particularly sensible, where the roots of cultivated plants touch
the rock, a circumstance which we often see in vineyards. The vine
frequently thrives remarkably on the declivities of mountains, in
which it sends its roots among fragments of stones. Experience shows,
that the quality of wine is influenced by the different conditions
of the stones, among which vines are planted. Albertus Magnus has
observed, that the vine thrives well in earth which is mixed with
fragments of black roofing slate; and Humboldt remarks, that the
vines which grow upon the mountains of the valley of the Rhine,
consisting of black clay-slate, afford an excellent wine. At the Cape
of Good Hope, also, the vine thrives well in a soil produced by the
decomposition of clay-slate, and mixed with fragments of it[413].
It is probable, that the adaptation of this sort of soil to the
cultivation of the vine, depends upon its slow conducting power, and
upon its rapidly imbibing the rays of the sun, on account of its dark
colour, and thus increasing the heat of the ground.
Hitherto we have only spoken of the _proximate_ influence of rocks
upon plants; but it cannot be denied, that the _remote_ effects which
they produce, (inasmuch as vegetable soil is derived from them, and,
therefore, the qualities of this soil depend in a great measure upon
their nature,) are of greater importance.
It is from the rocks which constitute the crust of the earth, that
the principal portion of productive soil is derived. Although other
substances belonging to the animal and vegetable kingdoms, are
necessary for the nourishment of plants, a soil consisting chiefly
of inorganic particles is still more necessary, both for sustaining
their roots, and for receiving, retaining, and partly also preparing
nutrition for them; for, according to accurate observations, some
inorganic substances exert an influence upon the decomposition of
animal and vegetable remains. These effects vary much according to
differences in the aggregation and chemical nature of the inorganic
parts; of which circumstances, however, the different qualities of
rocks are the ultimate cause.
Two kinds of productive soil may be distinguished with regard to
their origin. The soil has either originated in the place in which
it now is from the subjacent rock, or it has been transported to the
places in which it is now found by some power, especially by that
of water. The first kind may be named _untransported_, the second
_transported soil_. To the first kind of soil is to be referred a
great part of the soil which covers the summits and declivities of
mountains, and to the other, the soil which fills the bottoms of
valleys, as well as a great part of the loose soil of extensive
strata in hilly countries and plains. Untransported soil is generally
thinner than the transported; and of the two the latter is that which
most frequently occurs in low land. The first kind of soil, the
untransported, is found to be more or less similar, in its principal
constituent parts, to the rocks from which it has originated; in the
other kind, the transported soil, on the contrary, the parts which
were originally in connection, have been variously separated and
mixed, by the agency of the powers by which its transportation was
effected.
The quantity and quality of the soil derived from the disintegration
of rocks, must depend upon the nature of these rocks; its quality
being determined by the constituent parts of the rock from which it
originated, and its quantity being proportioned to the greater or
less degree in which the rock may resist decomposition.
The disintegration of rocks, and their conversion into loose earth,
are partly _mechanical_, and partly _chemical_. The principal
mechanical powers, by which disintegration is effected, are, _1st_,
The weight of the loosened parts; _2d_, Water, not merely in its
liquid and mobile state, but also, and that chiefly, in the state
of ice; _3d_, The roots of vegetables in general, and especially of
trees. These powers usually act more or less in conjunction, and the
effects produced by this union are in many cases almost incredible.
The disintegration of rocks commences in those parts where the
power of cohesion is least energetic. Rents take place owing to the
unequal attraction of parts, and also in the direction of planes,
in which heterogeneous parts are in contact; and in this manner
the original structure of rocks determines the first steps of
their disintegration. Water, which enters into the minute fissures
of rocks, by the power of capillary attraction, is expanded by
congelation, and thus overcomes the cohesion of parts, and produces
rents. The roots of trees acting as wedges, produce the same
effect in a wonderful degree, a phenomenon which has been so well
illustrated by Annæus Seneca, in his Natural Questions. “Let us
consider,” says he, “how great a power is exerted by the most minute
seeds, which, although at first small as they are, can scarcely
find a place in the crevices of rocks, yet at length grow to such
a size as to rend asunder vast rocks, overturning crags and cliffs,
by the power of their very minute and delicate roots.” The parts
of rocks loosened by these powers, are entirely separated, and are
carried to a great or less distance, by streams of water, and in
the higher regions, by the power of winds. In cliffs and precipices
which have been formed by the splitting of masses of rock, effected
in the manner above described, the loosened parts often lose their
stability; and, following the direction of gravity, fall to the
ground, an effect which has also been described by Seneca in another
place. “Nor is it alone probable,” says he, “that rocks are split
asunder by their mere weight, but also when streams of water are
carried over them, the continual moisture works into the joinings of
the rock, and daily takes away a portion of the connecting matter,
and, if I may so speak, abrades the skin by which it is contained.
At length, in the course of ages, this gradual detrition so much
diminishes the supporting parts, that they can no longer sustain the
weight. Then masses of vast size fall down, and the rock tumbling
from its ancient seat, overwhelms whatever lies below.” The cohesion
of some rocks, especially argillaceous ones, is so slight, and their
porosity so great, that their smallest parts imbibe water, and are
sensibly softened by it, an effect which is much assisted by the
freezing of the water. This mechanical change is experienced by the
different varieties of common clay, slate-clay, and some other rocks.
_Chemical_ powers often act in conjunction with mechanical ones, in
breaking down rocks, the former, the chemical, frequently finishing
what had been begun by the latter. Mechanical powers only changing
the _aggregation_ of rocks, may break down their parts to a certain
size, according to their different nature; chemical powers, again,
which change the _nature_ of substances, destroy the connection of
the minute parts of rocks. When chemical is preceded by mechanical
action, it is much assisted by it. The latter has a much more general
effect, as all rocks are subjected to its influence; chemical
decomposition, on the other hand, acts only upon some rocks, and in
these only upon certain parts. The chemical decomposition of rocks is
chiefly effected by the oxygen of atmospheric air and of water; but
we are also persuaded, that certain cryptogamic plants, intimately
attached to the surface of stones, Lichens namely, assist in their
destruction.
The oxygen of air and water can only affect the constituent parts
of rocks, which have a great affinity to it, such as the iron and
sulphur forming pyrites, oxydulous iron, oxydulous manganese, or
the same substances mixed with earth or carbonic acid, charcoal and
bitumen. Very solid and compact masses of rock, such as greenstone,
which are not easily affected by other means, are sometimes corroded
by the chemical change of the pyrites contained in them, by which
it is converted into a hydrate of iron[414]. In certain other
rocks, which are also readily broken down by mechanical agents,
clay-slate for instance, the disintegration is much accelerated by
the decomposition of the pyrites. The oxydulous iron of felspar is
commonly converted by decomposition into a hydrate or ochre. The
carbonate of iron, as well as of manganese, which sometimes occur
in rocks, in limestone rock for example, are deprived of carbonic
acid by the oxidation of their bases. Charcoal and bitumen, which
are sometimes contained in rocks, limestone and argillaceous ones
for example, are dissipated by the contact of air, so that rocks
which were originally of a dark colour, lose it, and become whitish.
Water, as a chemical agent, contributes so much to the decomposition
of certain rocks, that, either in a pure state, or in combination
with carbonic acid, it dissolves their parts, of which gypsum and
limestone afford examples. In certain other minerals, in felspar
for instance, a separation of the constituent parts, produced by
the contact of air and water, is observed, the proximate cause of
which has not hitherto been discovered. The mass is decomposed, its
lamellar structure is converted into an earthy nature, the alkali
contained in the felspar is extracted by the water, a mineral is
produced, to which the Chinese have given the name of _Kaolin_, and
which is adapted for the manufacture of porcelain. Granite and gneiss
occur in some districts, the felspar of which is decomposed in this
manner through the whole mass,--a circumstance which must be of great
importance in regard to the formation of productive soil.
Cryptogamic plants covering the surface of rocks, and thriving well
in this situation, where more perfect vegetables could not grow,
seem also destined to promote the chemical decomposition of rocks,
an effect which they produce both directly and indirectly. As they
imbibe the water of the atmosphere, and retain it like a sponge,
they keep up a constant application of this substance to the rock,
and in this manner contribute indirectly to its decomposition. There
are some cryptogamic plants also, which consume certain portions of
the rocks with which they are in contact, corrode their surface, and
destroy the cohesion of its parts, effects which may chiefly be seen
in certain cryptogamic plants attached to calcareous rocks. In this
manner one sort of vegetation prepares a place for another, and the
most imperfect vegetables are subservient to the growth of the more
perfect.
After premising thus much, we shall now proceed to the examination
of the principal rocks, in so far as regards their connection with
the formation of productive soil, beginning with those which resist
decomposition in the highest degree, and ending with those which are
the most conducive to the formation of loose earth and soil.