chemical decomposition, in so far as regards their principal mass,
and whose cohesion of parts is so great that mechanical powers can
only open their natural fissures to a greater extent, and thus break
them down into fragments. Of this kind are _vitreous lava_, _pure
quartz_, _compact quartz_, _flinty slate_, and _porphyry with a
siliceous basis_. On mountains consisting of these rocks, scarcely
any productive soil is found, and frequently none at all. They are
usually characterized by sterile rocks and cliffs, the bases of which
are covered with innumerable rough fragments of stones, retaining
their sharp edges for a great length of time, the heaps of which
seldom produce any thing else than mosses, which frequently cover the
interstices of fragments, occasionally a few grasses, and sometimes
a solitary shrub or tree. Examples, Bennevis, Paps of Jura, and
Morven Hills. Of all rocks, vitreous volcanic productions are the
least capable of contributing to the formation of productive soil.
Their dark coloured tracts descend from volcanic mountains to the
valleys in frightful sterility, the chinks of their rugged masses
scarcely affording sufficient water for the roots of mosses[415].
To the second class we refer _compact limestone_, a rock which
contributes extensively to the formation of the solid crust of
the globe. In so far as regards its principal constituent parts,
it is not affected by atmospheric water or air; but, as its parts
have but comparatively little cohesion, and are usually separated
in a considerable degree by minute fissures, they are more liable
to be broken down and crumbled by mechanical powers, than those
of the rocks belonging to the first class. In districts where the
fundamental rock is limestone, the layers of loose original soil
or subsoil are thin, and filled with numerous fragments. As the
soil arising from the disintegration of limestone contains a great
proportion of calcareous matter, it is neither favourable to the
growth of plants in general, nor to that of the greater number of
vegetables which are the object of cultivation. Soil of this kind
is too hot, dry and stony; hence the reason why districts, in which
pure limestone rocks predominate, are often sterile. The case is
different, however, where a portion of clay enters as an ingredient
into the composition of calcareous rocks, for here the soil is
usually very productive; or, where rocks of a different nature
alternate with masses of pure limestone, having a greater capability
than it of contributing to the formation of productive soil. When
water, containing carbonic acid, passes through limestone rocks, it
dissolves portions of it, and deposits them in other places, by which
the decomposition of the limestone and the formation of loose earth
may be in some measure accelerated.
To the third class belong _chalk_ and _gypsum_; which, in so far
as regards their decomposition by chemical means, are of a similar
nature with compact limestone; but possessing a much slighter
cohesion of parts, are more liable to be broken down by mechanical
means. Water also dissolves gypsum, and thus assists in its
disintegration. The soil arising from these rocks resembles that
produced by compact limestone, which explains the want of fertility,
observable in certain gypseous tracts of the North of Germany, and in
the chalk districts of France. The fertility which we see in certain
places where chalk is the fundamental rock, as in the Isle of Wight,
Island of Rugen, &c. is to be attributed as well to argillaceous and
marly strata alternating with the chalk, as to the greater humidity
of the atmosphere, by which the dryness and heat of the soil are
diminished.
In the fourth class we place certain rocks, composed of different
minerals, but compact in appearance, which, although they resist
mechanical disintegration, are yet subject to chemical action, and
are, by means of it, converted into a loose, compound productive
soil. Of this kind are _basalt_, and some other rocks very nearly
allied to it.
To the fifth class we refer those rocks which have a crystalline,
granular, or slaty texture. The mutual adhesion of the heterogeneous
parts, of which they consist, being, in general, inconsiderable,
they are easily broken down by mechanical means, and thus contribute
in a high degree to the formation of productive soil. The felspar
contained in these rocks, on account of the chemical decomposition
which it readily undergoes, has a great effect not only upon the
quantity, but also the fertility of the soil produced. The quartz,
on the contrary, as well as the mica and hornblende, long resist
chemical decomposition; they are, however, useful in this respect,
that the argillaceous soil arising from the felspar, has its
tenacity diminished; and is consequently rendered better adapted for
vegetation, by being intermixed with them. _Granite_ and _gneiss_,
of all truly granular crystalline rocks, afford the deepest and
most fertile soil, aptly compounded of different substances,
sufficiently loose in its aggregation, and capable of retaining the
necessary moisture. Soil arising from the disintegration of granite
is unfavourable to vegetation only, where the rock abounds much in
quartz, and where the superfluous water cannot run off, and so gives
rise to marshes, which produce only vegetables of inferior quality;
of which we have examples in the granite districts of Aberdeen. In
such places as these, peat is easily generated, which, although of
great use, is yet much less advantageous than wood. _Syenite_, which
abounds much in hornblende, is inferior to granite, with respect to
the production of fertile soil; and primitive _greenstone_, which
resists disintegration and decomposition in the highest degree,
occupies the last place in this class. In the series of slaty
crystalline rocks, _mica-slate_ is next to gneiss: but on account of
the small proportion of felspar which enters into its composition,
it does not afford so productive a soil.
In the sixth class may be placed the slaty rocks, whether simple,
or intimately compounded, which do not readily undergo chemical
decomposition, but which easily separate at their natural fissures,
and are mechanically resolved into an earthy mass, forming a paste
with water, circumstances which are observed chiefly in _clay-slate_,
a rock of much importance in the formation of productive soil,
usually passing into a clayey sort of earth.
To the seventh class belong the conglomerated rocks, whose parts
indeed undergo very little, if any, chemical change, but are easily
separated by mechanical means, and are thus converted into a
gravelly, sandy, or earthy mass. Of this kind are _greywacke_, _old
red sandstone_, and sandstones of various kinds. Much diversity is
exhibited by these rocks, with regard to the facility with which they
undergo disintegration, as well as the nature of the soil arising
from them; circumstances which chiefly depend upon the nature of the
cement, and its relation to the parts cemented. The disintegration of
these rocks is the more easily effected that the cement is abundant,
and less intimately connected with the other parts, that is, the more
they depart from a crystalline nature; on which account greywacke
is less easily converted into soil, than the common varieties of
sandstone. By the decomposition of greywacke, a loose and fertile
soil is formed, containing particles of quartz and clay in due
proportion; on the other hand, by the decomposition of red sandstone,
a soil is frequently produced, abounding in argillaceous particles
impregnated with iron, and therefore stiff and cold. The _variegated
sandstone_, with a marly cement, not unfrequently affords a pretty
fertile soil; the _quadersandstein_, on the contrary, commonly
presents a sandy and arid soil.
Lastly, in the eighth class we shall place those rocks, whether
simple or intimately compounded, whose nature is so loose, or whose
parts are so separated, that they fall with great facility into an
earthy mass, and are also in part mechanically reduced by water. To
this class belong the different varieties of _marl_, _slate-clay_,
_basaltic_ and _volcanic tuffa_. These rocks, many of which are
extensively diffused, are of much importance in the formation of
productive soil, although the quality of the earth produced by them
varies much, according to their different natures. Slate-clay affords
an argillaceous soil; in earth produced by the decomposition of marl,
the clay is diminished in proportion to the greater abundance of the
calcareous or sandy parts; while a mixed and very fertile soil is
usually generated from basaltic and volcanic tufas.
The various relations which exist in the stratification and position
of rocks, have much influence in producing a diversity in the
soil formed immediately from their decomposition. This diversity
cannot be so great when different rocks of various ages occur in a
determinate order in horizontal strata; in which case, the uppermost
bed may exhibit a great extent of surface of the same nature. When,
on the other hand, strata of rocks of different natures, forms,
and dimensions, placed at different angles of inclination, and
in different directions, appear at the surface, it will easily
be understood how it may happen that the soil produced by their
decomposition may occur of very different qualities, in places
not very distant from each other. The manner in which the soil is
influenced by a difference in the arrangement and position of the
strata, will become evident, on comparing districts in which one
particular sort of rock lies beneath the surface in horizontal
strata, with others in which the solid substratum is composed of
various rocks differing in their inclination towards the horizon.
In districts of the former kind, the qualities of the soil vary
in general but little; in such as are of the latter kind, on the
contrary, they are often found extremely different. The great
diversity of soil seen in England, as well as in Germany, may,
in fact, be partly explained by the circumstance, that, in those
countries, the nature and position of the strata vary every where.
On the other hand, the great similarity which pervades the soil of
Southern Russia, is without doubt produced by a uniformity in the
position and inclination of the limestone which lies immediately
under the soil.
The nature of the principal mass of the strata usually exerts a
great degree of influence over the qualities of the soil. When the
solid substratum is sandstone, its effect upon the soil is, in
general, as evidently seen, though not perhaps in an equal degree,
as when it is marl. Exceptions, however, to this rule sometimes
occur; as, for instance, when the principal mass of a rock which
resists disintegration in a high degree contains beds that are
easily reduced to earth. This is the case with the shell-limestone
(muschelkalkstein) of Germany, the mountains of which are not
unfrequently covered with a clayey soil, which has not been produced
by the decomposition of the principal strata themselves, but by that
of the slate-clay and argillaceous marl alternating with them.
Hitherto we have considered untransported soil, or that produced
from the disintegration or decomposition of the subjacent rocks in
the places where it occurs; we have now to examine the relations
which exist between the subjacent rock, and the _transported soil_
lying upon it. The nature of the rock does not indeed influence,
excepting in a more remote degree, the transported soil, which has
been carried to a greater or less distance from the places of its
production, by the agency of moving powers, and again deposited of
various forms and compositions. However, it may often be plainly
seen, that the materials of this soil have been derived from
particular rocks, and that these rocks have exerted some degree of
influence over the formation and distribution of the transported
soil. The examination of these relations is of great importance,
because it is with secondary or transported soil that agriculture
is principally concerned. The varieties of transported soil depend
chiefly upon three circumstances: _1st_, The nature of the rocks from
which they are derived; _2dly_, The quality and effect of the moving
powers; _3dly_, The changes which they may have undergone after their
formation.
The origin of the materials which enter into the composition of
transported soil, has been already considered. From their difference
may be easily explained why soil generated from the debris of
primitive crystalline rocks has different qualities from soil which
has been derived from strata of sandstone or marl.
The principal powers which contribute to the transportation of soil,
are, The weight of loose masses, ice, and water. The weight of loose
masses is a cause of transportation which we frequently see in
operation. By it the huge cones of debris at the base and upon the
declivities of precipices and mountains, are gradually carried off
toward the bottom of the valleys; a phenomenon which can scarcely any
where be better seen than in the valleys of the Alps, where mountains
sometimes occur evidently consisting of debris, and clothed with
trees and shrubs, or covered with pastures, the masses of which are
gradually moved, as upon inclined planes, by the action of the water
which percolates through them.
Ice effects the transportation of rocks and debris, with a power
which nothing can resist. This is no where more conspicuous than
among the glaciers of the Alps, by the falling of which great heaps
of stones and rubbish are produced. The transportation of large
stones by means of ice may also be seen in our mountain torrents in
winter. Huge masses of stone, scattered over the plains of the north
of Germany and the islands of Denmark, and often very prejudicial to
agriculture, whose northern origin appears to be established, may
have been carried by the same powerful agent from Finland, Sweden and
Norway, into those countries, at a time when the plains of northern
Germany, with the other flat districts along the shores of the
Baltic, were still covered by the waves of the ocean.
In the formation of transported soil, water usually exerts a great
degree of power. By means of it, not only are vast masses transported
to the greatest distances, but their parts are at the same time
crumbled down and mingled together. To these operations are to be
attributed the various terminations of different soils at horizontal
distances, as well as the different alternations of their strata at
vertical ones. The power of water in the formation of transported
soil varies, not only according to the different inclinations of its
channel, but also in regard to the form, size, and weight of the
parts carried off by it; for which reason, in the formation of such
soils, the same phenomena take place on a large scale, that we see
on a smaller, in performing the operations of breaking and washing
the ores of metals. For the same reason that, in these processes,
the larger particles subside, while the smaller are propelled, from
which again the heavier particles of ore are sooner deposited than
the lighter; in plains in the vicinity of a mountain, covered with
transported soil, stones and debris are usually seen first, then
earth, clay, and sand mixed together, and farther on, finer sand,
with strata of clay.
Transported or secondary soil, produced by water, according to the
mode of its formation, is divided into four classes, viz.--1. Soil of
Valleys; 2. River Soil; 3. Lake Soil; 4. Marine Soil.
1. _Soil of Valleys._--It is washed down by rain and snow water,
and partly also produced by rivulets, which carry off the loose
parts from the declivities of mountains to the plains. The nature of
this soil in general clearly shews the nearness of its origin. Its
depth is always greatest in the bottom of the valley, and gradually
diminishes toward the declivities of the mountains. The curvature
of the different strata is usually accommodated to the irregularity
of its external form, so that when a section is made of them, they
exhibit a series of parallel curved lines.
2. _River Soil_, or the soil found in the beds and banks of rivers,
and which is produced by the continual propelling power of large
rivers. To this class belong two different kinds; _1st_, Soil
containing pebbles of various sizes, produced by the power of
torrents in the vicinity of mountains; and, _2d_, Earth or mud,
deposited in the beds of rivers, in places at a distance from
mountains. A peculiarity of river soil in general is, that it is much
extended in length, while its breadth is comparatively but small.
The different layers have neither so much irregularity as in the
preceding kind, nor are they so precise in arrangement as in the
following.
3. _Lake Soil_, deposited at the bottom of still water. To this class
is to be referred the soil in the bottoms of valleys, which had
formerly been lakes, either separate or connected with rivers. The
horizontal dimensions of this kind of soil are often more or less
equal. Sometimes, indeed, the length is greater than the breadth;
not, however, in the same degree as in soil deposited in the bed
of rivers. The surface is usually plane, and the different strata
alternate in a parallel manner.
4. _Marine Soil_, that is to say, the mud of the ancient ocean.
It is the greatest of all in its extent, both in a horizontal and
a vertical direction. Its surface is more or less undulated, very
seldom even. Its masses are both very thick and very uniform in
composition. Different and alternating strata, however, do occur,
whose forms and dimensions are usually more or less regular, and
which are not unfrequently undulated.
Soil, after being formed, is acted upon by natural powers in various
ways. The atmosphere is perpetually modifying it; rivers, waves, and
winds, act here and there upon its surface, and alter its external
form; water introduces into it the substances which it holds in
solution. The different constituent parts of soil act upon each
other chemically, and in this manner new decompositions and mixtures
are produced; and this chemical change is increased by the action
of vegetables, as well as of bodies deriving their origin from both
organic kingdoms.
From what has been said of the relations existing between the
masses of which the solid crust of the globe is composed, and the
loose earth or soil by which it is covered, it appears evident
enough (Hausmann concludes) that they have great influence over its
formation and nature, and therefore upon the more perfect vegetables,
and especially those which are the objects of cultivation; and
that although the fertility of the soil is much increased by these
vegetables themselves, yet the first foundation of their vigour is
derived from the disintegration and decomposition of rocks. If this
be correct, the constitution of the solid crust of the earth has a
much more extended influence. For, by preparing a habitation for the
greater and most important parts of plants, it also exerts a high
degree of influence upon the animals which derive their sustenance
from them, and, at the same time, affords the means of subsistence to
man[416].
NOTE.
ACCOUNT OF THE IRISH ELK, FOSSIL ELEPHANT OR MAMMOTH, AND THE
MASTODON.
As the Irish Elk, the Fossil Elephant or Mammoth, and the Mastodon,
are among the most remarkable of the fossil and extinct species of
quadrupeds mentioned in the preceding pages of this work, we, with
the view of farther gratifying the curiosity of our readers, now lay
before them the following additional details from the writings of
Cuvier, Goldfuss, and others.
1. _Fossil Elk of Ireland_, Cervus megaceros[417].
(_Noticed at_ p. 286.)
One of the most magnificent of the bisulcated animals met with in
a fossil state in the British Islands is the Elk of Ireland, the
_Cervus megaceros_. Bones and horns of vast size of this species are
almost daily dug out of the bogs and marl pits of Ireland. Similar
remains have been met with in alluvial strata in Britain, and also in
the Isle of Man.
“So frequently do these remains,” Mr Hart remarks, “occur in most
parts of Ireland, that there are very few of the peasantry who are
not, either from personal observation or report, acquainted with
them by the familiar name of the horns of the ‘old deer.’ Indeed in
some parts of the country they have been found so often, that far
from being regarded as objects of any extraordinary interest, they
have been either thrown aside as lumber, or applied to the commonest
economical uses[418].
“I have made diligent but fruitless search for an account of the
particular time when any of these remains were first discovered. As
they generally occur in marl, it is most likely that they did not
begin to attract attention until the advanced state of agriculture
had created an increased demand for that mineral as a manure. We can
very easily imagine the astonishment which the appearance of horns so
large, and of such strange form, must have excited in the minds of
those who discovered them for the first time, and how readily they
obtained a place in the hall of some adjoining mansion, where they
were deposited as an ornament of great curiosity, from the contrast
which they formed with the horns of the species of deer known at
present. In this way we may account for the preservation of so many
specimens as are found in the possession of the gentry in different
parts of this country.
“Very lately an entire skeleton of the Irish Elk was dug up in that
country. The following statement of the circumstances under which
the bones were found, with their geological position, was laid before
the Dublin Society, in a letter from Archdean Maunsell to the Right
Hon. George Knox.
“_Middleton Lodge, March 8. 1825._
“MY DEAR SIR,