11' ■'
t
u
. . ■ t\\
necessaiy to begin by considering the geological stnicture of tlicir tcn-itorial surface, and
the maimer in which earths and soils are produced. We sh.-dl next consider m succession
the Nomenclatin-e, Quality, Use, mid Improvement of Soils.
Sect. I. O fth e Geological Structure o f the Ghhe, ami the Formation o f Earths and Soils.
1138. The crust o f the earth is idl that is attempted to bo described by geologists, as
nothing certain is known of the earth’s mtcrnal structure ; and this crast consists partly
of rocks ivhich .at some distant period havo heen in a state of igneous fusion, and partly
of earthy or luiuoral matters, wliich have been held in suspension by water, and afterwards
deposited in strata or layers. Rocks of the first kind ai-e sometimes called ciystallino
or non-fossilifci-ous, bocanso tlicir structure is orystaUine, and they contain no fossils; and
sometimes igneous or uiistratifiod, because they boar .strong marks of having been subjected
to intense heat, and because they are not deposited in distinct strata or layers. The othci-
dass of rocks, that is, those which arc deposited in strata or layers, are of various kinds.
Tlic oldest ai-e evidently formed from particles washed off the crystaUine rocks and
deposited when tho water iu which they were held in suspension was removed; and hence
these are sometimes called scdimentaiy ciystallino rooks. Above these lie rocks of
various kinds of slate : these form what ivcro formerly cidled transition rocks, or the uppcr
Graiiwacke system, but which ar-o now tbe Silurian rocks of Sir Roderick Murchison.
Tlieso rocks abound iu fossU remains, but chiefly of cor.als, Encrinitcs, and other in-
vcrtcbratcd animals. Tho -ivholc of these rocks wore included in the primary strata
of tlie older geologists, and togotbor with tlie old red sandstone and oai-boniforous series
form what is now cousidorcd to belong to the older or Pdaiozoio period. Above these
lies the new red sandstone. Iligbcv still ai-o the lias, tho various kinds of oolite,
and tho Wealden c la y ; and above these lie the rocks of the cretaceous system con!
taining groen sand, gault, and chalk. Those are all the rocks included hi tho
SBCondai-y .strata, and they all contain fossil remains belonging to extinct species. 'TIio
tertiary strata consist of tho plastic and London clays, and Suffolk crag. Thoso rocks
contain niiinci-ous fossils, some of which are identical with existing species ; and above
them lies only the gravel and wliat is generally called surface soil, fonned by the deposit
of river sediments, and tho decay of vegetable and animal substances.
1139. Igneous unstratifed rochs. The principal of these is granite, which is composed
of quartz, feldspai-, .and mica, with the occasional addition of hornblende, and
some other luiiieral substances. This rock generally forms the basis of .all other rocks
in mountain chains ; though it is also occasionally found ou the surface, having been
cither washed hai-e by the -violent action of water, or forced up by some internal fli-c throngh
tho strtata deposited upon it. There are various kinds of granite, and the constituent
parts of granite m-e occasionally found cither separately or two combined together.
'Thus hornblende is sometimes found mixed with feldspai- only, forming what is caUcd
syenite ; and sometimes ciystals of feldspar are found imbedded in massive feldspar,
forming what is callod porphyry. Serpentino is another rock fonned from the materials
of granite ; and talc sometimes replaces mica. Granite is found in great abundance in
various parts of England, particulai-ly Devonshire and Cornwall, and it is also found
abundantly in Scotland. Tlio other igneous rocks are apparently of volcanic origin.
1140. Sedimentary non-fossiHferous rochs. I f powdered granite be mixed with wator
and then suffered to settle and tho water poured off, tho residuum, when dry, will
closely resemble gneiss, which, as Professor Ansted obsei-vos, “ is, indeed, nothing else
than sti-atiflcd granite. I f tho water in which the pounded rock is thrown is moving
along at a slow rate, and that part of the granite called feldspar happens to bo somewhat
decomposed, as it often is, then the feldspar (which is so trnly clay, that it makes the
best possible material for tho nse of the potteries), .and the thin shining plates of mica
will be c.ai-i-ied farther by the water than the lumps of white quartz or flint sand, which
with the other two ingredients made up the granite, and the two former will be deposited
in layers, which, by passing a galvanic cim-cnt throngh them, would in time
become mica-schist. If the mica wore absent, or if the clay wore deposited without it,
owing to any cause, then a similar g.alvamc current would turn the deposit into somc-
tnilig like clay-slato.” {Ancient World, p. 19.) Thus were formed the flrst sedimentary
01- deposited rocks, still showing marks of their igneous origin, and “ often in themselves
crystalline, but bearing evident marks of what is called ‘ mechanical structm-c ’ or in
other words, of having been deposited from water.” {Ibid.) Gneiss and mica-slate often
form mountain masses in association with the different varieties of granite, particularly
in Scotland in the lofty mountain of Ben Nevis. The bods of gneiss in vai-ious parts of
Great Britain are found of onoi-inons depth, and they frequently contain large cracks
whicli are fllled up with trap, basalt, whin stone, and other igneous rocks. The diftbrent.
kinds of slates found associated with tho gneiss are often considoi-.ably distorted, and
sometimes they arc found in wavy contortions. Associated with thoso rocks are those
be,autiful varieties of limestone known as statuaiy marble, whicb arc generally white,
but sometimes streaked by the admixture of various minerals. ’
1141. Fassiliferous rocks. These are all deposited in strata, and though very frc-
qiiemly some of the scries arc deficient, what are caUcd the lower rocks are never found
above tho higher ones ; and in tliis respect they show a marked difl'ereiice to tho
igneous rocks, which seem to be acted upon from time to time by internal fires, .and
pushed np violently through the strata which havo boon deposited above them, tearing
tho stratified rocks asunder, and occasioning those cin-ious faults or breaks which frc-
qucntly occasion so muck trouble and annoyance to minors. In the lowest slate ami
liine.stonc rocks, a few fossils have been discovered, but a far greater mnnbcr arc found
in a scries of rocks whicli are called Silurian from thch- being best developed in that
part of England and Wales which was formerly included in the ancient British kingdom
of tho Silurcs. Some of tho rocks belonging to this system are found occasionally
ill comparatively thin strata, which are veiy much twisted and distorted. Sometimes
the intermediate rocks arc altogether w.anting, and “ tho beds resting upon
the gneiss, mica-schist, and other old rocks, consist, for tho most part, of coarse conglomerate
or pudding-stone, evidently made np of the broken fr.agmonts of the old
granitic rocks, rolled and tossed about for ages in a troubled sea, the hardest stones
being i-onnded into biillct-shaped pebbles by thch- long and incessant attrition against
one another.” {Ancient World, p. 57.) Tlras, the rock known as the old rod sandstone,
which is below the carboniferous system, has often more the appearance of a conglomerate
than a sand.stonc, and the Devonian rocks consist of gritty and slaty beds, the gi-ains of
sand appearing to be fi-agmonts of wliito rolled quartz, sm-rounded by a red peroxide
of iron like a varni.sh, the oxide of iron hcing an abundant substance in volcanic ejections.
The mountain or solid limestone which lies above these rocks, appears to have
been tho receptacle of great masses of vegetable remains which time has changed into
the substance wo call coal. This cai-boniferou5 limestone forms the midcrlying rock
in the immense tract caUcd the British Co.al Field, wliich extends from Bristol and South
Wales to tho north and oast, forming the limestone and coal districts of the midland
and nortlici-n districts of EngLand. Tho immense thickness of these carboniferous strata,
which is sometimes two thousand yards and upwards, renders it difficult to comprehend
how such enormous masses of vegetable matter can havo heen accinnidatcd and buried.
In some places the mountain limestone is absent, and tho coal mo.asm-cs rest immediately
on the old ro ck ; and sometimes muddy and sandy beds alternate with one another,
and -with tho coal itself. Some of these appear to bo of freshwatci- origin, and others
deposits from tho sea, and they generally contain remains of the leaves of ferns and
fcrn-likc trees. “ The trees,” says Professor Ansted, “ which, in many cases, contributed
largely to the fonnation of tlio coal, seem to h.ave boon .almost entirely succnlcnt, and
capable of being squeezed into a small compass during partial decomposition. This
squeezing process must have been conducted on a grand scale, both during and aftci-
thc fonnation of separate beds, and each bed in succession was probably soon covered
up by muddy and sandy accumubations, now alteniating with tbe coal in the foi-m of
shale and grit-stone.” {Ancient World, p. 79.) I t is a singular fact that in several
places in tho north of England, the mountain limestone serves as a receptacle for lead,
and otlicr metals. Above tho coal moasm-es is gcner.ally found magnosian limestone,
and above this the now red sandstone including various kinds of marl, and also
gypsmn, and rock-salt. Above this is tho lias, B.ath, and otlicr oolitic limestone, Portland
stone, and various kinds of clay and sand. Above this lies gi-een sandstone, gault, and
chalk, the Latter being intersected with rows of flints; and above these are the tcrtiaiy
strata, including the London cLay, &c., which are covered by the beds of loose sand and
gravel, &c., and by the loose cai-tli on the surface, wliich is what is usually denominated
soil.
1142. The majiner in which rochs are converted into soils. Sir II. Davy observes {Elem.
o f Agric. Chem. 188.), may bo easily conceived by rcfcn-ing to the instance of soft
gr.anito, or porcelain granite. This substance consists of three iiigredionts — quartz,
feldspar, .and mic.a. The quartz is almost pure siliceous earth in a ciystallmc fonn.
'I'he feldspar and mica arc vci-y compound substances; both contain silica, alumiina,
and oxide of iron ; in the feldspar there is usually lime and potassa ; in the mic.a, lime
and magnesia. When a granite rock of this kind has been long exposed to the influence
of air .and water, the lune and the potassa contained in its constituent parts arc acted
upon by -water or carbonie acid ; and the oxide of iron, which is .almost .always in its
least oxidised state, tends to combine with more oxygen ; the consequence is, that the
fchLspar decomposes, and likewise the mica ; but tlio first the most rapidly. The feldspar,
which is as it were the cement of the stone, forms a fine clay ; the mica, partially decomposed,
mixes with it .as s.and ; and the midocomposcd quartz appears as gi-avcl, or s,and
of different degrees of fmoncs.s. As soon as the smallest layer of earth is formed on the
surface of a rock, the seeds of lichens, mosses, and other imperfect vegetables which ai-e