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Coming Soon to Listen to Genius:
Charles Lyell
English Scientist
1797-1875 A selection from
THE STUDENT'S ELEMENTS OF GEOLOGY
Narrated by Michael Kramer
downloadable mp3 file coming soon
Of what materials is the earth composed, and in what manner are these materials
arranged? These are the first inquiries with which Geology is occupied, a
science which derives its name from the Greek ge, the earth, and logos, a
discourse. Previously to experience we might have imagined that investigations
of this kind would relate exclusively to the mineral kingdom, and to the various
rocks, soils, and metals, which occur upon the surface of the earth, or at
various depths beneath it. But, in pursuing such researches, we soon find
ourselves led on to consider the successive changes which have taken place in
the former state of the earth's surface and interior, and the causes which have
given rise to these changes; and, what is still more singular and unexpected, we
soon become engaged in researches into the history of the animate creation, or
of the various tribes of animals and plants which have, at different periods of
the past, inhabited the globe.
All are aware that the solid parts of the earth consist of distinct substances,
such as clay, chalk, sand, limestone, coal, slate, granite, and the like; but
previously to observation it is commonly imagined that all these had remained
from the first in the state in which we now see them— that they were created in
their present form, and in their present position. The geologist soon comes to a
different conclusion, discovering proofs that the external parts of the earth
were not all produced in the beginning of things in the state in which we now
behold them, nor in an instant of time. On the contrary, he can show that they
have acquired their actual configuration and condition gradually, under a great
variety of circumstances, and at successive periods, during each of which
distinct races of living beings have flourished on the land and in the waters,
the remains of these creatures still lying buried in the crust of the earth.
By the "earth's crust," is meant that small portion of the exterior of our
planet which is accessible to human observation. It comprises not merely all of
which the structure is laid open in mountain precipices, or in cliffs
overhanging a river or the sea, or whatever the miner may reveal in artificial
excavations; but the whole of that outer covering of the planet on which we are
enabled to reason by observations made at or near the surface. These reasonings
may extend to a depth of several miles, perhaps ten miles; and even then it may
be said, that such a thickness is no more than 1/400 part of the distance from
the surface to the centre. The remark is just: but although the dimensions of
such a crust are, in truth, insignificant when compared to the entire globe, yet
they are vast, and of magnificent extent in relation to man, and to the organic
beings which people our globe. Referring to this standard of magnitude, the
geologist may admire the ample limits of his domain, and admit, at the same
time, that not only the exterior of the planet, but the entire earth, is but an
atom in the midst of the countless worlds surveyed by the astronomer.
The materials of this crust are not thrown together confusedly; but distinct
mineral masses, called rocks, are found to occupy definite spaces, and to
exhibit a certain order of arrangement. The term ROCK is applied indifferently
by geologists to all these substances, whether they be soft or stony, for clay
and sand are included in the term, and some have even brought peat under this
denomination. Our old writers endeavoured to avoid offering such violence to our
language, by speaking of the component materials of the earth as consisting of
rocks and SOILS. But there is often so insensible a passage from a soft and
incoherent state to that of stone, that geologists of all countries have found
it indispensable to have one technical term to include both, and in this sense
we find ROCHE applied in French, ROCCA in Italian, and FELSART in German. The beginner, however, must constantly bear in mind that the term rock by no means implies that a mineral mass is in an indurated or stony condition.
The aqueous rocks, sometimes called the sedimentary, or fossiliferous, cover a
larger part of the earth's surface than any others. They consist chiefly of
mechanical deposits (pebbles, sand, and mud), but are partly of chemical and
some of them of organic origin, especially the limestones. These rocks are
STRATIFIED, or divided into distinct layers, or strata. The term STRATUM means
simply a bed, or any thing spread out or STREWED over a given surface; and we
infer that these strata have been generally spread out by the action of water,
from what we daily see taking place near the mouths of rivers, or on the land
during temporary inundations. For, whenever a running stream charged with mud or sand, has its velocity checked, as when it enters a lake or sea, or overflows a
plain, the sediment, previously held in suspension by the motion of the water,
sinks, by its own gravity to the bottom. In this manner layers of mud and sand
are thrown down one upon another.
If we drain a lake which has been fed by a small stream, we frequently find at
the bottom a series of deposits, disposed with considerable regularity, one
above the other; the uppermost, perhaps, may be a stratum of peat, next below a
more dense and solid variety of the same material; still lower a bed of shell-
marl, alternating with peat or sand, and then other beds of marl, divided by
layers of clay. Now, if a second pit be sunk through the same continuous
lacustrine FORMATION at some distance from the first, nearly the same series of
beds is commonly met with, yet with slight variations; some, for example, of the
layers of sand, clay, or marl, may be wanting, one or more of them having
thinned out and given place to others, or sometimes one of the masses first
examined is observed to increase in thickness to the exclusion of other beds.
The term "FORMATION," which I have used in the above explanation, expresses in
geology any assemblage of rocks which have some character in common, whether of origin, age, or composition. Thus we speak of stratified and unstratified,
fresh-water and marine, aqueous and volcanic, ancient and modern, metalliferous
and non-metalliferous formations.
In the estuaries of large rivers, such as the Ganges and the Mississippi, we may
observe, at low water, phenomena analogous to those of the drained lakes above
mentioned, but on a grander scale, and extending over areas several hundred
miles in length and breadth. When the periodical inundations subside, the river
hollows out a channel to the depth of many yards through horizontal beds of clay
and sand, the ends of which are seen exposed in perpendicular cliffs. These beds
vary in their mineral composition, or colour, or in the fineness or coarseness
of their particles, and some of them are occasionally characterised by
containing drift-wood. At the junction of the river and the sea, especially in
lagoons nearly separated by sand-bars from the ocean, deposits are often formed
in which brackish and salt-water shells are included.
In Egypt, where the Nile is always adding to its delta by filling up part of the
Mediterranean with mud, the newly deposited sediment is STRATIFIED, the thin
layer thrown down in one season differing slightly in colour from that of a
previous year, and being separable from it, as has been observed in excavations
at Cairo and other places. (See "Principles of Geology" by the Author Index
"Nile" "Rivers" etc.)
When beds of sand, clay, and marl, containing shells and vegetable matter, are
found arranged in a similar manner in the interior of the earth, we ascribe to
them a similar origin; and the more we examine their characters in minute
detail, the more exact do we find the resemblance. Thus, for example, at various
heights and depths in the earth, and often far from seas, lakes, and rivers, we
meet with layers of rounded pebbles composed of flint, limestone, granite, or
other rocks, resembling the shingles of a sea-beach or the gravel in a torrent's
bed. Such layers of pebbles frequently alternate with others formed of sand or
fine sediment, just as we may see in the channel of a river descending from
hills bordering a coast, where the current sweeps down at one season coarse sand
and gravel, while at another, when the waters are low and less rapid, fine mud
and sand alone are carried seaward.
If a stratified arrangement, and the rounded form of pebbles, are alone
sufficient to lead us to the conclusion that certain rocks originated under
water, this opinion is farther confirmed by the distinct and independent
evidence of FOSSILS, so abundantly included in the earth's crust. By a FOSSIL is
meant any body, or the traces of the existence of any body, whether animal or
vegetable, which has been buried in the earth by natural causes. Now the remains
of animals, especially of aquatic species, are found almost everywhere imbedded
in stratified rocks, and sometimes, in the case of limestone, they are in such
abundance as to constitute the entire mass of the rock itself. Shells and corals
are the most frequent, and with them are often associated the bones and teeth of
fishes, fragments of wood, impressions of leaves, and other organic substances.
Fossil shells, of forms such as now abound in the sea, are met with far inland,
both near the surface, and at great depths below it. They occur at all heights
above the level of the ocean, having been observed at elevations of more than
8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above
18,000 feet in the Himalaya.
These shells belong mostly to marine testacea, but in some places exclusively to
forms characteristic of lakes and rivers. Hence it is concluded that some
ancient strata were deposited at the bottom of the sea, and others in lakes and
estuaries.
We have now pointed out one great class of rocks, which, however they may vary
in mineral composition, colour, grain, or other characters, external and
internal, may nevertheless be grouped together as having a common origin. They
have all been formed under water, in the same manner as modern accumulations of
sand, mud, shingle, banks of shells, reefs of coral, and the like, and are all
characterised by stratification or fossils, or by both. More information about Charles Lyell from Wikipedia
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