the past condition of organic nature-第2节
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qualifications;is made up of such layers of mud; so hard; the
majority of them; that we call them rock whether limestone or
sandstone; or other varieties of rock。 And; seeing that every part of
the crust of the earth is made up in this way; you might think that the
determination of the chronology; the fixing of the time which it has
taken to form this crust is a comparatively simple matter。 Take a
broad average; ascertain how fast the mud is deposited upon the bottom
of the sea; or in the estuary of rivers; take it to be an inch; or two;
or three inches a year; or whatever you may roughly estimate it at;
then take the total thickness of the whole series of stratified rocks;
which geologists estimate at twelve or thirteen miles; or about seventy
thousand feet; make a sum in short division; divide the total thickness
by that of the quantity deposited in one year; and the result will; of
course; give you the number of years which the crust has taken to form。
Truly; that looks a very simple process! It would be so except for
certain difficulties; the very first of which is that of finding how
rapidly sediments are deposited; but the main difficultya difficulty
which renders any certain calculations of such a matter out of the
questionis this; the sea…bottom on which the deposit takes place is
continually shifting。
Instead of the surface of the earth being that stable; fixed thing that
it is popularly believed to be; being; in common parlance; the very
emblem of fixity itself; it is incessantly moving; and is; in fact; as
unstable as the surface of the sea; except that its undulations are
infinitely slower and enormously higher and deeper。
Now; what is the effect of this oscillation? Take the case to which I
have previously referred。 The finer or coarser sediments that are
carried down by the current of the river; will only be carried out a
certain distance; and eventually; as we have already seen; on reaching
the stiller part of the ocean; will be deposited at the bottom。
Let C y (Fig。 4) be the sea…bottom; y D the shore; x y the sea…level;
then the coarser deposit will subside over the region B; the finer over
A; while beyond A there will be no deposit at all; and; consequently;
no record will be kept; simply because no deposit is going on。 Now;
suppose that the whole land; C; D; which we have regarded as stationary;
goes down; as it does so; both A and B go further out from the shore;
which will be at yl; x1; y1; being the new sea…level。 The consequence
will be that the layer of mud (A); being now; for the most part;
further than the force of the current is strong enough to convey even
the finest 'debris'; will; of course; receive no more deposits; and
having attained a certain thickness will now grow no thicker。
We should be misled in taking the thickness of that layer; whenever it
may be exposed to our view; as a record of time in the manner in which
we are now regarding this subject; as it would give us only an
imperfect and partial record: it would seem to represent too short a
period of time。
Fig。4。
Suppose; on the other hand; that the land (C D) had gone on rising
slowly and graduallysay an inch or two inches in the course of a
century;what would be the practical effect of that movement? Why;
that the sediment A and B which has been already deposited; would
eventually be brought nearer to the shore…level; and again subjected to
the wear and tear of the sea; and directly the sea begins to act upon
it; it would of course soon cut up and carry it away; to a greater or
less extent; to be re…deposited further out。
Well; as there is; in all probability; not one single spot on the whole
surface of the earth; which has not been up and down in this way a
great many times; it follows that the thickness of the deposits formed
at any particular spot cannot be taken (even supposing we had at first
obtained correct data as to the rate at which they took place) as
affording reliable information as to the period of time occupied in its
deposit。 So that you see it is absolutely necessary from these facts;
seeing that our record entirely consists of accumulations of mud;
superimposed one on the other; seeing in the next place that any
particular spots on which accumulations have occurred; have been
constantly moving up and down; and sometimes out of the reach of a
deposit; and at other times its own deposit broken up and carried away;
it follows that our record must be in the highest degree imperfect; and
we have hardly a trace left of thick deposits; or any definite
knowledge of the area that they occupied; in a great many cases。 And
mark this! That supposing even that the whole surface of the earth had
been accessible to the geologist;that man had had access to every
part of the earth; and had made sections of the whole; and put them all
together;even then his record must of necessity be imperfect。
But to how much has man really access? If you will look at this Map you
will see that it represents the proportion of the sea to the earth:
this coloured part indicates all the dry land; and this other portion
is the water。 You will notice at once that the water covers
three…fifths of the whole surface of the globe; and has covered it in
the same manner ever since man has kept any record of his own
observations; to say nothing of the minute period during which he has
cultivated geological inquiry。 So that three…fifths of the surface of
the earth is shut out from us because it is under the sea。 Let us look
at the other two…fifths; and see what are the countries in which
anything that may be termed searching geological inquiry has been
carried out: a good deal of France; Germany; and Great Britain and
Ireland; bits of Spain; of Italy; and of Russia; have been examined;
but of the whole great mass of Africa; except parts of the southern
extremity; we know next to nothing; little bits of India; but of the
greater part of the Asiatic continent nothing; bits of the Northern
American States and of Canada; but of the greater part of the continent
of North America; and in still larger proportion; of South America;
nothing!
Under these circumstances; it follows that even with reference to that
kind of imperfect information which we can possess; it is only of about
the ten…thousandth part of the accessible parts of the earth that has
been examined properly。 Therefore; it is with justice that the most
thoughtful of those who are concerned in these inquiries insist
continually upon the imperfection of the geological record; for; I
repeat; it is absolutely necessary; from the nature of things; that
that record should be of the most fragmentary and imperfect character。
Unfortunately this circumstance has been constantly forgotten。 Men of
science; like young colts in a fresh pasture; are apt to be exhilarated
on being turned into a new field of inquiry; to go off at a
hand…gallop; in total disregard of hedges and ditches; losing sight of
the real limitation of their inquiries; and to forget the extreme
imperfection of what is really known。 Geologists have imagined that
they could tell us what was going on at all parts of the earth's
surface during a given epoch; they have talked of this deposit being
contemporaneous with that deposit; until; from our little local
histories of the changes at limited spots of the earth's surface; they
have constructed a universal history of the globe as full of wonders and
portents as any other story of antiquity。
But what does this attempt to construct a universal history of the globe
imply? It implies that we shall not only have a precise knowledge of
the events which have occurred at any particular point; but that we
shall be able to say what events; at any one spot; took place at the
same time with those at other spots。
Let us see how far that is in the nature of things practicable。 Suppose
that here I make a section of the Lake of Killarney; and here the
section of another lakethat of Loch Lomond in Scotland for instance。
The rivers that flow into them are constantly carrying down deposits of
mud; and beds; or strata; are being as constantly formed; one above the
other; at the bottom of those lakes。 Now; there is not a shadow of
doubt that in these two lakes the lower beds are all older than the
upperthere is no doubt about that; but what does 'this' tell us about
the age of any given bed in Loch Lomond; as compared with that of any
given bed in the Lake of Killarney? It is; indeed; obvious that if any
two sets of deposits are separated and discontinuous; there is
absolutely no means whatever given you by the nature of the deposit of
saying whether one is much younger or older than the other; but you may
say; as many have said and think; that the case is very much altered if
the beds which we are comparing are continuous。 Suppose two beds of
mud hardened into rock;A and B…are seen in section。 (Fig。 5。)
'Fig。 5。'
Well; you say; it is admitted that the lowermost bed is always the
older。 Very well; B; therefore; is older than A。 No doubt; 'as a
whole'; it is so; or if any parts of the two beds which are in the same
vertical line are compared; it is so。