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Both are real causes. Aerolites fall on the earth and generate heat,
the smaller ones, or shooting stars, being set on fire and burnt up by the
friction of the atmosphere; the larger ones reaching the earth in masses of
stone, singularly like those ejected from deep-seated valcanoes, and with
their surfaces glazed by intense heat. If such meteors fall on the earth,
it is reasonable to suppose that vastly more must fall on the sun, with ite


vastly greater surface and attracting power. And it is to be noted that
comparatively small masses might generate large amounts of heat, for the
amount of mechanical force, and therefore of heat, generated by arrested
motion, increases with the square of the velocity. A body weighing 8 -339
kilogrammes falling from a height which gave it a velocity of one metre
per second, would generate one calory of heat, or enough to raise the
temperature of one kilogramme of water by i Centigrade. But the same
body moving with the velocity of a cannon-ball, or 500 metres per second,
would generate two hundred and fifty thousand times as much heat; and
if moving with a velocity of 700,000 metres per second, which is about
the velocity with which a body would fall into the sun from the distance
of the earth, the heat produced would be nearly two million times as

Sir W. Thomson has calculated that a quantity of matter equal to about
one-hundredth of the mass of the earth falling annually with this velocity
on the sun's surface, would maintain its present radiation indefinitely.
It is clear therefore that if this amount of meteoric matter really falls on
the sun its heat might be maintained. But many objections have been
raised to such a supposition.

To explain the sun's heat we must have a cause that is not only suffi-
cient to generate its total amount, but also one which generates it uniform-
ily. If the sun were a target kept at an intense white heat by showers of
meteoric small shot peppering into it, how is it that this stream of small
shot is incessant and uniform ?

Only small portions of the total meteoric mass revolving round the sun
can be captured by it gradually, as their orbits are contracted. An extra
supply, as some solid body or enormous comet with its attendant meteo-
ric train falling into the sun, would raise its temperature above, while a
deficient supply would lower it below the average, and a comparatively
slight variation in the sun's temperature would destroy existing conditions
of life on the earth.

Another objection to the meteoric theory is, that it would require such
a large mass of meteoric matter revolving in space as might be expected
to exercise a perceptible effect on the motions of the planets, both by the
law of gravity and by the retardation due to a resisting medium. And this
is specially true of the orbits of comets which approach the sun very close-
ly. As meteors do not fall from a state of rest straight into the sun, but
revolve around it with planetary velocities, they can only fall into it by
being drawn inwards in gradually contracting spirals, until they reach a
point where they impinge on the sun or its atmosphere. Hence a vastly
greater amount of meteoric matter must be revolving round the sun in
the space near it, than can be captured and generate heat in any single year.
But several comets are known to almost have grazed the sun's atmosphere,
and emerged from it to continue to describe their elliptic orbits and return
true to time, as predicted by calculations based on the known laws of


gravity acting on them from the sun and planets alone, in a /xn-resisting

Consider what this means. Comets are bodies of such immense volume
and extreme rarity that one of them got entangled among Jupiter's satel-
lites and thrown out of its course, without affecting in the slightest per-
ceptible degree the motions of those satellites. How could such comets,
rushing closely round the sun with enormous velocities, avoid showing
perturbations, if they encountered any considerable mass of meteoric
matter ?

The theory of meteorites, to which reference will be made in a future
chapter, meets many of these difficulties, and strengthens the case for a
meteoric origin of a large part of solar heat, but it hardly accounts for the
uniformity of the supply, and is hardly yet so generally accepted as to su-
persede the older theory that the main source of the sun's heat is to be
sought in the transformation of the mechanical energy of gravity, as its
volume contracts.

Assuming this theory, the principle on which the supply of solar heat
is calculated is the following. We know the amount of heat given out
by each square metre of the sun's surface, and we know the height from
which a given weight must fall to generate this heat when its motion is
arrested. We know also that this heat will be the same whether the
motion is suddenly or gradually arrested. Now in this case the given
weight is that of a long narrow cone of matter, whose base is one square
metre at the sun's surface, and its apex a point at the sun's centre. Know-
ing the sun's diameter and mean density, it is easy to calculate the weight
of such a cone if we suppose it to be solid. Its weight is equivalent to
that of 244,000,000 tons of solar heaviness at the sun's surface. To re-
duce this to terrestrial tons, and their equivalent in horse-power, we must
allow for the difference of weight or gravity, at the respective surface of
the sun and earth.

Reduced to terrestrial figures, in which one horse-power is 270 metre-
tons per hour (i.e. a ton lifted 270 metres in an hour), the horse-power at
the sun's surface is 10 metre-tons. But the radiation from each square
metre of the solar surface in heat per hour is equivalent to 78,000 horse-
power in energy, or to that of 780,000 metre-tons. An easy calculation
shows that to supply energy at this rate for a year, our supposed cone of
244,000,000 tons must fall one metre in 313 hours, or about 35 metres in
a year. Refined mathematical calculations are requisite to show how this
result is effected, if we suppose, as is probable, that the mass of matter
forming the sun, instead of being solid, existed first in the nebulous or
gaseous state, and gradually contracted into a fluid mass in which con-
vection currents are constantly carrying down surface layers which have
become cooler by radiation, and replacing them by ascending currents
from the hotter and denser interior. These calculations have been made
by mathematicians of undoubted competence, with the result that the


dynamical equivalent of the heat radiated from the sun in a given time ia
practically the same as if it were solid.

This resu) f shows that if the sun has contracted to its present size,
from a volume extending far beyond the orbit of the remotest planet,
Neptune, it has furnished about eighteen million times as much heat as it
now supplies in a year; and that with its present dimensions it must con-
tract at the rate of 35 metres per year, or one per cent of its radius in
200,000 years.

Allowing for the increasing density of the sun as shrinkage proceeds,
the problem works out that if the sun's radiation of heat has been uniform
for the last fifteen millions of years, the solar radius must then have been
four times greater than it is now; and that if the present supply were
maintained by shrinkage alone, for the next twenty millions of years, the
sun must have shrunk to half its present size. But these figures must be
greatly reduced by several considerations. They are based on Herschell's
and Pouillet's figures for the total activity of solar radiation, but Forbes
and Langley have shown that the allowance made for absorption of solar
heat by the earth's atmosphere was insufficient, and that the real amount
of heat radiated by the sun is greater than was supposed by Pouillet in
the ratio of 17 to i. This diminishes the past and future periods of
solar radiation in the same proportion, reducing the past period from
fifteen to nine millions of years, and the future from twenty millions to
twelve. Moreover, when the sun's surface was four times larger, it must
have given out more heat than at present, and more than existing condi-
tions of life in geological times could support. If, therefore, the sun's
shrinkage from gravity has been the sole or principal source of its supply
of heat, it is difficult to see how life and the existing order of things on
the earth can have lasted for more than ten millions of years at the out-

So far the mathematicians seem to have it all their own way, and, as
often happens when the plaintiffs case only has been heard, it seems to
be conclusive. But what say the defendants the geologists ? They also
base their case on an undoubted principle, and on undeniable facts. The
principle is that of the uniformity of existing causes ; the facts, those of
actual experiment and observation.

Geology, in the pre-Lyellite days, passed through two stages, the theo-
logical and the theologico-scientific. The theological, which prevailed
universally until the present century, was based on the belief that the book
of Genesis, instead of being a sort of poetical prelude to a collection of
ancient writings of religious and moral import, was a strictly literal and
scientific narration of what actually took place, every word of which was
imparted by a Divine revelation, which it was impious to explain away or
to dispute. Geology was therefore confined very much to searching for
facts in Nature confirming this narrative. Thus when fossil-shells were
observed on mountain-tops, they were adduced as incontrovertible proofs


of Noah's deluge; and even a sceptical and encyclopaedic mind like that
of Voltaire could only attempt to palliate this proof by suggesting thai the
shells were dropped from pilgrims' hats while crossing the Alps on their
way to Rome. The period when such a ridiculous suggestion could be
made by an accomplished scholar seems thousands of years from us, and
yet it occurred in the last century. The naive and infantile narrative of
the Noachian deluge is now taken no more seriously than are the little
wooden arks, with their contents of pigmy animals, which with other toys
amuse the nursery.

The next stage was what may be called the theologico-scientific, when
the facts and laws of Nature began to be recognized; but the old dogmatic
faith was still so prevalent, that these facts and laws were viewed through
a theological medium, and attempts were made to reconcile the Bible and
science, by distorting the conclusions of science, and giving the state-
ments of Genesis a general and allegorical, rather than a literal meaning.
This was the era when days were expanded into periods, universal deluges
contracted into local floods, and when miraculous catastrophes and crea-
tions were invoked ad libitum, to bring geological and zoological facts
into some sort of possible accordance with the non-natural versions of
plain words into which Scriptural texts were evaporated. This school
included, in its time, some eminent men, such as Buckland and Hugh
Miller, and it still lingers on the outskirts of science, as may be seen by
Mr. Gladstone's essay on the Proem to Genesis. But with all the leader*
of science it is quite extinct, and the prevailing tone of thought has be-
come Darwinian, as universally as a century ago it was theological.
Differences may exist as to the details of Darwin's theory, and the extent
of its application in some of the more recondite causes of variation, but
no one of any authority in science doubts that evolution, under fixed
laws, is the key to the secrets of the universe, and that one original im-
press, and not perpetual miracle, or secondary interference, has been the
real course of Nature.

In geology this conviction has been embodied in what is known as
Lvell's Law of Uniformity. If any one wants to get a clear idea of what
this means, let him go to the British Museum and look at a slab of sand-
stone from the Silurian formation. He will see precisely what he may see
to-day on the sands of Southend or Margate. Ripple marks of a gently
flowing or ebbing tide, worm castings, or even little pits showing where
rain-drops had fallen on the wet sand, and these pits higher on one side
than the other, showing the size of the drops, the force of the wind, and
the direction from which it was blowing. The inference is irresistible that
at this immensely remote period the winds blew, the rain fell, the tides
ebbed and flowed, sand-banks were formed, and worms or sand-eels bur-
rowed in them, as they do at the present day. Or look at a piece of
chaiK through a microscope, and you will find it mainly composed of the
microscopic shells of a minute form of animal life, the Globigerina, which,


gradually falling to the bottom of a deep ocean like the finest dust, have
accumulated more deep than a thousand feet in thickness. Precisely
the same thing is going on in the Atlantic to-day, where deep-sea dredg-
ings bring up a Globigerina ooze, which affords a safe bed for the
submarine telegraph. Or take another instance. A shell called the
Lingula, about the size of a small mussel, is found abundantly in the
Silurian, and even in the earlier Cambrian formations; and another shell,
the Terebratula, in the Devonian. Both are found living at the present
day, not only of the same genus, but identically of the same species. It
is evident that no great change can have taken place in the conditions of
oceanic,, life since these mollusks lived and flourished in Silurian and
Devonian seas.

Nor can the condition of the atmosphere have greatly changed since
the time of the air-breathing Silurian scorpion, whose fossil remains show
him to be scarcely distinguishable from the present scorpion.

In fact, the atmosphere affords one of the most conclusive proofs of the
uninterrupted maintenance of existing conditions during an enormous
period. When we say enormous time, the term is used with reference to
any recent or historical standard as applicable to the period when geology
practically commences; that is, with the first dawn of life disclosed by
fossils in the Cambrian era, or beyond that with formations like the
Laurentian, which can be clearly proved to be sedimentary and metamor-
phic. But no geologist ventures to extend this doctrine of uniformity be-
yond the date when fossils appear, or to deny that though the laws of
Nature are the same, the conditions must have been totally different in the
earlier stages of the planet, when it was cooling and condensing into its
present form. Nor could he deny that, even within this comparatively
recent period, there may have been changes of existing conditions, as we
know indeed from the alterations between the Glacial period and those of
higher and more uniform temperature. But his position is that such
changes have been of the same order, and owing to similar causes as
those which now prevail; and that when a known cause, given a sufficient
time, will produce an effect, it is unphilosophical to 'assume miracles,
catastrophes, or a totally different order of things, in order to reduce the
time to some procrustean standard of theoretical prepossession.

To Sir C. Lyell belongs the credit of having established this doctrine
of uniformity on an unassailable basis, and made it the fundamental ax-
iom of geological science. By an exhaustive survey of the whole field of
geology, from the earliest formations in which life appears, down to the
present day, he has shown conclusively that while causes identical with,
or of the same order as, existing causes, will, if given sufficient time,
account for all the facts hitherto observed, there is not a single fact which
proves the occurrence of a totally different order of causes. This, of
course, applies only to the geological record commencing with the com-
mencement of organic life on the earth, and not to the earlier astronomi-


cal period when the planet was condensing from nebulous matter, and
slowly cooling and contracting. Nor does it imply absolute uniformity
with existing conditions, for changes in climate, temperature, distribution
of sea and land, and otherwise, have doubtless occurred from the slow
operation of existing causes. But it excludes all fanciful theories of ca-
taclysms, annihilating each successive era with its life, and introducing a
new one ; earthquakes throwing up mountain chains at a shock ; deluges
sweeping over the face of the earth, and so forth, in which even eminent
geologists used to indulge thirty or forty years ago. While no competent
geologist of the present day would like to affirm positively that there may
not have been, in past ages, explosions more violent than that of Kraka-
toa, lava streams more extensive than that of Skaptar-Jokul, and earth-
quakes more powerful than that which uplifted five or six hundred miles
of the Pacific coast of South America six or seven feet, it may be doubt-
ful if he could point out a single instance since the Silurian epoch where
such was demonstrably the case.

Assuming the principle of uniformity, the time requisite to explain the
facts of geology becomes a matter for approximate calculation. Not
readily in years or centuries, for our historical measuring-yard does not
extend beyond seven thousand years, when we find a dense population
and high civilization already existing in Egypt ; but in periods of which
we can form some approximate idea.

To understand the full force of the evidence, it is necessary to study
carefully the works of Lyell, Croll, Geikie, and other authorities on ge-
ology ; but some idea of the sort of periods which are required for gaug-
ing Time back to the commencement of life may be arrived at from a few

The tests of geological time are mainly from two sources denudation
and deposition. The present rate of denudation of a continent is known
with considerable accuracy, from careful measurements of the quantity of
solid matter carried down by rivers. The Mississippi affords the best
test, both because the measurements have been made with the greatest ac-
curacy, and because the conditions of the vast area drained by it and its
tributary rivers afford a better average of the rate of continental denuda-
tion, including as it does a great variety of climates and geological for-
mations, and being singularly free from exceptional influences. The rate
thus deduced is one foot from the general surface of the basin in six
thousand years. Now the measured thickness of the known sedimentary
strata is about 177,000 feet. The proportion of sea to land is three to
one, and the bulk of the deposition of the waste of land must have been
laid down within a comparatively narrow margin of the sea nearest to
land. On these data Wallace calculates that the time required to deposit
this 177,000 feet would be 28,000,000 years, taking the rate of denuda-
tion at one foot in 3000 years, or 56,000,00x3 years, taking the rate de-
duced from the Mississippi. But it must have been more than this, for


the stratified rocks are to a great extent composed of the debris of older
strata, which have been deposited, upheaved, and again denuded. Most
of the known stratified rocks must have been in this way denuded and
deposited many times over. Nor is there any good reason for supposing
that the rate of denudation was materially greater in former, than in recent
geological eras. On the contrary, the recent Glacial period, by grinding
down solid rock into loose materials, and, as the ice and snow melted,
causing more torrential inundations of rivers, must have tended to accel-
erate denudation.

Another proof of the enormous amount of solid rock which has been
removed by denudation, is afforded by the faults or cracks in the earth's
crust, which have in many cases displaced strata by thousands of feet, all
traces of which displacement have been subsequently planed down to one
uniform surface. Thus the great fault which separates the Silurians of
the south of Scotland from the Devonian and Carboniferous region to the
north of it, is estimated by the Geological Survey at 15,000 feet. A moun-
tain mass of this height, terminating in a steep cliff at the fault, must
have existed to the south of it, composed mainly of the Devonian strata
which now stop abruptly at the north edge of the fault. At present there
is no inequality of the surface at the fault, and /therefore 15,000 feet 01
nearly three miles of rock must have been removed by denudation. And
what is most important, the time in which this denudation was effected is
fixed as having occurred in the interval between the Devonian and Car-
boniferous periods, for while no trace of the former formation is found
south of the fault, the limestones and coal-measures of the latter lie directly
on the Silurian rocks. At the rate of denudation deduced from the Miss-
issippi observations of one foot in 6000 years, the removal of those three
miles of rock would have required 90,000,000 years for the interval be-
tween two of the geological formations.

Croll, in his recent work on Stellar Evolution, gives a number of simi-
lar instances, one in the Appalachian Mountains, in which the vertical
displacement is not less than 20,000 feet, bringing the upper Devonian
strata on one side opposite to the lowest Cambrian on the other. Of
course we cannot assume these enormous intervals of time to have actually
occurred, but they are quite sufficient to show the absolute impossibility of
reconciling geological facts with any estimate of the duration of solar heat
derived from the theory of contradiction by gravitation.

Take another instance from a more recent period. There is a dried-up
Eocene lake in North America, which once occupied an extensive area in
the States of Wyoming and Nebraska, formed by streams running down
from the Wahsatch. Uintah, and other mountain ranges, which are
Eastern outliers of the great backbone of the continent the Rocky Moun-
tains. It was gradually silted up by a deposit of more than 5000 feet, or
a mile thick of clays and sands, a portion of which has since been carved
by the rain and weather into the singular formation of isolated castle-like


bluffs and pyramids, known as the " bad lands." It is full of remains of
Eocene animals, often of huge size and of a peculiar type. How long
must it have taken to silt up a lake larger than Lake Superior, with tran-
quil deposits to fine mud and sand ? The nearest approximation towards
such a calculation is afforded by the silting up of the Lake of Geneva.
Swiss geologists have calculated from the rate of advance of the delta in
historical times, that it may have taken 90,000 or 100,000 years since the
silting process began, which could only be after the first Rhone glacier,
which once extended to the Juras, had shrunk back to the head of the lake.
This calculation may be right or wrong, but certainly a vastly longer time
must have been required to silt up a vastly larger lake to a depth of 5000
feet And if anything, one would expect the process of silting up to have
been slower, for in the Eocene period there were no glaciers, or melting
snow-fields, to accelerate the denudation which must have gone on pan
passu with the deposit If we consider the geological evidence more in
detail, we find it all pointing to the same conclusion of immense anti-

Thus, if we take the coal-measures which form only a part of one
formation the Carboniferous. Each seam of coal consists of the con-
solidated debris of a forest. With every seam there is an under-clay in
which the trees and ferns grow; and a roof of shale or sandstone deposited
on it when this floor was submerged. The bulk of the coal is frequently
composed of the microscopic spores of the ferns and club-mosses which
formed the principal vegetation of these forests. The time required is
therefore that for the accumulation of vegetable matter, consisting mainly
of fine spore-dust, to a depth sufficient, under great compression, to give
the seam of solid coal. In Nova Scotia, and other localities, the coal-
measures have a thickness of 12,000 feet, made up of seam upon seam of
coal, each with its under-clay and roof, implying a separate growth, sub-
mergence, and elevation.

Sir J. Dawson and Professor Huxley, who have studied the subject
minutely, calculate that the time represented by the coal-measures alone

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