_EVIDENCE IN SUPPORT OF THE THEORY FROM THE AGE OF THE SUN’S HEAT._
TESTIMONY OF GEOLOGY AND BIOLOGY AS TO THE AGE OF THE SUN’S HEAT.
The question which we have now to consider is—to which of the two
theories does geology lend its testimony? Will the length of time which,
according to the gravitation theory, can possibly be allotted satisfy
the requirements of geology? In short, are the facts of geology
reconcilable with the theory? If not, the theory must be abandoned.
Before the period when geologists felt that they were limited to time by
physical considerations, the most extravagant opinions prevailed in
regard to the length of geological epochs. So long as the physicist
continued to state in a loose and general way that the sun might have
been supplying our earth with heat at his present rate for the past
100,000,000 years, no very serious difficulty was felt; but when
geologists came to understand that ten or twenty millions of years were
all that could be granted to them, the condition of matters was entirely
altered. The belief that the mathematical physicist must be right in his
views as to the age of the sun’s heat, and that there is no possibility
of a longer period being admitted, seems at present to be leading
geologists towards the opposite extreme in regard to the length of
geological time. Attempts have been recently made to compress the
geological history of our globe into the narrow space allotted by the
physicist. The attempt is hopeless, as well as injurious to geological
science. What misleads is not the belief that gravitation could not
possibly afford a supply of heat sufficient for more than 20,000,000
years, for this is true; it is the belief that there was no other source
of heat than gravity.
We shall now consider the evidence which geology seems to afford as to
the age of the sun’s heat. Geology is quite competent to render aid on
this point, for the sun’s heat must be at least as old as life on this
globe; and the record of the rocks tells us when this life first
appeared. We require, however, to be able to measure the time which has
elapsed since these records were left. What we want is absolute time;
not relative time. Much has been done by geologists in regard to
relative time; but this can be of no service to us in our present
inquiry. Unfortunately very little trustworthy work has been done in the
way of determining the absolute length of geological periods. Happily,
however, great exactness of measure is not required. A rough
approximation to the truth will suffice for our present purpose. If it
can be shown to be more than fifteen or twenty millions of years since
life first appeared on the earth, it will as effectually prove that
gravitation alone could not have been the source from which the sun
derived his heat as if it were shown that that period was a thousand
times more remote. All we have to do is simply to assign an _inferior
limit_ to the age of life on the earth; and this can be effectually done
by means of the methods, imperfect though they be, which we have at
command. As the question of geological time is of some importance in
relation to our present inquiry, I shall consider it at some length.
_Testimony of Geology: method employed._—What has subsequently proved to
be a pretty successful method of measuring geological time suggested
itself to my mind during the summer of 1865. It then occurred to me that
we might obtain a tolerably accurate measure of absolute geological time
from the present rate of subaërial denudation, which might be
ascertained in the following way: The rate of subaërial denudation must
be equal to the rate at which materials are carried off the land into
the sea; and this is measured by the rate at which sediment is carried
down by our river systems. _Consequently, in order to determine the
present rate of subaërial denudation, we have only to ascertain the
quantity of sediment annually carried down by the river systems._ This
gives us the time required to remove any given quantity, say one foot,
off the face of the country. If we assume the rate to be pretty much the
same during past geological ages, we have a means of telling the time
that was occupied in removing any known thickness of strata. But as we
never can be perfectly certain that the rate is the same in both cases,
the results can, of course, be regarded as only approximately true.
Taking the quantity of sediment discharged into the sea annually by the
Mississippi river, as determined by Messrs. Brown and Dickson,[23] I
found that it amounted to one foot off the face of the country in 1,388
years, and that, at this rate of denudation, our continents, even if
they had an elevation of 1,000 feet, would not remain above sea-level
over 1,500,000 years.[24] This was an exaggerated estimate of the
quantity of sediment, for I shortly afterwards found that far more
accurate determinations were made by Messrs. Humphreys and Abbot,[25]
who were employed by the United States Government to report upon the
physics and hydraulics of the Mississippi. Messrs. Brown and Dickson had
estimated the quantity of sediment at 28,188,083,892 cubic feet, whereas
Messrs. Humphreys and Abbot found it to be only 6,724,000,000 cubic
feet, or less than one-fourth that amount. This gives one foot in 6,000
years as the rate of denudation.
[Footnote 23: _Proceedings of the American Association for the
Advancement of Science_ for 1848.]
[Footnote 24: _Philosophical Magazine_, February 1867. I was not aware
at this time that Mr. Alfred Tylor had previously applied the same
method to determine an entirely different point, viz.: how much the
sea-level is being raised by the sediment deposited on the sea-bottom.
Mr. Tylor’s paper, entitled “On Changes of the Sea-Level effected by
existing Physical Causes during stated Periods of Time,” appeared in the
_Phil. Mag._ for April 1853. Mr. Tylor came to the conclusion that the
sea-level was being raised, from this cause, about 3 inches in 10,000
years.]
[Footnote 25: _Report upon the Physics and Hydraulics of the
Mississippi._]
At this time Dr. Archibald Geikie took up the question and went into the
consideration of the subject in a most thorough manner; and it is mainly
through the instrumentality of his writings on the matter[26] that the
method under consideration has gained such wide-spread acceptance among
geologists. After an examination of nearly all that is known regarding
the amount of sediment carried down by rivers, he drew up the following
table, showing the number of years required by seven rivers to remove
one foot of rock from the general surface of their basins.
Danube 6,846 years
Mississippi 6,000 „
Nith 4,723 „
Ganges 2,358 „
Rhone 1,528 „
Hoang-Ho 1,464 „
Po 729 „
----- -----
Mean 3,378 years
This gives a mean of 3,378 years to remove one foot, or a little over
one-half the time taken by the Mississippi. This mean appears to be
generally taken as representing the average rate of subaërial denudation
of the whole earth, but it has, I fear, been rather too hastily adopted.
To estimate correctly the quantity of sediment annually discharged by a
large river is a most difficult and laborious undertaking. A perusal of
the voluminous report of Messrs. Humphreys and Abbot, extending over 690
pages, which Dr. Geikie justly styles a model of patient and exhaustive
research, will clearly show this, and at the same time prove how
skilfully and accurately the task allotted to them was performed.
[Footnote 26: _Trans. of Geol. Soc. of Glasgow_, vol. iii.; Jukes &
Geikie’s _Manual of Geology_, chap. xxv.; _Text Book of Geology_, p.
441.]
The risk of making very serious errors in computing the amount of
sediment discharged, unless proper precautions are taken, is well
illustrated in the case of the determinations made by Messrs. Brown and
Dickson, to which reference has already been made. Although their report
shows that they took great pains in order to arrive at correct
results—in fact, they computed the total annual quantity of sediment
discharged to within a cubic foot—after all, instead of being correct to
this minute quantity, they gave a total more than fourfold what it ought
to be. A somewhat similar discrepancy exists in reference to the
denudation of the basin of the Ganges. The time required to lower its
surface by one foot is, according to one estimate, 2,358 years;
according to another, 1,751; and according to a third, only 1,146 years.
The first figure is probably nearest the truth. Still, these differences
show both the difficulty of the problem and the necessity of caution in
adopting any of these results as correct.
By far the most trustworthy determinations of the whole are those of the
Mississippi by Messrs. Humphreys and Abbot, which may be relied upon as
not far from the truth. But, supposing the estimates in the foregoing
table to be perfectly correct, can we assume that their mean may be
safely taken as probably representing the average rate of denudation of
the whole earth? I would most unhesitatingly reply, Certainly not. The
Rhone and Po are full of glacier mud from the Alps; and the amount of
sediment which they carry down may give us the rate of denudation of
Switzerland, but certainly not that of the whole earth, or even of
Europe. The same may be said of the Ganges, which is charged with the
mud which it brings down from the Himalaya Mountains. The Hoang-Ho, or
Yellow River, is an exceptionally muddy river; in fact, it derives its
name from the vast quantity of yellow mud held by its waters in a state
of solution. It was probably the exceptionally muddy character of the
Po, the Rhone, the Ganges, and the Yellow River which attracted
attention, and led to observations being made of the sediment they
contain. Rivers more unsuitable than these to give us the average
denudation of the earth’s surface could not well be selected. Among the
seven rivers in the table, leaving out of account the small Scottish
stream, the Nith, with its basin of only 200 square miles, there are
only two, the Mississippi and the Danube, that drain countries which may
be regarded as in every way resembling the average condition of the
earth’s surface. I would choose the Mississippi as being superior to the
Danube, for two reasons: (1) because the rate of denudation of its basin
has been more accurately determined; and (2) because the area of its
basin not only exceeds that of the Danube as five to one, but better
fulfils the necessary conditions, as Sir Charles Lyell has so clearly
shown. “That river,” says Sir Charles, “drains a country equal to more
than half the continent of Europe, extends through twenty degrees of
latitude, and therefore through regions enjoying a great variety of
climate, and some of its tributaries descend from mountains of great
height. The Mississippi is also more likely to afford us a fair test of
ordinary denudation, because, unlike the St. Lawrence and its
tributaries, there are no great lakes in which the fluviatile sediment
is thrown down and arrested on its way to the sea.”[27] There is no
other river in the globe which to my mind better fulfils the required
conditions. It is no doubt true that the rate of denudation of the basin
of the Mississippi is probably less than that of Switzerland, Norway,
and the Himalayas, where glaciers abound, and certainly less than that
of Greenland and the Antarctic continent; but, on the other hand, this
rate is certainly much greater than that of the whole continent of
Africa, Australia, and large tracts of Asia, where the rainfall is much
smaller. One foot in 6,000 years may, therefore, I think, be safely
taken as the average rate of denudation of the whole surface of the
globe.
[Footnote 27: _Student’s Elements of Geology_, p. 91.]
_The average rate of denudation in the past probably not much greater
than in the present._—The belief has long prevailed that the rate of
denudation was much greater in past ages than it is now; but I am unable
to perceive any good grounds for concluding that such was the case at
any time since the beginning of the Palæozoic period. Various reasons
have, however, been assigned for this supposed greater rate; and to the
consideration of these I shall now very briefly refer.
It has been thought that at some remote epoch of the earth’s history,
when the moon was much nearer and the day much shorter than now, the
rate of denudation would, owing to the erosive power of the enormous
tides which would then prevail, be much greater than at the present day.
This, however, is very doubtful. There is nothing in the stratified
rocks which affords any support to the idea of great tidal waves having
swept over the land, at least since the time when life began on our
globe. Such a state of things would have destroyed all animal life. “The
Palæozoic sediments,” as Professor A. Winchell remarks, “have been
deposited, for the chief part, in quiet seas. The deep beds of
limestones and shales are spread out in sheets continent-wide, which
testify unmistakably to placid waters and slow deposition.”[28] But high
tides, not sweeping over the land, would not increase the rate of
denudation to the extent supposed. High tides silt up a river channel
more readily than they deepen it. A higher tide would probably produce a
greater destruction of sea-coast: it would tend to increase the rate of
marine denudation, but this would not materially affect the general rate
of denudation. For, as the present rate of marine denudation is to that
of subaërial denudation only as 1 to about 1,700,[29] it would take a
very large increase in the rate of marine denudation to affect sensibly
the general result. Suppose the rate of marine denudation to have been,
for example, ten times as great during the Palæozoic age as it is now
(which it certainly was not), it would only have shortened the time
required to effect a given amount of denudation of the whole earth by 9
years in 1,700, i.e. by little more than one-half per cent.
[Footnote 28: _World Life_, p. 265.]
[Footnote 29: See _Climate and Time_, p. 337.]
Again, it is assumed that the greater rate of terrestrial rotation in
the early ages would produce certain influences which would in turn
bring about a greater amount of denudation. The rate of rotation has
been slowly decreasing for ages, and in Palæozoic times it must, of
course, have been greater than at present. A more rapid rotation would
increase the velocity of the trade and anti-trade winds, and would thus
tend to augment the action of those meteorological agents chiefly
effective in the work of subaërial denudation. Here again the testimony
of geology is negative. We have no geological grounds to conclude that
the winds of Palæozoic times were stronger than those at the present
day. The heat was no doubt greater, and perhaps there was more rain;
but, on the other hand, there would be less frost, snow, ice, and other
denuding agents.
There is one cause which would, perhaps, be more effective than any of
the foregoing: viz. the periodic occurrence of glacial epochs. When a
country is buried under ice, the erosion of the surface is great. But it
must be borne in mind that the influence of rain, rivers, and other
denuding agents now in operation would then, in the glaciated regions,
be almost _nil_. Besides, the greater part of the materials ground off
the rocks would be left on the land; and, when the ice disappeared, it
would be found in the form of a thick mantle of boulder clay—a mantle
which would protect the rocky surface of the country for thousands and
tens of thousands of years from further denudation. This is shown by the
fine striæ on the rocky surface, made perhaps more than 50,000 years
ago, remaining under the boulder clay as perfect as the day on which
they were engraved. But, more than all this, a very considerable part of
the 1 foot presently being removed off the country in 6,000 years
consists of the loose materials belonging to the glacial epoch, such as
sands, gravels, and boulder clay, which are being swept off the surface
by rain and river action. Were it not for this, the present rate of
subaërial denudation would not be so high as it actually is. Taking all
things into consideration, it is, I think, obvious that the average rate
of denudation since the beginning of Palæozoic times was probably not
much greater than at the present day.
_How the method has been applied._—Having determined what appears to be
the probable average rate of subaërial denudation, we may now proceed to
consider the way in which this rate has been applied to measure past
geological time. There are two ways in which it may be applied for this
purpose. It may (1) be applied directly: knowing the thickness of strata
which may have been removed by denudation, we can easily tell, from that
rate, the time it required to effect their removal. If we have evidence,
for example, that at some epoch 1,000 feet of stratified rock were
carried away, then, on the assumption that the rate of denudation was
the same at that epoch as now, we have 1,000 × 6,000 = 6,000,000 years
as the required time. (2) It may be applied indirectly: knowing the
thickness of the strata, we may estimate the time required for their
formation. This is the way in which it has usually been applied, but, as
we shall see, it is the less satisfactory way of the two.
Dr. A. Geikie gives the land area of the globe as 52,000,000 square
miles, and that of water as 144,712,000 square miles.[30] We may thus
take the proportion of land to water roughly as 1 to 3; about
one-quarter of the earth’s surface being land, and three-quarters water.
One foot, therefore, removed off the surface of the land would cover the
whole globe with a layer 3 inches thick, or the entire sea-bottom with a
layer 4 inches thick.
[Footnote 30: _Physical Geography_, p. 103.]
If we knew the total quantity of stratified rock on the globe, we could
easily tell the time that would be required for its formation. Most
geologists would, I believe, be inclined to admit that, if spread
uniformly over the entire globe, it would form a layer of at least 1,000
feet in thickness. In such a case the time required for its deposition
would be as follows:
1,000 × 6,000 × 4 = 24,000,000 years.
This would not, however, represent the age of the stratified rocks. It
would only represent the time required to deposit the rocks which we
have assumed to be now in existence. The greater mass of sedimentary
rocks has been formed out of previously existing sedimentary rocks, and
these again out of sedimentary rocks still older. The oldest known
sedimentary rocks are the Laurentian; but these are believed by
geologists to have been formed from still older sedimentary rocks. It is
therefore evident that the materials composing our stratified beds must
have passed through many cycles of destruction and re-formation. The
materials of some of the recent formations, for example, may have passed
through denudation and deposition a dozen of times over.[31] The time
required to have deposited at a given rate the present existing mass of
sedimentary rocks is probably but a small fraction of the time required
to have deposited at the same rate the total mass that has actually been
formed. Few geologists, I think, who will duly reflect on the subject,
will deem it too much to say that the present existing stratified rocks
have on an average passed at least thrice through the cycle of
destruction and re-formation. If this be admitted, then the 1,000 feet
of stratified rock represent, not a period of 24,000,000 years, but a
period three times as great, viz. 72,000,000 years.
[Footnote 31: It is this destruction of the stratified rocks which makes
it so difficult to detect the marks of former glacial epochs, and which
has led to such prevailing misconceptions regarding the evidence which
we ought to expect of those epochs. See paper read before the Geological
Society, “On Prevailing Misconceptions regarding the Evidence which we
ought to expect of former Glacial Periods,” January 23, 1889.]
It is impossible to tell from geological data the actual age of the
stratified rocks; but this is not required. What we require is, as
already stated, not their _actual_ age, but an _inferior limit_ to that
age.
_Method as applied by Professor Haughton._—Professor Haughton estimates
the mass of the stratified rocks down to the time of the Miocene
Tertiary period as being 177,200 feet in thickness, and covering an area
equal to that of the sea. The present rate of subaërial denudation he
considers to be equal to one foot removed off the surface of the land in
3,090 years. If the proportion of land to water be taken as 52 to 145,
it thus requires 8,616 years to deposit one foot of sediment over the
bed of the ocean, and consequently this is the rate at which strata are
at present being formed. This would give 8,616 × 177,200 = 1,526,750,000
years for the age of the stratified rocks. But he assumes the rate of
denudation to have been _ten times_ greater in geological time than at
present. This consequently reduces the age of the rocks to 152,675,000
years. By adding one-third for the time which has elapsed since the
Miocene Tertiary period he gets 200,000,000 years as a minimum length of
geological time.[32]
[Footnote 32: _Physical Geography_, p. 94.]
The validity of this result rests upon what appear to me to be two very
doubtful assumptions. It is assumed in his calculations that the total
amount of strata formed during past ages (not the amount presently
remaining) was equal to a mass 177,200 feet in thickness, covering the
entire area of the ocean. This is certainly doubtful. It may have been
as great, for anything that can be proved to the contrary; but we have
no evidence that it was so. Certainly there is no evidence that the rate
of subaërial denudation during past ages was ever ten times as great as
it is now. But how is a length of 200,000,000 years to be reconciled
with the age of the sun’s heat? The stratified rocks may be as old as
this, but assuredly they are not if gravitation was the only source from
which the sun derived his energy.
_Method as applied by Mr. Alfred R. Wallace._—Mr. Wallace adopts
Professor Haughton’s estimate of 177,200 feet for the maximum thickness
of the sedimentary rocks. But, instead of supposing, like Professor
Haughton, the products of denudation to be uniformly spread over the
entire sea-bottom, he supposes them spread over a belt of merely 30
miles broad, extending along the entire coast-line of the globe, which
he estimates at 100,000 miles. This gives an area of 3,000,000 square
miles on which the denuded matter of the whole land area of 57,000,000
square miles is deposited. These two areas are to one another as 1 to
19, and thus it follows that deposition goes on 19 times as fast as
denudation. The rate of denudation he takes as one foot removed off the
surface of the land in 3,000 years, so that the rate of deposition would
be about one foot in 158 years, and consequently the time required to
deposit the 177,200 feet of rock would be
177,200 × 158 = 27,997,600 years.
This is a period double what the gravitation theory of the source of the
sun’s energy can afford. And if the rate of denudation be taken at one
foot in 6,000 years, which is, as we have seen, probably nearer the
truth, then this would make the age of the stratified rocks 56,000,000
years.
There seems to be a little ambiguity about Mr. Wallace’s result. Do the
177,200 feet represent the quantity of rock which presently exists, or
do they represent the total quantity which has been formed during all
past ages? If the former, then the 28,000,000 years are but a fraction
of the time which must have been required; for, as we have been shown,
the materials composing the stratified rocks have, on an average, been
deposited at least three or four times over. If, on the other hand, the
thickness is meant to represent the total quantity of rock which has
been formed during the whole of past geological time, then the question
arises, by what means could this quantity possibly be ascertained? In
other words, how was the relation between the present quantity and the
total quantity ascertained? But in either case the result is wholly
irreconcilable with the gravitation theory of the source of the sun’s
heat.
_Method as applied directly._—We have seen that it is impossible to
determine the actual age of the earth from the stratified rocks, even if
we knew with perfect accuracy their present total amount. We have also
seen that from the rate of deposition we cannot fix with any degree of
certainty a minimum value for the age of these rocks. We can, however,
by means of the first or direct application of the method, assign with
tolerable accuracy, as was shown on a former occasion,[33] a minimum age
to the earth. We can be far more certain of the time which must have
been required to remove by denudation, say, a thousand feet of rock than
we can possibly be of the time required to have deposited a thousand
feet of sediment. The thousand feet of sediment may, under certain
conditions, have been deposited in a hundred years, while under other
conditions they may have required a million of years. In fact, nothing
can be more uncertain than the rate of deposition: it depends upon such
a multitude of circumstances. At the mouth of a great river, for
example, a foot of sediment may be deposited in a single day, whereas in
some places, as in mid-ocean, it may require a million of years to
deposit the same amount. But in reference to subaërial denudation no
such uncertainty exists.
[Footnote 33: _Quart. Journ. of Science_, July 1877; _Climate and
Cosmology_, chap. xvii.]
The utter inadequacy of a period of 20,000,000 years for the age of our
earth is demonstrable from the enormous thickness of rock which is known
to have been removed off certain areas by denudation. I shall now
briefly refer to a few of the many facts which might be adduced on this
point.
_Evidence from “faults.”_—One plain and obvious method of showing the
great extent to which the general surface of the country has been
lowered by denudation is furnished, as is well known, by the way in
which the inequalities of surface produced by faults or dislocations
have been effaced. It is quite common to meet with faults where the
strata on the one side have been depressed several hundreds—and in some
cases thousands—of feet below those on the other; but we seldom find any
indications of such on the surface, the inequalities on the surface
having been all removed by denudation. Now, in order to effect this, a
mass of rock must have been removed equal in thickness to the extent of
the dislocation. The following are a few examples of large faults:
The great Irwell fault, described by Professor Hull,[34] which stretches
from the Mersey west of Stockport to the north of Bolton, has a throw of
upwards of 3,000 feet.
Some remarkable faults have been found by Professor Ramsay in North
Wales. For example, near Snowdon, and about a mile E.S.E. of Beddgelert,
there is a fault with a downthrow of 5,000 feet; and in the Berwyn
Hills, between Bryn-mawr and Post-gwyn, there is one of 5,000 feet. In
the Aran Range there is a great fault, designated the Bala fault, with a
downthrow of 7,000 feet. Again, between Aran Mowddwy and Careg Aderyn
the displacement of the strata amounts to no less than from 10,000 to
11,000 feet.[35] Here we have evidence that a mass of rock, varying from
one to two miles in vertical thickness, must have been denuded in many
places from the surface of the country in North Wales.
The fault which passes along the east side of the Pentlands is estimated
to have a throw of upwards of 3,000 feet.[36] Along the flank of the
Grampians a great fault runs from the North Sea at Stonehaven to the
estuary of the Clyde, throwing the Old Red Sandstone on end sometimes
for a distance of two miles from the line of dislocation. The amount of
the displacement, Dr. A. Geikie[37] concludes, must in some places be
not less than 5,000 feet, as indicated by the position of occasional
outliers of conglomerate on the Highland side of the fault.
The great fault crossing Scotland from near Dunbar to the Ayrshire
coast, which separates the Silurians of the South of Scotland from the
Old Red Sandstone and Carboniferous tracts of the North, has been found
by Mr. B. N. Peach, of the Geological Survey,[38] to have in some places
a throw of fully 15,000 feet. This great dislocation is older than the
Carboniferous period, as is shown by the entire absence of any Old Red
Sandstone on the south side of the fault, and by the occurrence of the
Carboniferous Limestone and Coal-measures lying directly on the Silurian
rocks. We obtain here some idea of the enormous amount of denudation
which must have taken place during a comparatively limited geological
epoch. So vast a thickness of Old Red Sandstone could not, as Mr. Peach
remarks, “have ended originally where the fault now is, but must have
swept southwards over the Lower Silurian uplands. Yet these thousands of
feet of sandstones, conglomerates, lavas, and tuffs were so completely
removed from the south side of the fault previous to the deposition of
the Carboniferous Limestone series and the Coal-measures, that not a
fragment of them is anywhere to be seen between these latter formations
and the old Silurian floor.”[39] This enormous thickness of nearly three
miles of Old Red Sandstone must have been carried away during the period
which intervened between the deposition of the lower members of the
Lower Old Red Sandstone and the accumulation of the Carboniferous
Limestone.
Near Tipperary, in the south of Ireland, there is a dislocation of the
strata of not less than 4,000 feet,[40] which brings down the
Coal-measures against the Silurian rocks. Here 1,000 feet of Old Red
Sandstone, 3,000 feet of Carboniferous Limestone, and 800 feet of
Coal-measures have been removed by denudation off the Silurian rocks.
Not only has this immense thickness of beds been carried away, but the
Silurian itself on which they rested has been eaten down in some places
into deep valleys several hundreds of feet below the surface on which
the Old Red Sandstone rested.
[Footnote 34: _Mem. Geol. Survey of Lancashire_, 1862.]
[Footnote 35: _Mem. Geol. Survey of Great Britain_, vol. iii.]
[Footnote 36: _Memoir_ to Sheet 32, Geol. Survey Map of Scotland.]
[Footnote 37: _Nature_, vol. xiii. p. 390.]
[Footnote 38: _Explanation_ to Sheet 15, Geol. Survey Map of Scotland.]
[Footnote 39: I have been informed by Mr. Peach that since the above was
written additional light has been cast on this immense fault. It has
been found, he says, that the fault consists of two sub-parallel
branches, the more southerly of which has the effect of bringing the
rocks of the Upper Silurian age against the Lower Silurian beds. The
northern branch brings the upper division of the Lower Old Red
Sandstones, in turn, against the Upper Silurian rocks. This, Mr. Peach
remarks, does not in the least invalidate the reasoning as to the amount
of material removed by denudation from this region in the time
specified. In fact, it shows, he says, that a greater amount must have
been removed than was at first suspected.]
[Footnote 40: Jukes’s and Geikie’s _Manual of Geology_, p. 441.]
Turning to the American continent, we find the amount of rock removed to
be even still greater. In the Valley of Thessolon, to the north of Lake
Huron, there is a dislocation of the strata to the extent of 9,000
feet.[41]
In front of the Chilowee Mountains there is a vertical displacement of
the strata of more than 10,000 feet.[42] Professor H. D. Rogers found in
the Appalachian coal-fields faults ranging from 5,000 feet to more than
10,000 feet of displacement.
In the Nova Scotia coal-fields one or two miles in thickness of strata
have been removed in some places.[43]
A great fracture runs along the axis of the Sierra Nevada for 300 miles,
accompanied by a dislocation of 3,000 to 10,000 feet.[44]
The anticlinal of the Park Range of the Rocky Mountains was cleft down
the axis, and the eastern half depressed 10,000 feet. And Mr. J. P.
Lesley gives an account of a fault in the Appalachians of not less than
20,000 feet, bringing the upper Devonian strata on the one side opposite
to the lowest Cambrian on the other.[45]
A fault with a vertical displacement of 20,000 feet was found in the
Uinta Mountains.[46]
In the Aqui range of mountains, Utah, there is a fault determined by Mr.
S. F. Emmons to be at least 10,000 feet.[47]
The Grand Cañon of Colorado, in some places 4,000, 5,000, and 6,000 feet
in depth, is cut, says Professor A. Winchell, in a plateau which has
itself been lowered by erosion to the extent of 10,000 feet; and this
plateau occupies an area of 13,000 to 15,000 square miles.[48]
The Grand “Wash Fault,” Colorado, has a downthrow to the west of 6,000
feet. The “Hurricane Fault,” close to it, has displaced the strata to
the extent of over 12,000 feet.[49]
In the Valley of East Tennessee, Appalachian Mountains, it has been
shown by Mr. J. P. Lesley that as much as 35,000 feet of rock have been
removed by denudation. But this being from an anticlinal arch, it does
not, of course, afford any measure of the extent of the denudation of
the surrounding country. Major J. W. Powell, Director of the U.S. Geol.
Survey, found that under a similar condition as much as three and a half
miles of strata have been removed by denudation from the top of
anticlinal beds in the Uinta Mountains.[50]
Probably the most enormous displacement of strata which has yet been
found is that of the “Wahsatch Fault,” Utah. This fault is about 100
miles in length, crossing the fortieth parallel of latitude from north
to south, with a downthrow to the west of not less than 40,000 feet. So
clear is the evidence regarding this fault that Mr. Clarence King says
“that there can be no doubt of the quantitative correctness of my
reading of this tremendous dislocation.”[51]
There are other modes than the foregoing by means of which geologists
are enabled to measure the thickness of strata which may have been
removed in places off the present surface of the country. Into the
details of these I need not here enter; but I may give a few examples of
the enormous extent to which the country, in some places, has been found
to have been lowered by denudation.
Dr. A. Geikie has shown[52] that the Pentlands must at one time have
been covered with Carboniferous rocks, upwards of a mile in thickness,
which have all been removed by denudation.
In the Bristol coal-fields, between the river Avon and the Mendips, Sir
Andrew C. Ramsay has shown[53] that about 9,000 feet of Carboniferous
strata have been removed by denudation from the present surface.
Between Bendrick Rock and Garth Hill, South Glamorganshire, a mass of
Carboniferous and Old Red Sandstone, of upwards of 9,000 feet, has been
removed. At the Vale of Towy, Caermarthenshire, about 6,000 feet of
Silurian and 5,000 feet of Old Red Sandstone—in all about 11,000
vertical feet—have been swept away. Between Llandovery and Aberaeron a
mass of about 12,000 vertical feet of the Silurian series has been
removed by denudation. Between Ebwy and the Forest of Dean, a distance
of upwards of 20 miles, a thickness of rock varying from 5,000 to 10,000
feet has been abstracted.
Prof. Hull found[54] on the northern flanks of the Pendle Range,
Lancashire, the Permian beds resting on the denuded edges of the
Millstone Grit, and these were again observed resting on the Upper
Coal-measures south of the Wigan coal-field. Now from the known
thickness of the Carboniferous series in this part of Lancashire he was
enabled to calculate approximately the quantity of Carboniferous strata
which must have been carried away between the period of the Millstone
Grit and the deposition of the Permian beds, and found that it actually
amounted to no less than 9,900 feet. He also found in the Vale of
Clitheroe, and at the base of the Pendle Range, that the Coal-measures,
the whole of the Millstone Grit, the Yoredale series, and part of the
Carboniferous Limestone, amounting in all to nearly 20,000 feet, had
been swept away—an amount of denudation which, as Prof. Hull remarks,
cannot fail to impress us with some idea of the prodigious lapse of time
necessary for its accomplishment.
[Footnote 41: _Geology of Canada_, 1863, p. 61.]
[Footnote 42: Safford’s _Geology of Tennessee_, p. 309.]
[Footnote 43: Lyell’s _Student’s Manual_, chap. xxiii.]
[Footnote 44: _Geological Studies_, by Prof. A. Winchell, p. 165.]
[Footnote 45: _Geological Studies_, pp. 93, 163.]
[Footnote 46: Powell’s _Geology of the Uinta Mountains_.]
[Footnote 47: _Geological Exploration of the Fortieth Parallel_, vol.
ii. p. 456.]
[Footnote 48: _Geological Studies_, p. 92; see also Dutton’s _Tertiary
History of the Cañon District_.]
[Footnote 49: _Tertiary History of the Cañon District_, pp. 20, 113;
_Second Annual Report, U.S. Geol. Survey_, p. 125.]
[Footnote 50: Powell’s _Geology of Uinta Mountains_.]
[Footnote 51: _Geological Exploration of the Fortieth Parallel_, vol. i.
p. 745.]
[Footnote 52: _Memoir_ to Sheet 32, Geol. Survey of Scotland.]
[Footnote 53: _Denudation of South Wales._ _Memoirs of Geol. Survey_,
vol. i.]
[Footnote 54: _Quart. Journ. Geol. Soc._ vol. xxiv. p. 323.]
It may be observed that, enormous as is the amount of denudation
indicated by the foregoing figures, these figures do not represent in
most cases the actual thickness of rock removed from the surface. We are
necessitated to conclude that a mass of rock equal to the thickness
stated must have been removed, but we are in most cases left in
uncertainty as to the total thickness which has actually been carried
away. It cannot be imagined that these great disruptions occurred first
when the surface became subject to denuding agencies, or that denudation
ceased to operate precisely when the inequality was smoothed away.
Moreover, during the time the surface on one side of the fault was being
reduced, some amount of denudation must also have been in progress on
the other and lower side. In the case of a fault, for example, with a
displacement of, say, one mile, where no indication of it is seen at the
surface of the ground, we know that on one side of the fault a thickness
of rock equal to one mile must have been denuded, but we do not know how
much more than that may have been removed. For anything which we know to
the contrary, hundreds of feet of rock may have been removed before the
dislocation took place, and as many more hundreds after all indications
of dislocation had been effaced at the surface.
But it must be observed that the total quantity of rock which has been
removed from the _present_ surface of the land is evidently small in
proportion to the total quantity removed during the past history of our
globe. For those thousands and thousands of feet of rock which have been
denuded were formed out of the waste of previously existing rocks, just
as these had been formed out of the waste of yet older rock-masses. In
short, as a general rule, the rocks of one epoch have been formed out of
those of preceding periods, and go themselves to form those of
subsequent epochs.
In many of the cases of enormous denudation to which we have referred,
the erosion has been effected during a limited geological epoch. We
have, for example, seen that upwards of a mile in thickness of
Carboniferous rock has been denuded in the area of the Pentlands. But
the Pentlands themselves, it can be proved, existed as hills, in much
their present form, before the Carboniferous rocks were laid down over
them; and as they are of Lower Old Red Sandstone age, and have been
formed by denudation, they must consequently have been carved out of the
solid rock between the period of the Old Red Sandstone and the beginning
of the Carboniferous age. This affords us some conception of the immense
lapse of time represented by the Middle and Upper Old Red Sandstone
periods.
Again, in the case of the great fault separating the Silurians of the
south of Scotland from the Old Red Sandstone tracts lying to the north,
a thickness of the latter strata of probably more than a mile, as we
have seen, must have been removed from the ground to the south of the
fault before the commencement of the Carboniferous period. And again, in
the case of the Lancashire coal-fields, to which reference has been
made, nearly two miles in thickness of strata had been removed in the
interval which elapsed between the Millstone Grit and the Permian
periods.
_Time required to effect the foregoing amount of denudation._—To lower
the country one mile by denudation would therefore require, according to
the rate which we have already established, about 15,000,000 years; but
we have seen that a thickness of rock more than equal to that must have
been swept away since the Carboniferous period; and even during the
Carboniferous period itself more than a mile in thickness of strata in
many places was removed. Again, there can be no doubt whatever that the
amount of rock removed during the Old Red Sandstone period was much
greater than one mile; for we know perfectly well that over large tracts
of country nearly a mile in thickness of rock was carried away _between
the period of the Lower Old Red Sandstone and the Carboniferous epoch_.
Further, all geological facts go to show that the time represented by
the Lower Old Red Sandstone itself must have been enormous.
Now, three miles of rock removed since the commencement of the Old Red
Sandstone period (which, doubtless, is an under-estimate) would give us
45,000,000 years.
Again, going farther back, we find the lapse of time represented by the
Silurian period to be even more striking than that of the Old Red
Sandstone. The unconformities in the Silurian series indicate that many
thousands of feet of these strata were denuded before overlying members
of the same great formations were deposited. And again, this immense
formation was formed in the ocean by the slow denudation of pre-existing
Cambrian continents, just as these had been built up out of the ruins of
the still prior Laurentian land. And even here we do not reach the end
of the series, for the Laurentians themselves resulted from the
denudation, not of the primary rocks of the globe, but of previously
existing sedimentary and probably igneous rocks, of which, perhaps, no
recognisable portion now remains.
It is the opinion of Mr. Darwin, and also of Mr. Wallace, that the
geological time which elapsed anterior to the Cambrian period was as
long as the whole interval from that period to the present day. This is
an opinion which I suppose is supported by most geologists. But, to err
on the safe side, I shall assume that the time which had elapsed prior
to the Old Red Sandstone was not greater than the time which has elapsed
since that period. Even on this assumption we have at least 90,000,000
years as a minimum duration of geological time.
_Age of the earth as determined by the date of the glacial
epoch._—Professor A. Winchell, by a most careful examination of the
probable relative lengths of geological periods, arrived at the
conclusion that the time which elapsed since the beginning of the
_glacial epoch_ is to the time which has elapsed since the
solidification of the earth’s surface as 1 to 250.[55] According to the
eccentricity theory of the cause of the glacial epoch, that epoch began
240,000 years ago; consequently this makes the time since solidification
took place 60,000,000 years, a period which agrees roughly with that
deduced from denudation, and is so far presumptive evidence of the truth
of that theory of the cause of the glacial cold.
[Footnote 55: _World Life_, p. 369.]
_Testimony of Biology_.—The time required for the variation and
modification of organic forms has, Mr. Alfred R. Wallace states, been
generally considered to require an even longer series of ages than might
satisfy the demands of physical geology. This is a point, however, on
which I am not qualified to venture an opinion. I shall simply refer to
the views held by our highest authorities on the subject.
Referring to Professor Huxley’s anniversary address to the Geological
Society in 1870, where he shows that almost all the higher forms of life
must have existed during the Palæozoic period, Mr. Wallace says: “Thus,
from the fact that almost the whole of the Tertiary period has been
required to convert the ancestral Orohippus into the true horse, he,
Professor Huxley, believes that, in order to have time for the much
greater change of the ancestral ungulata into the two great odd-toed and
even-toed divisions (of which change there is no trace even among the
earliest Eocene mammals), we should require a large portion, if not the
whole, of the Mesozoic or Secondary period. Another case is furnished by
the bats and whales, both of which strange modifications of the
mammalian type occur perfectly developed in the Eocene formation. What
countless ages back must we, then, go for the origin of these groups,
the whales from some ancestral carnivorous animal, and the bats from the
insectivora! And even then we have to seek for the common origin of
carnivora, insectivora, ungulata, and marsupials at a far earlier
period; so that, on the lowest estimate, we must place the origin of the
mammalia very far back in Palæozoic times.”[56]
“If the very small differences,” says Professor Huxley,[57] “which are
observable between the _Crocodilia_ of the older Mesozoic formations and
those of the present day furnish any sort of approximation towards an
estimate of the average rate of change among the _Sauropsida_, it is
almost appalling to reflect how far back in Palæozoic times we must go
before we can hope to arrive at that common stock from which the
_Crocodilia_, _Lacertilia_, _Ornithoscelida_, and _Plesiosauria_, which
had attained so great a development in the Triassic epoch, must have
been derived.
“The _Amphibia_ and _Pisces_ tell the same story. There is not a single
class of vertebrated animals which, when it first appears, is
represented by analogues of the lowest known members of the same class.
Therefore, if there is any truth in the doctrine of evolution, every
class must be vastly older than the first record of its appearance upon
the surface of the globe. But if considerations of this kind compel us
to place the origin of vertebrated animals at a period sufficiently
distant from the Upper Silurian, in which the first Elasmobranchs and
Ganoids occur, to allow of the evolution of such fishes as these from a
vertebrate as simple as the _Amphioxus_, I can only repeat that it is
appalling to speculate upon the extent to which that origin must have
preceded the epoch of the first recorded appearance of vertebrate life.”
“If the theory be true,” says Mr. Darwin, “it is indisputable that
before the lowest Cambrian stratum was deposited long periods elapsed—as
long as, or probably far longer than, the whole interval from the
Cambrian age to the present day; and that during these vast periods the
world swarmed with living creatures.”[58]
In referring to the abundant and well-developed fauna of the Cambrian
period, Sir Andrew C. Ramsay remarks:[59] “In this earliest known varied
life we find no evidence of its having lived near the beginning of the
Zoological series. In a broad sense, compared with what must have gone
before, both biologically and physically, all the phenomena connected
with this old period seem, to my mind, to be quite of a recent
description; and the climates of seas and lands were of the very same
kind as those that the world enjoys at the present day—one proof of
which, in my opinion, is the existence of great glacial boulder beds in
the Lower Silurian strata of Wigtonshire, west of Loch Ryan.”
Professor Haeckel remarks that “Darwin’s theory, as well as that of
Lyell, renders the assumption of immense periods absolutely necessary.
If the theory of development be true at all, there must certainly have
elapsed immense periods, utterly inconceivable to us.”
In reference to the foregoing, Mr. Wallace says:[60] “These opinions,
and the facts on which they are founded, are so weighty that we can
hardly doubt that, if the time since the Cambrian epoch is correctly
estimated at 200,000,000 of years,[61] the date of the commencement of
life on the earth cannot be much less than 500,000,000; while it may not
improbably have been longer, because the reaction of the organism under
changes of the environment is believed to have been less active in low
and simple than in high and complex forms of life, and thus the
processes of organic development may for countless ages have been
excessively slow.”
[Footnote 56: _Island Life_, p. 204.]
[Footnote 57: _Quart. Journ. of Geol. Soc._ vol. xxvi. p. 53.]
[Footnote 58: _Origin of Species_, p. 286.]
[Footnote 59: _Proceedings of the Royal Society_, No. 152, 1874, p.
342.]
[Footnote 60: _Island Life_, p. 205.]
[Footnote 61: Of course, Mr. Wallace does not believe that it is
actually 200,000,000 years since the Cambrian period.]
I think it must now be perfectly evident that the facts both of geology
and of biology are utterly irreconcilable with the theory that the sun’s
heat was derived from the condensation of its mass by gravitation; and
that the mistake in regard to geological time has been committed by the
physicist, and not by the geologist. The grounds upon which the
geologists and the biologists found the conclusion that it is more than
20 or 30 millions of years since life began on the earth are far more
certain and reliable than the grounds upon which the physicist concludes
that the period must be less. The only real ground that the physicist
has is that according to the theory which he holds of the origin of the
sun’s heat a longer period is not possible. This might be considered
good evidence were no other theory possible; but there is another
theory, which accords with all the facts, and consequently has a strong
presumption in its favour.