The recent discovery of the Cretaceous f_Climate and Time in Their Geological Relations A Theory of Secular Changes of the Earth's Climate

that during that period a mild and temperate condition of climate

must have prevailed in that continent up to high latitudes. “This

formation in Greenland,” says Dr. Robert Brown, “has only been recently

separated from the Miocene formation, with which it is associated and

was supposed to be a part of. It is, as far as we yet know, only found

in the vicinity of Kome or Koke, near the shores of Omenak Fjord, in

about 70° north latitude, though traces have been found elsewhere

on Disco, &c. The fossils hitherto brought to Europe have been very

few, and consist of plants which are now preserved in the Stockholm

and Copenhagen Museums. From these there seems little doubt that the

age assigned to this limited deposit (so far as we yet know) by the

celebrated palæontologist, Professor Oswald Heer, of Zurich, is the

correct one.”[189] Dr. Brown gives a list of the Cretaceous flora found

in Greenland.

EOCENE PERIOD.

_Switzerland._—In a coarse conglomerate belonging to the “_flysch_”

of Switzerland, an Eocene formation, there are found certain immense

blocks, some of which consist of a variety of granite which is not

known to occur _in situ_ in any part of the Alps. Some of the blocks

are 10 feet and upwards in length, and one at Halekeren, at the Lake of

Thun, is 105 feet in length, 90 feet in breadth, and 45 feet in height.

Similar blocks are found in the Apennines. These unmistakably indicate

the presence of glaciers or floating ice. This conclusion is further

borne out by the fact that the “_flysch_” is destitute of organic

remains. But the hypothesis that these huge masses were transported

to their present sites by glaciers or floating ice has been always

objected to, says Sir Charles Lyell, “on the ground that the Eocene

strata of Nummulitic age in Switzerland, as well as in other parts of

Europe, contain genera of fossil plants and animals characteristic of a

warm climate. And it has been particularly remarked,” he continues, “by

M. Desor that the strata most nearly associated with the ‘_flysch_’ in

the Alps are rich in echinoderms of the _Spatangus_ family which have a

decided tropical aspect.”[190]

But according to the theory of Secular Changes of Climate, the very

fact that the “_flysch_” is immediately associated with beds indicating

a warm or even tropical condition of climate, is one of the strongest

proofs which could be adduced in favour of its glacial character, for

the more severe a cold period of a glacial epoch is, the warmer will be

the periods which immediately precede and succeed. These crocodiles,

tortoises, and tropical flora probably belong to a warm Eocene

inter-glacial period.

MIOCENE PERIOD.

_Italy._—We have strong evidence in favour of the opinion that a

glacial epoch existed during the Miocene period. It has been shown

by M. Gastaldi, that during that age Alpine glaciers extended to the

sea-level.

Near Turin there is a series of hills, rising about 500 or 600 feet

above the valleys, composed of beds of Miocene sandstone, marl, and

gravel, and loose conglomerate. These beds have been carefully examined

and described by M. Gastaldi.[191] The hill of the Luperga has been

particularly noticed by him. Many of the stones in these beds are

striated in a manner similar to those found in the true till or boulder

clay of this country. But what is most remarkable is the fact that

large erratic blocks of limestone, many of them from 10 to 15 feet in

diameter, are found in abundance in these beds. It has been shown by

Gastaldi that these blocks have all been derived from the outer ridge

of the Alps on the Italian side, namely, from the range extending from

Ivrea to the Lago Maggiore, and consequently they must have travelled

from twenty to eighty miles. So abundant are these large blocks, that

extensive quarries have been opened in the hills for the sake of

procuring them. These facts prove not only the existence of glaciers

on the Alps during the Miocene period, but of glaciers extending to

the sea and breaking up into icebergs; the stratification of the beds

amongst which the blocks occur sufficiently indicating aqueous action

and the former presence of the sea.

That the glaciers of the Southern Alps actually reached to the sea,

and sent their icebergs adrift over what are now the sunny plains of

Northern Italy, is sufficient proof that during the cold period of

Miocene times the climate must have been very severe. Indeed, it may

well have been as severe as, if not even more excessive than, the

intensest severity of climate experienced during the last great glacial

epoch.

_Greenland._—Of the existence of warm conditions during Miocene times,

geology affords us abundant evidence. I shall quote the opinion of Sir

Charles Lyell on this point:—

“We know,” says Sir Charles, “that Greenland was not always covered

with snow and ice; for when we examine the tertiary strata of Disco

Island (of the Upper Miocene period), we discover there a multitude

of fossil plants which demonstrate that, like many other parts of the

arctic regions, it formerly enjoyed a mild and genial climate. Among

the fossils brought from that island, lat. 70° N., Professor Heer has

recognised _Sequoia Landsdorfii_, a coniferous species which flourished

throughout a great part of Europe in the Miocene period. The same

plant has been found fossil by Sir John Richardson within the Arctic

Circle, far to the west on the Mackenzie River, near the entrance of

Bear River; also by some Danish naturalists in Iceland, to the east.

The Icelandic surturband or lignite, of this age, has also yielded a

rich harvest of plants, more than thirty-one of them, according to

Steenstrup and Heer, in a good state of preservation, and no less than

fifteen specifically identical with Miocene plants of Europe. Thirteen

of the number are arborescent; and amongst others is a tulip-tree

(_Liriodendron_), with its fruit and characteristic leaves, a plane

(_Platanus_), a walnut, and a vine, affording unmistakable evidence

of a climate in the parallel of the Arctic Circle which precludes the

supposition of glaciers then existing in the neighbourhood, still less

any general crust of continental ice like that of Greenland.”[192]

At a meeting of the British Association, held at Nottingham in August

1866, Professor Heer read a valuable paper on the “Miocene Flora of

North Greenland.” In this paper some remarkable conclusions as to the

probable temperature of Greenland during the Miocene period were given.

Upwards of sixty different species brought from Atanekerdluk, a place

on the Waigat opposite Disco, in lat. 70° N., have been examined by him.

A steep hill rises on the coast to a height of 1,080 feet, and at

this level the fossil plants are found. Large quantities of wood in

a fossilized or carbonized condition lie about. Captain Inglefield

observed one trunk thicker than a man’s body standing upright. The

leaves, however, are the most important portion of the deposit. The

rock in which they are found is a sparry iron ore, which turns reddish

brown on exposure to the weather. In this rock the leaves are found, in

places packed closely together, and many of them are in a very perfect

condition. They give us a most valuable insight into the nature of the

vegetation which formed this primeval forest.

He arrives at the following conclusions:—

1. _The fossilized plants of Atanekerdluk cannot have been drifted from

any great distance. They must have grown on the spot where they were

found._

This is shown—

(_a_) By the fact that Captain Inglefield and Dr. Ruik observed trunks

of trees standing upright.

(_b_) By the great abundance of the leaves, and the perfect state of

preservation in which they are found.

(_c_) By the fact that we find in the stone both fruits and seeds of

the trees whose leaves are also found there.

(_d_) By the occurrence of insect remains along with the leaves.

2. _The flora of Atanekerdluk is Miocene._

3. _The flora is rich in species._

4. _The flora proves without a doubt that North Greenland, in the

Miocene epoch, had a climate much warmer than its present one. The

difference must be at least_ 29° F.

Professor Heer discusses at considerable length this proposition. He

says that the evidence from Greenland gives a final answer to those

who objected to the conclusions as to the Miocene climate of Europe

drawn by him on a former occasion. It is quite impossible that the

trees found at Atanekerdluk could ever have flourished there if

the temperature were not far higher than it is at present. This is

clear from many of the species, of which we find the nearest living

representative 10° or even 20° of latitude to the south of the locality

in question.

The trees of Atanekerdluk were not, he says, all at the extreme

northern limit of their range, for in the Miocene flora of Spitzbergen,

lat. 78° N., we find the beech, plane, hazelnut, and some other species

identical with those from Greenland, and we may conclude, he thinks,

that the firs and poplars which we meet at Atanekerdluk and Bell Sound,

Spitzbergen, must have reached up to the North Pole if land existed

there in the tertiary period.

“The hills of fossilized wood,” he adds, “found by McClure and his

companions in Banks’s Land (lat. 74° 27′ N.), are therefore discoveries

which should not astonish us, they only confirm the evidence as to the

original vegetation of the polar regions which we have derived from

other sources.”

The _Sequoia landsdorfii_ is the most abundant of the trees of

Atanekerdluk. The _Sequoia sempervirens_ is its present representative.

This tree has its extreme northern limit about lat. 53° N. For its

existence it requires a summer temperature of 59° or 61° F. Its fruit

requires a temperature of 64° for ripening. The winter temperature must

not fall below 34°, and that of the whole year must be at least 49°.

The temperature of Atanekerdluk during the time that the Miocene flora

grew could not have been under the above.[193]

Professor Heer concludes his paper as follows:—

“I think these facts are convincing, and the more so that they are not

insulated, but confirmed by the evidence derivable from the Miocene

flora of Iceland, Spitzbergen, and Northern Canada. These conclusions,

too, are only links in the grand chain of evidence obtained from the

examination of the Miocene flora of the whole of Europe. They prove to

us that we could not by any re-arrangement of the relative positions

of land and water produce for the northern hemisphere a climate which

would explain the phenomena in a satisfactory manner. We must only

admit that we are face to face with a problem, whose solution in all

probability must be attempted, and, we doubt not, completed by the

astronomer.”

CHAPTER XIX.

GEOLOGICAL TIME.—PROBABLE DATE OF THE GLACIAL EPOCH.

Geological Time measurable from Astronomical Data.—M. Leverrier’s

Formulæ.—Tables of Eccentricity for 3,000,000 Years in the

Past and 1,000,000 Years in the Future.—How the Tables have

been computed.—Why the Glacial Epoch is more recent than had

been supposed.—Figures convey a very inadequate Conception

of immense Duration.—Mode of representing a Million of

Years.—Probable Date of the Glacial Epoch.

If those great Secular variations of climate which we have been

considering be indirectly the result of changes in the eccentricity

of the earth’s orbit, then we have a means of determining, at least

so far as regards recent epochs, when these variations took place.

If the glacial epoch be due to the causes assigned, we have a means

of ascertaining, with tolerable accuracy, not merely the date of its

commencement, but the length of its duration. M. Leverrier has not

only determined the superior limit of the eccentricity of the earth’s

orbit, but has also given formulæ by means of which the extent of the

eccentricity for any period, past or future, may be computed.

A well-known astronomer and mathematician, who has specially

investigated the subject, is of opinion that these formulæ give results

which may be depended upon as approximately correct for _four millions

of years_ past and future. An eminent physicist has, however, expressed

to me his doubts as to whether the results can be depended on for a

period so enormous. M. Leverrier in his Memoir has given a table of the

eccentricity for 100,000 years before and after 1800 A.D., computed

for intervals of 10,000 years. This table, no doubt, embraces a period

sufficiently great for ordinary astronomical purposes, but it is by far

too limited to afford information in regard to geological epochs.

With the view of ascertaining the probable date of the glacial epoch,

as well as the character of the climate for a long course of ages,

Table I. was computed from M. Leverrier’s formulæ.[194] It shows the

eccentricity of the earth’s orbit and longitude of the perihelion for

3,000,000 of years back, and 1,000,000 of years to come, at periods

50,000 years apart.

On looking over the table it will be seen that there are three

principal periods when the eccentricity rose to a very high value,

with a few subordinate maxima between. It will be perceived also that

during each of those periods the eccentricity does not remain at the

same uniform value, but rises and falls, in one case twice, and in the

other two cases three times. About 2,650,000 years back we have the

eccentricity almost at its inferior limit. It then begins to increase,

and fifty thousand years afterwards, namely at 2,600,000 years ago, it

reaches ·0660; fifty thousand years after this period it has diminished

to ·0167, which is about its present value. It then begins to increase,

and in another fifty thousand years, namely at 2,500,000 years ago, it

approaches to almost the superior limit, its value being then ·0721. It

then begins to diminish, and at 2,450,000 years ago it has diminished

to ·0252. These two maxima, separated by a minimum and extending over a

period of 200,000 years, constitute the first great period of high

eccentricity. We then pass onwards for upwards of a million and a half

years, and we come to the second great period. It consists of three

maxima separated by two minima. The first maximum occurred at 950,000

years ago, the second or middle one at 850,000 years ago, and the

third and last at 750,000 years ago—the whole extending over a period

of nearly 300,000 years. Passing onwards for another million and half

years, or to about 800,000 years in the future, we come to the third

great period. It also consists of three maxima one hundred thousand

years apart. Those occur at the periods 800,000, 900,000, and 1,000,000

years to come, respectively, separated also by two minima. Those three

great periods, two of them in the past and one of them in the future,

included in the Table, are therefore separated from each other by an

interval of upwards of 1,700,000 years.

[Illustration: PLATE IV

W. & A. K. Johnston, Edinb^r. and London.

DIAGRAM REPRESENTING THE VARIATIONS IN THE ECCENTRICITY OF THE

EARTH’S ORBIT FOR THREE MILLION OF YEARS BEFORE 1800 A.D. ONE

MILLION OF YEARS AFTER IT.

_The Ordinates are joined by straight lines where the values, at

intervals of 10,000 years, between them have not been determined._]

In this Table there are seven periods when the earth’s orbit becomes

nearly circular, four in the past and three in the future.

The Table shows also four or five subordinate periods of high

eccentricity, the principal one occurring 200,000 years ago.

The variations of eccentricity during the four millions of years, are

represented to the eye diagrammatically in Plate IV.

In order to determine with more accuracy the condition of the earth’s

orbit during the three periods of great eccentricity included in Table

I., I computed the values for periods of ten thousand years apart, and

the results are embodied in Tables II., III., and IV.

There are still eminent astronomers and physicists who are of opinion

that the climate of the globe never could have been seriously affected

by changes in the eccentricity of its orbit. This opinion results, no

doubt, from viewing the question as a purely astronomical one. Viewed

from an astronomical standpoint, as has been already remarked, there

is actually nothing from which any one could reasonably conclude with

certainty whether a change of eccentricity would seriously affect

climate or not. By means of astronomy we ascertain the extent of the

eccentricity at any given period, how much the winter may exceed

the summer in length (or the reverse), how much the sun’s heat is

increased or decreased by a decrease or an increase of distance,

and so forth; but we obtain no information whatever regarding how

these will actually affect climate. This, as we have already seen,

must be determined wholly from physical considerations, and it is

an exceedingly complicated problem. An astronomer, unless he has

given special attention to the physics of the question, is just as

apt to come to a wrong conclusion as any one else. The question

involves certain astronomical elements; but when these are determined

everything then connected with the matter is purely physical. Nearly

all the astronomical elements of the question are comprehended in the

accompanying Tables.

TABLE I.

THE ECCENTRICITY AND LONGITUDE OF THE PERIHELION OF THE EARTH’S

ORBIT FOR 3,000,000 YEARS IN THE PAST AND 1,000,000 YEARS IN

THE FUTURE, COMPUTED FOR INTERVALS OF 50,000 YEARS.

+---------------------------------------------+

| PAST TIME. |

+------------------+-------------+------------+

| Number of years |Eccentricity.|Longitude of|

|before epoch 1800.| |perihelion. |

+------------------+-------------+------------+

| | | ° ′ |

| −3,000,000 | 0·0365 | 39 30 |

| −2,950,000 | 0·0170 | 210 39 |

| −2,900,000 | 0·0442 | 200 52 |

| −2,850,000 | 0·0416 | 0 18 |

| −2,800,000 | 0·0352 | 339 14 |

| −2,750,000 | 0·0326 | 161 22 |

| −2,700,000 | 0·0330 | 65 37 |

| −2,650,000 | 0·0053 | 318 40 |

| −2,600,000 | 0·0660 | 190 4 |

| −2,550,000 | 0·0167 | 298 34 |

| −2,500,000 | 0·0721 | 338 36 |

| −2,450,000 | 0·0252 | 109 33 |

| −2,400,000 | 0·0415 | 116 40 |

| −2,350,000 | 0·0281 | 308 23 |

| −2,300,000 | 0·0238 | 195 25 |

| −2,250,000 | 0·0328 | 141 18 |

| −2,200,000 | 0·0352 | 307 6 |

| −2,150,000 | 0·0183 | 307 5 |

| −2,100,000 | 0·0304 | 98 40 |

| −2,050,000 | 0·0170 | 334 46 |

| −2,000,000 | 0·0138 | 324 4 |

| −1,950,000 | 0·0427 | 120 32 |

| −1,900,000 | 0·0336 | 188 31 |

| −1,850,000 | 0·0503 | 272 14 |

| −1,800,000 | 0·0334 | 354 52 |

| −1,750,000 | 0·0350 | 65 25 |

| −1,700,000 | 0·0085 | 95 13 |

| −1,650,000 | 0·0035 | 168 23 |

| −1,600,000 | 0·0305 | 158 42 |

| −1,550,000 | 0·0239 | 225 57 |

| −1,500,000 | 0·0430 | 303 29 |

| −1,450,000 | 0·0195 | 57 11 |

| −1,400,000 | 0·0315 | 97 35 |

| −1,350,000 | 0·0322 | 293 38 |

| −1,300,000 | 0·0022 | 0 48 |

| −1,250,000 | 0·0475 | 105 50 |

| −1,200,000 | 0·0289 | 239 34 |

| −1,150,000 | 0·0473 | 250 27 |

| −1,100,000 | 0·0311 | 55 24 |

| −1,050,000 | 0·0326 | 4 8 |

| −1,000,000 | 0·0151 | 248 22 |

| −950,000 | 0·0517 | 97 51 |

| −900,000 | 0·0102 | 135 2 |

| −850,000 | 0·0747 | 239 28 |

| −800,000 | 0·0132 | 343 49 |

| −750,000 | 0·0575 | 27 18 |

| −700,000 | 0·0220 | 208 13 |

| −650,000 | 0·0226 | 141 29 |

| −600,000 | 0·0417 | 32 34 |

| −550,000 | 0·0166 | 251 50 |

| −500,000 | 0·0388 | 193 56 |

| −450,000 | 0·0308 | 356 52 |

| −400,000 | 0·0170 | 290 7 |

| −350,000 | 0·0195 | 182 50 |

| −300,000 | 0·0424 | 23 29 |

| −250,000 | 0·0258 | 59 39 |

| −200,000 | 0·0569 | 168 18 |

| −150,000 | 0·0332 | 242 56 |

| −100,000 | 0·0473 | 316 18 |

| −50,000 | 0·0131 | 50 14 |

+------------------+-------------+------------+

+---------------------------------------------+

| FUTURE TIME. |

+------------------+-------------+------------+

| Number of years |Eccentricity.|Longitude of|

|before epoch 1800.| |perihelion. |

+------------------+-------------+------------+

| | | ° ′ |

| A.D. 1800 | 0·0168 | 99 30 |

| +50,000 | 0·0173 | 38 12 |

| +100,000 | 0·0191 | 114 50 |

| +150,000 | 0·0353 | 201 57 |

| +200,000 | 0·0246 | 279 41 |

| +250,000 | 0·0286 | 350 54 |

| +300,000 | 0·0158 | 172 29 |

| +350,000 | 0·0098 | 201 40 |

| +400,000 | 0·0429 | 6 9 |

| +450,000 | 0·0231 | 98 37 |

| +500,000 | 0·0534 | 157 26 |

| +550,000 | 0·0259 | 287 31 |

| +600,000 | 0·0395 | 285 43 |

| +650,000 | 0·0169 | 144 3 |

| +700,000 | 0·0357 | 17 12 |

| +750,000 | 0·0195 | 0 53 |

| +800,000 | 0·0639 | 140 38 |

| +850,000 | 0·0144 | 176 41 |

| +900,000 | 0·0659 | 291 16 |

| +950,000 | 0·0086 | 115 13 |

| +1,000,000 | 0·0528 | 57 31 |

+------------------+-------------+------------+

TABLE II.

ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF

10,000 YEARS, FROM 2,650,000 TO 2,450,000 YEARS AGO.

THE GLACIAL EPOCH OF THE _Eocene period_ IS PROBABLY COMPREHENDED

WITHIN THIS TABLE.

+---------+------------+-----------+-----------+-------------------------------------------+

| I. | II. | III. | IV. | Winter occurring in aphelion. |

| | | | +---------+---------+-----------+-----------+

| | | | | V. | VI. | VII. | VIII. |

| | | |at periods | | =1000. | | |

+---------+------------+-----------+-----------+---------+---------+-----------+-----------+

| | | ° | | | | | |

|2,650,000| 0·0053 | 318 40 | ° ′ | | | F. | F. |

|2,640,000| 0·0173 | 54 25 | 95 45 | | | ° | ° |

|2,630,000| 0·0331 | 93 37 | 39 12 | 15·4 | 906 | 26·2 | 12·8 |

|2,620,000| 0·0479 | 127 12 | 33 35 | 22·2 | 884 | 33·3 | 5·7 |

|2,610,000| 0·0591 | 158 36 | 31 24 | 27·4 | 862 | 38·3 | 0·7 |

|2,600,000| 0·0660 | 190 4 | 31 28 | 30·6 | 851 | 41·5 | −2·5 |

|2,590,000| 0·0666 | 220 28 | 30 24 | 30·9 | 850 | 41·8 | −2·8 |

|2,580,000| 0·0609 | 249 56 | 29 28 | 28·3 | 859 | 39·2 | −0·2 |

+---------+------------+-----------+-----------+---------+---------+-----------+-----------+

|2,570,000| 0·0492 | 277 24 | 27 28 | 22·9 | 878 | 33·9 | 5·1 |

|2,560,000| 0·0350 | 305 2 | 27 38 | 16·2 | 902 | 27·1 | 11·9 |

|2,550,000| 0·0167 | 298 34 | R 6 28 | | | | |

|2,540,000| 0·0192 | 253 58 | R 44 36 | | | | |

|2,530,000| 0·0369 | 259 19 | 5 21 | 17·1 | 899 | 28·0 | 11·0 |

|2,520,000| 0·0537 | 283 7 | 23 48 | 25·0 | 871 | 35·9 | 3·1 |

|2,510,000| 0·0660 | 310 4 | 26 57 | 30·6 | 851 | 41·5 | −2·5 |

|2,500,000| 0·0721 | 338 36 | 28 32 | 33·5 | 841 | 44·2 | −5·2 |

|2,490,000| 0·0722 | 7 36 | 29 0 | 33·6 | 841 | 44·3 | −5·3 |

|2,480,000| 0·0662 | 35 46 | 28 10 | 30·8 | 850 | 41·7 | −2·7 |

|2,470,000| 0·0553 | 63 26 | 27 40 | 25·7 | 868 | 36·6 | 2·4 |

|2,460,000| 0·0410 | 89 13 | 25 47 | 19·1 | 892 | 30·0 | 9·0 |

|2,450,000| 0·0252 | 109 33 | 20 20 | 11·7 | | | |

+---------+------------+-----------+-----------+---------+---------+-----------+-----------+

TABLE III.

ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF

10,000 YEARS, FROM 1,000,000 TO 750,000 YEARS AGO.

THE GLACIAL EPOCH OF THE _Miocene period_ IS PROBABLY COMPREHENDED

WITHIN THIS TABLE.

+---------+------------+-----------+-----------+-------------------------------------------+

| I. | II. | III. | IV. | Winter occurring in aphelion. |

| | | | +---------+---------+-----------+-----------+

| | | | | V. | VI. | VII. | VIII. |

| | | |at periods | | =1000. | | |

+---------+------------+-----------+-----------+---------+---------+-----------+-----------+

| | | ° ′ | | | | | |

|1,000,000| 0·0151 | 248 22 | ° ′ | | | F. | F. |

| 990,000| 0·0224 | 313 50 | 65 28 | | | ° | ° |

| 980,000| 0·0329 | 358 2 | 44 12 | 15·3 | 906 | 26·1 | 12·9 |

| 970,000| 0·0441 | 32 40 | 34 38 | 20·5 | 887 | 31·5 | 7·5 |

| 960,000| 0·0491 | 66 49 | 34 9 | 22·8 | 878 | 33·8 | 5·2 |

| 950,000| 0·0517 | 97 51 | 31 2 | 24·0 | 874 | 35·0 | 4·0 |

| 940,000| 0·0495 | 127 42 | 29 51 | 23·0 | 878 | 34·0 | 5·0 |

| 930,000| 0·0423 | 156 11 | 28 29 | 19·7 | 890 | 30·6 | 8·4 |

| 920,000| 0·0305 | 181 40 | 25 29 | 14·2 | 910 | 25·0 | 14·0 |

| 910,000| 0·0156 | 194 15 | 12 35 | | | | |

| 900,000| 0·0102 | 135 2 | R 59 13 | | | | |

| 890,000| 0·0285 | 127 1 | R 8 1 | | | | |

| 880,000| 0·0456 | 152 33 | 25 32 | 21·2 | 884 | 32·2 | 6·8 |

| 870,000| 0·0607 | 180 23 | 27 50 | 28·2 | 859 | 39·0 | 0·0 |

| 860,000| 0·0708 | 209 41 | 29 18 | 32·9 | 843 | 43·6 | −4·6 |

| 850,000| 0·0747 | 239 28 | 29 47 | 34·7 | 837 | 45·3 | −6·3 |

| 840,000| 0·0698 | 269 14 | 29 46 | 32·4 | 845 | 43·2 | −4·2 |

| 830,000| 0·0623 | 298 28 | 29 14 | 29·0 | 857 | 40·0 | −1·0 |

| 820,000| 0·0476 | 326 4 | 27 36 | 22·1 | 881 | 33·1 | 5·9 |

| 810,000| 0·0296 | 348 30 | 22 26 | | | | |

| 800,000| 0·0132 | 343 49 | R 4 41 | | | | |

| 790,000| 0·0171 | 293 19 | R 50 30 | | | | |

| 780,000| 0·0325 | 303 37 | 10 18 | 15·2 | 907 | 26·0 | 13·0 |

| 770,000| 0·0455 | 328 38 | 25 1 | 21·2 | 884 | 32·2 | 6·8 |

| 760,000| 0·0540 | 357 12 | 28 34 | 25·1 | 870 | 36·0 | 3·0 |

| 750,000| 0·0575 | 27 18 | 30 6 | 26·7 | 864 | 37·7 | 1·3 |

| 740,000| 0·0561 | 58 30 | 31 12 | 26·1 | 867 | 37·0 | 2·0 |

| 730,000| 0·0507 | 90 55 | 32 25 | 23·6 | 876 | 34·6 | 4·4 |

| 720,000| 0·0422 | 125 14 | 34 19 | 19·6 | 890 | 30·6 | 8·4 |

| 710,000| 0·0307 | 177 26 | 52 12 | 14·3 | 910 | 25·0 | 14·0 |

| 700,000| 0·0220 | 208 13 | 30 47 | | | | |

+---------+------------+-----------+-----------+---------+---------+-----------+-----------+

TABLE IV.

ECCENTRICITY, LONGITUDE OF THE PERIHELION, &C., &C., FOR INTERVALS OF

10,000 YEARS, FROM 250,000 YEARS AGO TO THE PRESENT DATE.

THE _Glacial epoch_ IS PROBABLY COMPREHENDED WITHIN THIS TABLE.

+---------+------------+-----------+-----------+-------------------------------------------+

| I. | II. | III. | IV. | Winter occurring in aphelion. |

| | | | +---------+---------+-----------+-----------+

| | | | | V. | VI. | VII. | VIII. |

| | | |at periods | | =1000. | | |

+---------+------------+-----------+-----------+---------+---------+-----------+-----------+

| | | ° ′ | | | | F. | F. |

| 250,000| 0·0258 | 59 39 | ° ′ | | | ° | ° |

| 240,000| 0·0374 | 74 58 | 15 19 | 17·4 | 898 | 28·3 | 10·7 |

|S 230,000| 0·0477 | 102 49 | 27 51 | 22·2 | 885 | 33·2 | 5·8 |

|S 220,000| 0·0497 | 124 33 | 21 44 | 23·2 | 877 | 34·1 | 4·9 |

|S 210,000| 0·0575 | 144 55 | 20 22 | 26·7 | 864 | 37·7 | 1·3 |

| 200,000| 0·0569 | 168 18 | 23 23 | 26·5 | 865 | 37·4 | 1·6 |

|S 190,000| 0·0532 | 190 4 | 21 46 | 24·7 | 871 | 35·7 | 3·3 |

|S 180,000| 0·0476 | 209 22 | 19 18 | 22·1 | 881 | 33·1 | 5·9 |

|S 170,000| 0·0437 | 228 7 | 18 45 | 20·3 | 887 | 31·3 | 7·7 |

| 160,000| 0·0364 | 236 38 | 8 31 | 16·9 | 900 | 27·8 | 11·2 |

| 150,000| 0·0332 | 242 56 | 6 18 | 15·4 | 905 | 26·2 | 12·8 |

| 140,000| 0·0346 | 246 29 | 3 33 | 16·1 | 903 | 26·9 | 12·1 |

| 130,000| 0·0384 | 259 34 | 13 5 | 17·8 | 896 | 28·8 | 10·2 |

| 120,000| 0·0431 | 274 47 | 15 13 | 20·1 | 888 | 31·0 | 8·0 |

| 110,000| 0·0460 | 293 48 | 19 1 | 21·4 | 883 | 32·4 | 6·6 |

| 100,000| 0·0473 | 316 18 | 22 30 | 22·0 | 881 | 33·0 | 6·0 |

|L 90,000| 0·0452 | 340 2 | 23 44 | 21·0 | 885 | 32·0 | 7·0 |

|L 80,000| 0·0398 | 4 13 | 24 11 | 18·5 | 894 | 29·4 | 9·6 |

|L 70,000| 0·0316 | 27 22 | 23 9 | 14·7 | 908 | 25·5 | 13·5 |

|L 60,000| 0·0218 | 46 8 | 18 46 | | | | |

| 50,000| 0·0131 | 50 14 | 4 6 | | | | |

|L 40,000| 0·0109 | 28 36 | R 21 38 | | | | |

|L 30,000| 0·0151 | 5 50 | R 22 46 | | | | |

|L 20,000| 0·0188 | 44 0 | 38 10 | | | | |

|L 10,000| 0·0187 | 78 28 | 34 28 | | | | |

|A.D. 1800| 0·0168 | 99 30 | 21 2 | | | | |

+---------+------------+-----------+-----------+---------+---------+-----------+-----------+

In Tables II., III., and IV., column I. represents the dates of the

periods, column II. the eccentricity, column III. the longitude of

the perihelion. In Table IV. the eccentricity and the longitude of

the perihelion of the six periods marked with an S are copied from a

letter of Mr. Stone to Sir Charles Lyell, published in the Supplement

of the Phil. Mag. for June, 1865; the eight periods marked L are copied

from M. Leverrier’s Table, to which reference has been made. For the

correctness of everything else, both in this Table and in the other

three, I alone am responsible.

Column IV. gives the number of degrees passed over by the perihelion

during each 10,000 years. From this column it will be seen how

irregular is the motion of the perihelion. At four different periods

it had a retrograde motion for 20,000 years. Column V. shows the

number of days by which the winter exceeds the summer when the winter

occurs in aphelion. Column VI. shows the intensity of the sun’s heat

during midwinter, when the winter occurs in aphelion, the present

midwinter intensity being taken at 1,000. These six columns comprehend

all the astronomical part of the Tables. Regarding the correctness of

the principles upon which these columns are constructed, there is no

diversity of opinion. But these columns afford no direct information

as to the character of the climate, or how much the temperature is

increased or diminished. To find this we pass on to columns VII. and

VIII., calculated on physical principles. Now, unless the physical

principles upon which these three columns are calculated be wholly

erroneous, change of eccentricity must undoubtedly very seriously

affect climate. Column VII. shows how many degrees Fahrenheit the

temperature is lowered by a decrease in the intensity of the sun’s heat

corresponding to column VI. For example, 850,000 years ago, if the

winters occurred then in aphelion, the direct heat of the sun during

midwinter would be only 837/1000 of what it is at present at the same

season of the year, and column VII. shows that this decrease in the

intensity of the sun’s heat would lower the temperature 45°·3 F.

The principle upon which this result is arrived at is this:--The

temperature of space, as determined by Sir John Herschel, is −239°

F. M. Pouillet, by a different method, arrived at almost the same

result. If we take the midwinter temperature of Great Britain at

39°, then 239° + 39° = 278° will represent the number of degrees of

rise due to the sun’s heat at midwinter; in other words, it takes a

quantity of sun-heat which we have represented by 1000 to maintain the

temperature of the earth’s surface in Great Britain 278° above the

temperature of space. Were the sun extinguished, the temperature of

our island would sink 278° below its present midwinter temperature,

or to the temperature of space. But 850,000 years ago, as will be

seen from Table III., if the winters occurred in aphelion, the heat

of the sun at midwinter would only equal 837 instead of 1000 as at

present. Consequently, if it takes 1,000 parts of heat to maintain the

temperature 278° above the temperature of space, 837 parts of heat will

only be able to maintain the temperature 232°·7 above the temperature

of space; for 232°·7 is to 278 as 837 is to 1,000. Therefore, if the

temperature was then only 232°·7 above that of space, it would be

45°·3 below what it is at present. This is what the temperature would

be on the supposition, of course, that it depended wholly on the

sun’s intensity and was not modified by other causes. This method has

already been discussed at some length in Chapter II. But whether these

values be too high or too low, one thing is certain, that a very slight

increase or a very slight decrease in the quantity of heat received

from the sun must affect temperature to a considerable extent. The

direct heat of the moon, for example, cannot be detected by the finest

instruments which we possess; yet from 238,000 observations made at

Prague during 1840−66, it would seem that the temperature is sensibly

affected by the mere change in the lunar perigee and inclination of the

moon’s orbit.[195]

Column VIII. gives the midwinter temperature. It is found by

subtracting the numbers in column VII. from 39°, the present midwinter

temperature.

I have not given a Table showing the temperature of the summers at

the corresponding periods. This could not well be done; for there is

no relation at the periods in question between the intensity of the

sun’s heat and the temperature of the summers. One is apt to suppose,

without due consideration, that the summers ought to be then as much

warmer than they are at present, as the winters were then colder than

now. Sir Charles Lyell, in his “Principles,” has given a column of

summer temperatures calculated from my table upon this principle.

Astronomically the principle is correct, but physically, as was shown

in Chapter IV., it is totally erroneous, and calculated to convey a

wrong impression regarding the whole subject of geological climate.

The summers at those periods, instead of being much warmer than they

are at present, would in reality be much colder, notwithstanding the

great increase in the intensity of the sun’s heat resulting from the

diminished distance of the sun.

What, then, is the date of the glacial epoch? It is perfectly obvious

that if the glacial epoch resulted from a high state of eccentricity,

it must be referred either to the period included in Table III. or

to the one in Table IV. In Table III. we have a period extending from

about 980,000 to about 720,000 years ago, and in Table IV. we have a

period beginning about 240,000 years ago, and extending down to about

80,000 years ago. As the former period was of greater duration than

the latter, and the eccentricity also attained to a higher value, I at

first felt disposed to refer the glacial epoch proper (the time of the

till and boulder clay) to the former period; and the latter period, I

was inclined to believe, must have corresponded to the time of local

glaciers towards the close of the glacial epoch, the evidence for which

(moraines) is to be found in almost every one of our Highland glens.

On this point I consulted several eminent geologists, and they all

agreed in referring the glacial epoch to the former period; the reason

assigned being that they considered the latter period to be much too

recent and of too short duration to represent that epoch.

Pondering over the subject during the early part of 1866, reasons soon

suggested themselves which convinced me that the glacial epoch must

be referred to the latter and not to the former period. Those reasons

I shall now proceed to state at some length, since they have a direct

bearing, as will be seen, on the whole question of geological time.

It is the modern and philosophic doctrine of uniformity that has

chiefly led geologists to over-estimate the length of geological

periods. This philosophic school teaches, and that truly, that the

great changes undergone by the earth’s crust must have been produced,

not by convulsions and cataclysms of nature, but by those ordinary

agencies that we see at work every day around us, such as rain, snow,

frost, ice, and chemical action, &c. It teaches that the valleys

were not produced by violent dislocations, nor the hills by sudden

upheavals, but that they were actually carved out of the solid rock

by the silent and gentle agency of chemical action, frost, rain, ice,

and running water. It teaches, in short, that the rocky face of our

globe has been carved into hill and dale, and ultimately worn down

to the sea-level, by means of these apparently trifling agents, not

only once or twice, but probably dozens of times over during past

ages. Now, when we reflect that with such extreme slowness do these

agents perform their work, that we might watch their operations from

year to year, and from century to century, if we could, without being

able to perceive that they make any very sensible advance, we are

necessitated to conclude that geological periods must be enormous. And

the conclusion at which we thus arrive is undoubtedly correct. It is,

in fact, impossible to form an adequate conception of the length of

geological time. It is something too vast to be fully grasped by our

minds. But here we come to the point where the fundamental mistake

arises; Geologists do not err in forming too great a conception of the

extent of geological periods, _but in the mode in which they represent

the length of these periods in numbers_. When we speak of units, tens,

hundreds, thousands, we can form some notion of what these quantities

represent; but when we come to millions, tens of millions, hundreds

of millions, thousands of millions, the mind is then totally unable

to follow, and we can only use these numbers as representations of

quantities that turn up in calculation. We know, from the way in which

they do turn up in our process of calculation, whether they are correct

representations of things in actual nature or not; but we could not,

from a mere comparison of these quantities with the thing represented

by them, say whether they were actually too small or too great.

At present, geological estimates of time are little else than mere

conjectures. Geological science has hitherto afforded no trustworthy

means of estimating the positive length of geological epochs.

Geological phenomena tell us most emphatically that these periods

must be long; but how long they have hitherto failed to inform us.

Geological phenomena represent time to the mind under a most striking

and imposing form. They present to the eye, as it were, a sensuous

representation of time; the mind thus becomes deeply impressed with

a sense of immense duration; and when one under these feelings is

called upon to put down in figures what he believes will represent that

duration, he is very apt to be deceived. If, for example, a million of

years as represented by geological phenomena and a million of years as

represented by figures were placed before our eyes, we should certainly

feel startled. We should probably feel that a unit with six ciphers

after it was really something far more formidable than we had hitherto

supposed it to be. Could we stand upon the edge of a gorge a mile and

a half in depth that had been cut out of the solid rock by a tiny

stream, scarcely visible at the bottom of this fearful abyss, and were

we informed that this little streamlet was able to wear off annually

only 1/10 of an inch from its rocky bed, what would our conceptions be

of the prodigious length of time that this stream must have taken to

excavate the gorge? We should certainly feel startled when, on making

the necessary calculations, we found that the stream had performed this

enormous amount of work in something less than a million of years.

If, for example, we could possibly form some adequate conception of a

period so prodigious as one hundred millions of years, we should not

then feel so dissatisfied with Sir W. Thomson’s estimate that the age

of the earth’s crust is not greater than that.

Here is one way of conveying to the mind some idea of what a million

of years really is. Take a narrow strip of paper an inch broad, or

more, and 83 feet 4 inches in length, and stretch it along the wall of

a large hall, or round the walls of an apartment somewhat over 20 feet

square. Recall to memory the days of your boyhood, so as to get some

adequate conception of what a period of a hundred years is. Then mark

off from one of the ends of the strip 1/10 of an inch. The 1/10 of the

inch will then represent one hundred years, and the entire length of

the strip a million of years. It is well worth making the experiment,

just in order to feel the striking impression that it produces on the

mind.

The latter period, which we have concluded to be that of the glacial

epoch, extended, as we have seen, over a period of 160,000 years. But

as the glaciation was only on one hemisphere at a time, 80,000 years

or so would represent the united length of the cold periods. In order

to satisfy ourselves that this period is sufficiently long to account

for all the amount of denudation effected during the glacial epoch,

let us make some rough estimate of the probable rate at which the

surface of the country would be ground down by the ice. Suppose the

ice to grind off only one-tenth of an inch annually this would give

upwards of 650 feet as the quantity of rock removed during the time.

But it is probable that it did not amount to one-fourth part of that

quantity. Whether one-tenth of an inch per annum be an over-estimate or

an under-estimate of the rate of denudation by the ice, it is perfectly

evident that the period in question is sufficiently long, so far as

denudation is concerned, to account for the phenomena of the glacial

epoch.

But admitting that the period under consideration is sufficiently

_long_ to account for all the denudation which took place _during_

the glacial epoch, we have yet to satisfy ourselves that it is also

sufficiently _remote_ to account for all the denudation which has taken

place _since_ the glacial epoch. Are the facts of geology consistent

with the idea that the close of the glacial epoch does not date back

beyond 80,000 years?

This question could be answered if we knew the present rate of

subaërial denudation, for the present rate evidently does not differ

greatly from that which has obtained since the close of the glacial

epoch.

CHAPTER XX.

GEOLOGICAL TIME.—METHOD OF MEASURING THE RATE OF SUBAËRIAL

DENUDATION.

Rate of Subaërial Denudation a Measure of Time.—Rate determined

from Sediment of the Mississippi.—Amount of Sediment carried

down by the Mississippi; by the Ganges.—Professor Geikie on

Modern Denudation.—Professor Geikie on the Amount of Sediment

conveyed by European Rivers.—Rate at which the Surface of

the Globe is being denuded.—Alfred Tylor on the Sediment

of the Mississippi.—The Law which determines the Rate of

Denudation.—The Globe becoming less oblate.—Carrying Power

of our River Systems the true Measure of Denudation.—Marine

Denudation trifling in comparison to Subaërial.—Previous

Methods of measuring Geological Time.—Circumstances which

show the recent Date of the Glacial Epoch.—Professor Ramsay

on Geological Time.

It is almost self-evident that the rate of subaërial denudation must

be equal to the rate at which the materials are carried off the land

into the sea, but the rate at which the materials are carried off the

land 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._

Knowing the quantity of sediment transported by a river, say annually,

and the area of its drainage, we have the means of determining the

rate at which the surface of this area is being lowered by subaërial

denudation. And if we know this in reference to a few of the great

continental rivers draining immense areas in various latitudes, we

could then ascertain with tolerable correctness the rate at which the

surface of the globe is being lowered by subaërial denudation, and

also the length of time which our present continents can remain above

the sea-level. Explaining this to Professor Ramsay during the winter

of 1865, I learned from him that accurate measurements had been made

of the amount of sediment annually carried down by the Mississippi

River, full particulars of which investigations were to be found

in the Proceedings of the American Association for the Advancement

of Science for 1848. These proceedings contain a report by Messrs.

Brown and Dickeson, which unfortunately over-estimated the amount of

sediment transported by the Mississippi by nearly four times what

was afterwards found by Messrs. Humphreys and Abbot to be the actual

amount. From this estimate, I was led to the conclusion that if the

Mississippi is a fair representative of rivers in general, our existing

continents would not remain longer than one million and a half years

above the sea-level.[196] This was a conclusion so startling as to

excite suspicion that there must have been some mistake in reference

to Messrs. Brown and Dickeson’s data. It showed beyond doubt, however,

that the rate of subaërial denudation, when accurately determined by

this method, would be found to be enormously greater than had been

supposed. Shortly afterwards, on estimating the rate from the data

furnished by Humphreys and Abbot, I found the rate of denudation to

be about one foot in 6,000 years. Taking the mean elevation of all

the land as ascertained by Humboldt to be 1,000 feet, the whole would

therefore be carried down into the ocean by our river systems in about

6,000,000 of years if no elevation of the land took place.[197] The

following are the data and mode of computation by which this conclusion

was arrived at. It was found by Messrs. Humphreys and Abbot that the

average amount of sediment held in suspension in the waters of the

Mississippi is about 1/1500 of the weight of the water, or 1/2900

by bulk. The annual discharge of the river is 19,500,000,000,000

cubic feet of water. The quantity of sediment carried down into the

Gulf of Mexico amounts to 6,724,000,000 cubic feet. But besides

that which is held in suspension, the river pushes down into the

sea about 750,000,000 cubic feet of earthy matter, making in all a

total of 7,474,000,000 cubic feet transferred from the land to the

sea annually. Where does this enormous mass of material come from?

Unquestionably it comes from the ground drained by the Mississippi. The

area drained by the river is 1,244,000 square miles. Now 7,474,000,000

cubic feet removed off 1,224,000 square miles of surface is equal to

1/4566 of a foot off that surface per annum, or one foot in 4,566

years. The specific gravity of the sediment is taken at 1·9, that of

rock is about 2·5; consequently the amount removed is equal to one foot

of rock in about 6,000 years. The average height of the North American

continent above the sea-level, according to Humboldt, is 748 feet;

consequently, at the present rate of denudation, the whole area of

drainage will be brought down to the sea-level in less than 4,500,000

years, if no elevation of the land takes place.

Referring to the above, Sir Charles Lyell makes the following

appropriate remarks:—“There seems no danger of our overrating the

mean rate of waste by selecting the Mississippi as our example, for

that river 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.”[198]

The rate of denudation of the area drained by the river Ganges is much

greater than that of the Mississippi. The annual discharge of that

river is 6,523,000,000,000 cubic feet of water. The sediment held in

suspension is equal to 1/510 by weight; area of drainage 432,480 square

miles. This gives one foot of rock in 2,358 years as the amount removed.

Rough estimates have been made of the amount of sediment carried down

by some eight or ten European rivers; and although those estimates

cannot be depended upon as being anything like perfectly accurate,

still they show (what there is very little reason to doubt) that it is

extremely probable that the European continent is being denuded about

as rapidly as the American.

For a full account of all that is known on this subject I must

refer to Professor Geikie’s valuable memoir on Modern Denudation

(Transactions of Geological Society of Glasgow, vol. iii.; also Jukes

and Geikie’s “Manual of Geology,” chap. xxv.) It is mainly through the

instrumentality of this luminous and exhaustive memoir that the method

under consideration has gained such wide acceptance amongst geologists.

Professor Geikie finds that at the present rate of erosion the

following is the number of years required by the undermentioned rivers

to remove one foot of rock from the general surface of their basins.

Professor Geikie thus shows that the rate of denudation, as determined

from the amount of sediment carried down the Mississippi, is certainly

not too high.

Danube 6,846 years.

Mississippi 6,000 〃

Nith 4,723 〃

Ganges 2,358 〃

Rhone 1,528 〃

Hoang Ho 1,464 〃

Po 729 〃

By means of subaërial agencies continents are being cut up into

islands, the islands into smaller islands, and so on till the whole

ultimately disappears.

No proper estimate has been made of the quantity of sediment carried

down into the sea by our British rivers. But, from the principles just

stated, we may infer that it must be as great in proportion to the area

of drainage as that carried down by the Mississippi. For example, the

river Tay, which drains a great portion of the central Highlands of

Scotland, carries to the sea three times as much water in proportion

to its area of drainage as is carried by the Mississippi. And any one

who has seen this rapidly running river during a flood, red and turbid

with sediment, will easily be convinced that the quantity of solid

material carried down by it into the German Ocean must be very great.

Mr. John Dougall has found that the waters of the Clyde during a flood

hold in suspension 1/800 by bulk of sediment. The observations were

made about a mile above the city of Glasgow. But even supposing the

amount of sediment held in suspension by the waters of the Tay to be

only one-third (which is certainly an under-estimate) of that of the

Mississippi, viz. 1/4500 by weight, still this would give the rate of

denudation of the central Highlands at one foot in 6,000 years, or

1,000 feet in 6 millions of years.

It is remarkable that although so many measurements have been made of

the amount of fluviatile sediment being transported seawards, yet that

the bearing which this has on the broad questions of geological time

and the rate of subaërial denudation should have been overlooked. One

reason for this, no doubt, is that the measurements were made, not

with a view to determine the rate at which the river basins are being

lowered, but mainly to ascertain the age of the river deltas and the

rate at which these are being formed.[199]

_The Law which determines the Rate at which any Country is being

denuded._—By means of subaërial agencies continents are being cut up

into islands, the islands into smaller islands, and so on till the

whole ultimately disappears.

So long as the present order of things remains, the rate of denudation

will continue while land remains above the sea-level; and we have no

warrant for supposing that the rate was during past ages less than it

is at the present day. It will not do to object that, as a considerable

amount of the sediment carried down by rivers is boulder clay and

other materials belonging to the Ice age, the total amount removed

by the rivers is on that account greater than it would otherwise be.

Were this objection true, it would follow that, prior to the glacial

period, when it is assumed that there was no boulder clay, the face of

the country must have consisted of bare rock; for in this case no soil

could have accumulated from the disintegration and decomposition of the

rocks, _since, unless the rocks of a country disintegrate more rapidly

than the river systems are able to carry the disintegrated materials

to the sea, no surface soil can form on that country_. The rate at

which rivers carry down sediment is evidently not determined by the

rate at which the rocks are disintegrated and decomposed, but by the

quantity of rain falling, and the velocity with which it moves off the

face of the country. Every river system possesses a definite amount of

carrying-power, depending upon the slope of the ground, the quantity of

rain falling per annum, the manner in which the rain falls, whether it

falls gradually or in torrents, and a few other circumstances. When it

so happens, as it generally does, that the amount of rock disintegrated

on the face of the country is greater than the carrying-power of the

river systems can remove, then a soil necessarily forms. But when the

reverse is the case no soil can form on that country, and it will

present nothing but barren rock. This is no doubt the reason why in

places like the Island of Skye, for example, where the rocks are

exceedingly hard and difficult to decompose and separate, the ground

steep, and the quantity of rain falling very great, there is so much

bare rock to be seen. If, prior to the glacial epoch, the rocks of

the area drained by the Mississippi did not produce annually more

material from their destruction under atmospheric agency than was being

carried down by that river, then it follows that the country must have

presented nothing but bare rock, if the amount of rain falling then was

as great as at present.

But, after all, one foot removed off the general level of the country

since the creation of man, according to Mosaic chronology, is certainly

not a very great quantity. No person but one who had some preconceived

opinions to maintain, would ever think of concluding that one foot of

soil during 6,000 years was an extravagant quantity to be washed off

the face of the country by rain and floods during that long period.

Those who reside in the country and are eye-witnesses of the actual

effects of heavy rains upon the soil, our soft country roads, ditches,

brooks, and rivers, will have considerable difficulty in actually

believing that only one foot has been washed away during the past 6,000

years.

Some may probably admit that a foot of soil may be washed off during

a period so long as 6,000 years, and may tell us that what they deny

is not that a foot of loose and soft soil, but a foot of solid rock

can be washed away during that period. But a moment’s reflection must

convince them that, unless the rocks of the country were disintegrating

and decomposing as rapidly into soil as the rain is carrying the soil

away, the surface of the country would ultimately become bare rock. It

is true that the surface of our country in many places is protected by

a thick covering of boulder clay; but when this has once been removed,

the rocks will then disintegrate far more rapidly than they are doing

at present.

But slow as is the rate at which the country is being denuded, yet

when we take into consideration a period so enormous as 6 millions of

years, we find that the results of denudation are really startling.

One thousand feet of solid rock during that period would be removed

from off the face of the country. But if the mean level of the country

would be lowered 1,000 feet in 6 millions of years, how much would our

valleys and glens be deepened during that period? This is a problem

well worthy of the consideration of those who treat with ridicule the

idea that the general features of our country have been carved out by

subaërial agency.

In consequence of the retardation of the earth’s rotation, occasioned

by the friction of the tidal wave, the sea-level must be slowly sinking

at the equator and rising at the poles. But it is probable that the

land at the equator is being lowered by denudation as rapidly as

the sea-level is sinking. _Nearly one mile must have been worn off

the equator during the past 12 millions of years_, if the rate of

denudation all along the equator be equal to that of the basin of the

Ganges. It therefore follows that we cannot infer from the present

shape of our globe what was its form, or the rate at which it was

rotating, at the time when its crust became solidified. Although it

had been as oblate as the planet Jupiter, denudation must in time have

given it its present form.

There is another effect which would result from the denudation of the

equator and the sinking of the ocean at the equator and its rise at

the poles. This, namely, that it would tend to increase the rate of

rotation; or, more properly, it would tend to _lessen_ the rate of

tidal retardation.

But if the rate of denudation be at present so great, what must it have

been during the glacial epoch? It must have been something enormous.

At present, denudation is greatly retarded by the limited power of

our river systems to remove the loose materials resulting from the

destruction of the rocks. These materials accumulate and form a thick

soil over the surface of the rocks, which protects them, to a great

extent, from the weathering effects of atmospheric agents. So long as

the amount of rock disintegrated exceeds that which is being removed

by the river systems, the soil will continue to accumulate till the

amount of rock destroyed per annum is brought to equal that which

is being removed. It therefore follows from this principle that the

CARRYING-POWER OF OUR RIVER SYSTEMS IS THE TRUE MEASURE OF DENUDATION.

But during the glacial epoch the thickness of the soil would have but

little effect in diminishing the waste of the rocks; for at that period

the rocks were not decomposed by atmospheric agency, but were ground

down by the mechanical friction of the ice. But the presence of a thick

soil at this period, instead of retarding the rate of denudation,

would tend to increase it tenfold, for the soil would then be used as

grinding-material for the ice-sheet. In places where the ice was, say,

2,000 feet in thickness, the soil would be forced along over the rocky

face of the country, exerting a pressure on the rocks equal to 50 tons

on the square foot.

It is true that the rate at which many kinds of rocks decompose and

disintegrate is far less than what has been concluded to be the mean

rate of denudation of the whole country. This is evident from the fact

which has been adduced by some writers, that inscriptions on stones

which have been exposed to atmospheric agency for a period of 2,000

years or so, have not been obliterated. But in most cases epitaphs on

monuments and tombstones, and inscriptions on the walls of buildings,

200 years old, can hardly be read. And this is not all: the stone on

which the letters were cut has during that time rotted in probably to

the depth of several inches; and during the course of a few centuries

more the whole mass will crumble into dust.

The facts which we have been considering show also how trifling is the

amount of denudation effected by the sea in comparison with that by

subaërial agents. The entire sea-coast of the globe, according to Dr.

A. Keith Johnston, is 116,531 miles. Suppose we take the average height

of the coast-line at 25 feet, and take also the rate at which the sea

is advancing on the land at one foot in 100 years, then this gives

15,382,500,000 cubic feet of rock as the total amount removed in 100

years by the action of the sea. The total amount of land is 57,600,000

square miles, or 1,605,750,000,000,000 square feet; and if one foot is

removed off the surface in 6,000 years, then 26,763,000,000,000 cubic

feet is removed by subaërial agency in 100 years, or about 1,740 times

as much as that removed by the sea. Before the sea could denude the

globe as rapidly as the subaërial agents, it would have to advance on

the land at the rate of upwards of 17 feet annually.

It will not do, however, to measure marine denudation by the rate at

which the sea is advancing on the land. There is no relation whatever

between the rate at which the sea is _advancing_ on the land and the

rate at which the sea is _denuding_ the land. For it is evident that as

the subaërial agents bring the coast down to the sea-level, all that

the sea has got to do is simply to advance, or at most to remove the

loose materials which may lie in its path. The amount of denudation

which has been effected by the sea during past geological ages,

compared with what has been effected by subaërial agency, is evidently

but trifling. Denudation is not the proper function of the sea. The

great denuding agents are land-ice, frost, rain, running-water,

chemical agency, &c. The proper work which belongs to the sea is the

transporting of the loose materials carried down by the rivers, and the

spreading of these out so as to form the stratified beds of future ages.

_Previous Methods of measuring Geological Time unreliable._—The method

which has just been detailed of estimating the rate of subaërial

denudation seems to afford the only reliable means of a geological

character of determining geological time in absolute measure. The

methods which have hitherto been adopted not only fail to give the

positive length of geological periods, but some of them are actually

calculated to mislead.

The common method of calculating the length of a period from the

thickness of the stratified rocks belonging to that period is one of

that class. Nothing whatever can be inferred from the thickness of a

deposit as to the length of time which was required to form it. The

thickness of a deposit will depend upon a great many circumstances,

such as whether the deposition took place near to land or far away in

the deep recesses of the ocean, whether it occurred at the mouth of a

great river or along the sea-shore, or at a time when the sea-bottom

was rising, subsiding, or remaining stationary. Stratified formations

10,000 feet in thickness, for example, may, under some conditions, have

been formed in as many years, while under other conditions it may have

required as many centuries. Nothing whatever can be safely inferred as

to the absolute length of a period from the thickness of the stratified

formations belonging to that period. Neither will this method give us a

trustworthy estimate of the _relative_ lengths of geological periods.

Suppose we find the average thickness of the Cambrian rocks to be,

say, 26,000 feet, the Silurian to be 28,000 feet, the Devonian to be

6,000 feet, and the Tertiary to be 10,000 feet, it would not be safe

to assume, as is sometimes done, that the relative duration of those

periods must have corresponded to these numbers. Were we sure that we

had got the correct average thickness of all the rocks belonging to

each of those formations, we might probably be able to arrive at the

relative lengths of those periods; but we can never be sure of this.

Those formations all, at one time, formed sea-bottoms; and we can only

measure such deposits as are now raised above the sea-level. But is

not it probable that the relative positions of sea and land during the

Cambrian, Silurian, Old Red Sandstone, Carboniferous, and other early

periods of the earth’s history, differed more from the present than the

distribution of sea and land during the Tertiary period differed from

that which obtains now? May not the greater portion of the Tertiary

deposits be still under the sea-bottom? And if this be the case, it may

yet be found at some day in the distant future, when these deposits

are elevated into dry land, that they are much thicker than we now

conclude them to be. Of course, it is by no means asserted that this

is so, but only that they _may_ be thicker for anything we know to the

contrary; and the possibility that they may, destroys our confidence

in the accuracy of this method of determining the relative lengths of

geological periods.

Neither does palæontology afford any better mode of measuring

geological time. In fact, the palæontological method of estimating

geological time, either absolute or relative, from the rate at which

species change, appears to be even still more unsatisfactory. If we

could ascertain by some means or other the time that has elapsed from

some given epoch (say, for example, the glacial) till the present

day, and were we sure at the same time that species have changed at a

uniform rate during all past ages, then, by ascertaining the percentage

of change that has taken place since the glacial epoch, we should

have a means of making something like a rough estimate of the length

of the various periods. But without some such period to start with,

the palæontological method is useless. It will not do to take the

historic period as a base-line. It is far too short to be used with

safety in determining the distance of periods so remote as those which

concern the geologist. But even supposing the palæontologist had a

period of sufficient length measured off correctly to begin with, his

results would still be unsatisfactory; for it is perfectly obvious,

that unless the climatic conditions of the globe during the various

periods were nearly the same, the rate at which the species change

would certainly not be uniform; but such has not been the case, as an

examination of the Tables of eccentricity will show. Take, for example,

that long epoch of 260,000 years, beginning about 980,000 years ago

and terminating about 720,000 years ago. During that long period the

changes from cold to warm conditions of climate every 10,000 or 12,000

years must have been of the most extreme character. Compare that

period with the period beginning, say, 80,000 years ago, and extending

to nearly 150,000 years into the future, during which there will be

no extreme variations of climate, and how great is the contrast! How

extensive the changes in species must have been during the first period

as compared with those which are likely to take place during the latter!

Besides, it must also be taken into consideration that organization was

of a far more simple type in the earlier Palæozoic ages than during the

Tertiary period, and would probably on this account change much more

slowly in the former than in the latter.

The foregoing considerations render it highly probable, if not

certain, that the rate at which the general surface of the globe is

being lowered by subaërial denudation cannot be much under one foot

in 6,000 years. How, if we assign the glacial epoch to that period of

high eccentricity beginning 980,000 years ago, and terminating 720,000

years ago, then we must conclude that as much as 120 feet must have

been denuded off the face of the country since the close of the glacial

epoch. But if as much as this had been carried down by our rivers into

the sea, hardly a patch of boulder clay, or any trace of the glacial

epoch, should be now remaining on the land. It is therefore evident

that the glacial epoch cannot be assigned to that remote period, but

ought to be referred to the period terminating about 80,000 years ago.

We have, in this latter case, 13 feet, equal to about 18 feet of drift,

as the amount removed from the general surface of the country since

the glacial epoch. This amount harmonizes very well with the direct

evidence of geology on this point. Had the amount of denudation since

the close of the glacial epoch been much greater than this, the drift

deposits would not only have been far less complete, but the general

appearance and outline of the surface of all glaciated countries would

have been very different from what they really are.

_Circumstances which show the Recent Date of the Glacial Epoch._—One

of the circumstances to which I refer is this. When we examine the

surface of any glaciated country, such as Scotland, we can easily

satisfy ourselves that the upper surface of the ground differs very

much from what it would have been had its external features been due

to the action of rain and rivers and the ordinary agencies which have

been at work since the close of the Ice period. Go where one will in

the Lowlands of Scotland, and he shall hardly find a single acre whose

upper surface bears the marks of being formed by the denuding agents

which are presently in operation. He will observe everywhere mounds

and hollows, the existence of which cannot be accounted for by the

present agencies at work. In fact these agencies are slowly denuding

pre-existing heights and silting up pre-existing hollows. Everywhere

one comes upon patches of alluvium which upon examination prove to be

simply old glacially formed hollows silted up. True, the main rivers,

streams, and even brooks, occupy channels which have been formed by

running water, either since or prior to the glacial epoch, but, in

regard to the general surface of the country, the present agencies may

be said to be just beginning to carve a new line of features out of

the old glacially formed surface. But so little progress has yet been

made, that the kames, gravel mounds, knolls of boulder clay, &c., still

retain in most cases their original form. Now, when we reflect that

more than a foot of drift is being removed from the general surface of

the country every 5,000 years or so, it becomes perfectly obvious that

the close of the glacial epoch must be of comparatively recent date.

There is another circumstance which shows that the glacial epoch must

be referred to the latest period of great eccentricity. If we refer the

glacial epoch to the penultimate period of extreme eccentricity, and

place its commencement one million of years back, then we must also

lengthen out to a corresponding extent the entire geological history

of the globe. Sir Charles Lyell, who is inclined to assign the glacial

epoch to this penultimate period, considers that when we go back as far

as the Lower Miocene formations, we arrive at a period when the marine

shells differed as a whole from those now existing. But only 5 per

cent. of the shells existing at the commencement of the glacial epoch

have since died out. Hence, assuming the rate at which the species

change to be uniform, it follows that the Lower Miocene period must

be twenty times as remote as the commencement of the glacial epoch.

Consequently, if it be one million of years since the commencement

of the glacial epoch, 20 millions of years, Sir Charles concludes,

must have elapsed since the time of the Lower Miocene period, and

60 millions of years since the beginning of the Eocene period, and

about 160 millions of years since the Carboniferous period, and about

240 millions of years must be the time which has elapsed since the

beginning of the Cambrian period. But, on the other hand, if we refer

the glacial epoch to the latest period of great eccentricity, and take

250,000 years ago as the beginning of that period, then, according

to the same mode of calculation, we have 15 millions of years since

the beginning of the Eocene period, and 40 millions of years since

the Carboniferous period, and 60 millions of years in all since the

beginning of the Cambrian period.

If the beginning of the glacial epoch be carried back a million years,

then it is probable, as Sir Charles Lyell concludes, that the beginning

of the Cambrian period will require to be placed 240 millions of years

back. But it is very probable that the length of time embraced by the

pre-Cambrian ages of geological history may be as great as that which

has elapsed since the close of the Cambrian period, and, if this be

so, then we shall be compelled to admit that nearly 500 millions of

years have passed away since the beginning of the earth’s geological

history. But we have evidence of a physical nature which proves that it

is absolutely impossible that the existing order of things, as regards

our globe, can date so far back as anything like 500 millions of years.

The arguments to which I refer are those which have been advanced by

Professor Sir William Thomson at various times. These arguments are

well known, and to all who have really given due attention to them must

be felt to be conclusive. It would be superfluous to state them here; I

shall, however, for reasons which will presently appear, refer briefly

to one of them, and that one which seems to be the most conclusive of

all, viz., the argument derived from the limit to the age of the sun’s

heat.

_Professor Ramsay on Geological Time._—In an interesting suggestive

memoir, “On Geological Ages as items of Geological Time,”[200]

Professor Ramsay discusses the comparative values of certain groups of

formations as representative of geological time, and arrives at the

following general conclusion, viz., “That the local continental era

which began with the Old Red Sandstone and closed with the New Red Marl

is comparable, in point of geological time, to that occupied in the

deposition of the whole of the Mesozoic, or Secondary series, later

than the New Red Marl and all the Cainozoic or Tertiary formations,

and indeed of all the time that has elapsed since the beginning of

the deposition of the Lias down to the present day.” This conclusion

is derived partly from a comparison of the physical character of

the formations constituting each group, but principally from the

zoological changes which took place during the time represented by them.

The earlier period represented by the Cambrian and Silurian rocks he

also, from the same considerations, considers to have been very long,

but he does not attempt to fix its relative length. Of the absolute

length of any or all of these great eras of geological time no

estimate or guess is given. He believes, however, that the whole time

represented by all the fossiliferous rocks, from the earliest Cambrian

to the most recent, is, geologically speaking, short compared with that

which went before it. After quoting Professor Huxley’s enumeration of

the many classes and orders of marine life (identical with those still

existing), whose remains characterize the lowest Cambrian rocks, he

says, “The inference is obvious that 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.”... “In the

words of Darwin, when discussing the imperfection of the geological

record of this history, ‘we possess the last volume alone relating

only to two or three countries,’ and the reason why we know so little

of pre-Cambrian faunas and the physical characters of the more ancient

formations as originally deposited, is that below the Cambrian strata

we get at once involved in a sort of chaos of metamorphic strata.’”

It seems to me that Professor Ramsay’s results lead to the same

conclusion regarding the _positive_ length of geological periods as

those derived from physical considerations. It is true that his views

lead us back to an immense lapse of unknown time prior to the Cambrian

period, but this practically tends to shorten geological periods. For

it is evident that the geological history of our globe must be limited

by the age of the sun’s heat, no matter how long or short its age may

be. This being the case, the greater the length of time which must

have elapsed prior to the Cambrian period, the less must be the time

which has elapsed since that period. Whatever is added to the one

period must be so much taken from the other. Consequently, the longer

we suppose the pre-Cambrian periods to have been, the shorter must we

suppose the post-Cambrian to be.

CHAPTER XXI.

THE PROBABLE AGE AND ORIGIN OF THE SUN.

Gravitation Theory.—Amount of Heat emitted by the Sun.—Meteoric

Theory.—Helmholtz’s Condensation Theory.—Confusion of

Ideas.—Gravitation not the chief Source of the Sun’s

Heat.—Original Heat.—Source of Original Heat.—Original Heat

derived from Motion in Space.—Conclusion as to Date of

Glacial Epoch.—False Analogy.—Probable Date of Eocene and

Miocene Periods.

_Gravitation Theory of the Origin and Source of the Sun’s Heat._—There

are two forms in which this theory has been presented: the first, the

meteoric theory, propounded by Dr. Meyer, of Heilbronn; and the second,

the contraction theory, advocated by Helmholtz.

It is found that 83·4 foot-pounds of heat per second are incident upon

a square foot of the earth’s surface exposed to the perpendicular rays

of the sun. The amount radiated from a square foot of the sun’s surface

is to that incident on a square foot of the earth’s surface as the

square of the sun’s distance to the square of his radius, or as 46,400

to 1. Consequently 3,869,000 foot-pounds of heat are radiated off every

square foot of the sun’s surface per second—an amount equal to about

7,000 horse power. The total amount radiated from the whole surface

of the sun per annum is 8,340 × 10^{30} foot-pounds. To maintain the

present rate of radiation, it would require the combustion of about

1,500 lbs. of coal per hour on every square foot of the sun’s surface;

and were the sun composed of that material, it would be all consumed in

less than 5,000 years. The opinion that the sun’s heat is maintained

by combustion cannot be entertained for a single moment. A pound of

coal falling into the sun from an infinite distance would produce by

its concussion more than 6,000 times the amount of heat that would be

generated by its combustion.

It is well known that the velocity with which a body falling from an

infinite distance would reach the sun would be equal to that which

would be generated by a constant force equal to the weight of the body

at the sun’s surface operating through a space equal to the sun’s

radius. One pound would at the sun’s surface weigh about 28 pounds.

Taking the sun’s radius at 441,000 miles,[201] the energy of a pound

of matter falling into the sun from infinite space would equal that

of a 28-pound weight descending upon the earth from an elevation of

441,000 miles, supposing the force of gravity to be as great at that

elevation as it is at the earth’s surface. It would amount to upwards

of 65,000,000,000 foot-pounds. A better idea of this enormous amount

of energy exerted by a one-pound weight falling into the sun will be

conveyed by stating that it would be sufficient to raise 1,000 tons to

a height of 5½ miles. It would project the _Warrior_, fully equipped

with guns, stores, and ammunition, over the top of Ben Nevis.

Gravitation is now generally admitted to be the only conceivable

source of the sun’s heat. But if we attribute the energy of the sun to

gravitation as a source, we assign it to a cause the value of which can

be accurately determined. Prodigious as is the energy of a single pound

of matter falling into the sun, nevertheless a range of mountains,

consisting of 176 cubic miles of solid rock, falling into the sun,

would maintain his heat for only a single second. A mass equal to that

of the earth would maintain the heat for only 93 years, and a mass

equal to that of the sun itself falling into the sun would afford but

33,000,000 years’ sun-heat.

It is quite possible, however, that a meteor may reach the sun with a

velocity far greater than that which it could acquire by gravitation;

for it might have been moving in a direct line towards the sun with

an original velocity before coming under the sensible influence of

the sun’s attraction. In this case a greater amount of heat would

be generated by the meteor than would have resulted from its merely

falling into the sun under the influence of gravitation. But then

meteors of this sort must be of rare occurrence. The meteoric theory

of the sun’s heat has now been pretty generally abandoned for the

contraction theory advanced by Helmholtz.

Suppose, with Helmholtz, that the sun originally existed as a nebulous

mass, filling the entire space presently occupied by the solar system

and extending into space indefinitely beyond the outermost planet. The

total amount of work in foot-pounds performed by gravitation in the

condensation of this mass to an orb of the sun’s present size can be

found by means of the following formula given by Helmholtz,[202]

3 _r_^{2}M^{2}

Work of condensation = — × ———————————— × _g_

5 R_m_

M is the mass of the sun, _m_ the mass of the earth, R the sun’s

radius, and _r_ the earth’s radius. Taking M = 4230 × 10^{27} lbs.,

_m_ = 11,920 × 10^{21} lbs., R = 2,328,500,000 feet, and _r_ =

20,889,272 feet; we have then for the total amount of work performed by

gravitation in foot-pounds,

3 (20,889,272·5)^2 × (4230 × 10^{27})^2

Work = — × —————————————————————————————————————

5 2,328,500,000 × 11,920 × 10^{21}

= 168,790 × 10^{36} foot-pounds.

The amount of heat thus produced by gravitation would suffice for

nearly 20,237,500 years.

These calculations are based upon the assumption that the density of

the sun is uniform throughout. But it is highly probable that the sun’s

density increases towards the centre, in which case the amount of work

performed by gravitation would be somewhat more than the above.

Some confusion has arisen in reference to this subject by the

introduction of the question of the amount of the sun’s specific heat.

If we simply consider the sun as an incandescent body in the process

of cooling, the question of the amount of the sun’s specific heat is

of the utmost importance; because the absolute amount of heat which

the sun is capable of giving out depends wholly upon his temperature

and specific heat. In this case three things only are required: (1),

the sun’s mass; (2), temperature of the mass; (3), specific heat of

the mass. But if we are considering what is the absolute amount of

heat which could have been given out by the sun on the hypothesis that

gravitation, either according to the meteoric theory suggested by Meyer

or according to the contraction theory advocated by Helmholtz, is the

only source of his heat, then we have nothing whatever to do with any

inquiries regarding the specific heat of the sun. This is evident

because the absolute amount of work which gravitation can perform in

the pulling of the particles of the sun’s mass together, is wholly

independent of the specific heat of those particles. Consequently, the

amount of energy in the form of heat thus imparted to the particles

by gravity must also be wholly independent of specific heat. That is

to say, the amount of heat imparted to a particle will be the same

whatever may be its specific heat.

Even supposing we limit the geological history of our globe to 100

millions of years, it is nevertheless evident that gravitation will not

account for the supply of the sun’s heat during so long a period. There

must be some other source of much more importance than gravitation.

What other source of energy greater than that of gravitation can there

be? It is singular that the opinion should have become so common even

among physicists, that there is no other conceivable source than

gravitation from which a greater amount of heat could have been derived.

_The Origin and Chief Source of the Sun’s Heat._—According to the

foregoing theories regarding the source of the sun’s heat, it is

assumed that the matter composing the sun, when it existed in space as

a nebulous mass, was not originally possessed of temperature, but that

the temperature was given to it as the mass became condensed under the

force of gravitation. It is supposed that the heat given out was simply

the heat of condensation. But it is quite conceivable that the nebulous

mass might have been possessed of an original store of heat previous to

condensation.

It is quite possible that the very reason why it existed in such a

rarefied or gaseous condition was its excessive temperature, and that

condensation only began to take place when the mass began to cool down.

It seems far more probable that this should have been the case than

that the mass existed in so rarefied a condition without temperature.

For why should the particles have existed in this separated form when

devoid of the repulsive energy of heat, seeing that in virtue of

gravitation they had such a tendency to approach to one another? But

if the mass was originally in a heated condition, then in condensing

it would have to part not only with the heat generated in condensing,

but also with the heat which it originally possessed, a quantity

which would no doubt much exceed that produced by condensation. To

illustrate this principle, let us suppose a pound of air, for example,

to be placed in a cylinder and heat applied to it. If the piston be so

fixed that it cannot move, 234·5 foot-pounds of heat will raise the

temperature of the air 1° C. But if the piston be allowed to rise as

the heat is applied, then it will require 330·2 foot-pounds of heat to

raise the temperature 1° C. It requires 95·7 foot-pounds more heat in

the latter case than in the former. The same amount of energy, viz.,

234·5 foot-pounds, in both cases goes to produce temperature; but in

the latter case, where the piston is allowed to move, 95·7 foot-pounds

of additional heat are consumed in the mechanical work of raising the

piston. Suppose, now, that the air is allowed to cool under the same

conditions: in the one case 234·5 foot-pounds of heat will be given

out while the temperature of the air sinks 1° C.; in the other case,

where the piston is allowed to descend, 330·2 foot-pounds will be given

out while the temperature sinks 1° C. In the former case, the air in

cooling has simply to part with the energy which it possesses in

the form of temperature; but in the latter case it has, in addition

to this, to part with the energy bestowed upon its molecules by the

descending piston. While the temperature of the gas is sinking 1°,

95·7 foot-pounds of energy in the form of heat are being imparted to

it by the descending piston; and these have to be got rid of before

the temperature is lowered by 1°. Consequently 234·5 foot-pounds of

the heat given out previously existed in the air under the form of

temperature, and the remaining 95·7 foot-pounds given out were imparted

to the air by the descending piston while the gas was losing its

temperature. 234·5 foot-pounds represent the energy or heat which the

air previously possessed, and 95·7 the energy or heat of condensation.

In the case of the cooling of the sun from a nebulous mass, there

would of course be no external force or pressure exerted on the mass

analogous to that of the piston on the air; but there would be, what

is equivalent to the same, the gravitation of the particles to each

other. There would be the pressure of the whole mass towards the centre

of convergence. In the case of air, and all perfect gases cooling

under pressure, about 234 foot-pounds of the original heat possessed

by the gas are given out while 95 foot-pounds are being generated by

condensation. We have, however, no reason whatever to believe that in

the case of the cooling of the sun the same proportions would hold

true. The proportion of original heat possessed by the mass of the sun

to that produced by condensation may have been much greater than 234 to

95, or it may have been much less. In the absence of all knowledge on

this point, we may in the meantime assume that to be the proportion.

The total quantity of heat given out by the sun resulting from the

condensation of his mass, on the supposition that the density of the

sun is uniform throughout, we have seen to be equal to 20,237,500

years’ sun-heat. Then the quantity of heat given out, which previously

existed in the mass as original temperature, must have been 49,850,000

years’ heat, making in all 70,087,500 years’ heat as the total amount.

The above quantity represents, of course, the total amount of heat

given out by the mass since it began to condense. But the geological

history of our globe must date its beginning at a period posterior to

that. For at that time the mass would probably occupy a much greater

amount of space than is presently possessed by the entire solar system;

and consequently, before it had cooled down to within the limits of

the earth’s present orbit, our earth could not have had an existence

as a separate planet. Previously to that time it must have existed as

a portion of the sun’s fiery mass. If we assume that it existed as a

globe previously to that, and came in from space after the condensation

of the sun, then it is difficult to conceive how its orbit should be so

nearly circular as it is at present.

Let us assume that by the time that the mass of the sun had condensed

to within the space encircled by the orbit of the planet Mercury (that

is, to a sphere having, say, a radius of 18,000,000 miles) the earth’s

crust began to form; and let this be the time when the geological

history of our globe dates its commencement. The total amount of heat

generated by the condensation of the sun’s mass from a sphere of this

size to its present volume would equal 19,740,000 years’ sun-heat.

The amount of original heat given out during that time would equal

48,625,000 years’ sun-heat,—thus giving a total of 68,365,000 years’

sun-heat enjoyed by our globe since that period. The total quantity may

possibly, of course, be considerably more than that, owing to the fact

that the sun’s density may increase greatly towards his centre. But we

should require to make extravagant assumptions regarding the interior

density of the sun and the proportion of original heat to that produced

by condensation before we could manage to account for anything like the

period that geological phenomena are supposed by some to demand.

The question now arises, by what conceivable means could the mass of

the sun have become possessed of such a prodigious amount of energy

in the form of heat previous to condensation? What power could have

communicated to the mass 50,000,000 years’ heat before condensation

began to take place?

_The Sun’s Energy may have originally been derived from Motion in

Space._—There is nothing at all absurd or improbable in the supposition

that such an amount of energy might have been communicated to the

mass. The Dynamical Theory of Heat affords an easy explanation of at

least _how_ such an amount of energy _may_ have been communicated. Two

bodies, each one-half the mass of the sun, moving directly towards

each other with a velocity of 476 miles per second, would by their

concussion generate in a _single moment_ the 50,000,000 years’ heat.

For two bodies of that mass moving with a velocity of 476 miles per

second would possess 4149 × 10^{38} foot-pounds of energy in the form

of _vis viva_; and this, converted into heat by the stoppage of their

motion, would give an amount of heat which would cover the present rate

of the sun’s radiation, for a period of 50,000,000 years.

Why may not the sun have been composed of two such bodies? And why may

not the original store of heat possessed by him have all been derived

from the concussion of these two bodies? Two such bodies coming into

collision with that velocity would be dissipated into vapour by such

an inconceivable amount of heat as would thus be generated; and when

they condensed on cooling, they would form one spherical mass like the

sun. It is perfectly true that two such bodies could never attain the

required amount of velocity by their mutual gravitation towards each

other. But there is no necessity whatever for supposing that their

velocities were derived from their mutual attraction alone. They might

have been approaching towards each other with the required velocity

wholly independent of gravitation.

We know nothing whatever regarding the absolute motion of bodies in

space. And beyond the limited sphere of our observation, we know

nothing even of their relative motions. There may be bodies moving

in relation to our system with inconceivable velocity. For anything

that we know to the contrary, were one of these bodies to strike our

earth, the shock might be sufficient to generate an amount of heat that

would dissipate the earth into vapour, though the striking body might

not be heavier than a cannon-ball. There is, however, nothing very

extraordinary in the velocity which we have found would be required

in the two supposed bodies to generate the 50,000,000 years’ heat. A

comet, having an orbit extending to the path of the planet Neptune,

approaching so near the sun as to almost graze his surface in passing,

would have a velocity of about 390 miles per second, which is within 86

miles of the required velocity.

But in the original heating and expansion of the sun into a gaseous

mass, an amount of work must have been performed against gravitation

equal to that which has been performed by gravitation during his

cooling and condensation, a quantity which we have found amounts to

about 20,000,000 years’ heat. The total amount of energy originally

communicated by the concussion must have been equal to 70,000,000

years’ sun-heat. A velocity of 563 miles per second would give this

amount. It must be borne in mind, however, that the 563 miles per

second is the velocity at the moment of collision; about one-half of

this velocity would be derived from the mutual attraction of the two

bodies in their approach to each other. Suppose each body to be equal

in volume to the sun, and of course one-half the density, the amount

of velocity which they would acquire by their mutual attraction would

be 274 miles per second, consequently we have to assume an original or

projected velocity of only 289 miles per second.

If we admit that gravitation is not sufficient to account for the

amount of heat given out by the sun during the geological history of

our globe, we are compelled to assume that the mass of which the sun is

composed existed prior to condensation in a heated condition; and if

so, we are further obliged to admit that the mass must have received

its heat from some source or other. And as the dissipation of heat into

space must have been going on, in all probability, as rapidly before

as after condensation took place, we are further obliged to conclude

that the heat must have been communicated to the mass immediately

before condensation began, for the moment the mass began to lose its

heat condensation would ensue. If we confine our speculations to causes

and agencies known to exist, the cause which has been assigned appears

to be the only conceivable one that will account for the production of

such an enormous amount of heat.

The general conclusion to which we are therefore led from physical

considerations regarding the age of the sun’s heat is, that the entire

geological history of our globe must be comprised within less than

100 millions of years, and that consequently the commencement of the

glacial epoch cannot date much farther back than 240,000 years.

The facts of geology, more especially those in connection with

denudation, seem to geologists to require a period of much longer

duration than 100 millions of years, and it is this which has so

long prevented them accepting the conclusions of physical science in

regard to the age of our globe. But the method of measuring subaërial

denudation already detailed seems to me to show convincingly that the

geological data, when properly interpreted, are in perfect accord with

the deductions of physical science. Perhaps there are now few who

have fairly considered the question who will refuse to admit that 100

millions of years are amply sufficient to comprise the whole geological

history of our globe.

_A false Analogy supposed to exist between Astronomy and

Geology._—Perhaps one of the things which has tended to mislead on

this point is a false analogy which is supposed to subsist between

astronomy and geology, viz., that geology deals with unlimited _time_,

as astronomy deals with unlimited _space_. A little consideration,

however, will show that there is not much analogy between the two cases.

Astronomy deals with the countless worlds which lie spread out in the

boundless infinity of space; but geology deals with only one world.

No doubt reason and analogy both favour the idea that the age of the

material universe, like its magnitude, is immeasurable; we have no

reason, however, to conclude that it is eternal, any more than we

have to infer that it is infinite. But when we compare the age of the

material universe with its magnitude, we must not take the age of one

of its members (say, our globe) and compare it with the size of the

universe. Neither must we compare the age of all the presently existing

systems of worlds with the magnitude of the universe; but we must

compare the past history of the universe as it stretches back into the

immensity of bygone _time_, with the presently existing universe as it

stretches out on all sides into limitless _space_. For worlds precede

worlds in time as worlds lie beyond worlds in space. Each world,

each individual, each atom is evidently working out a final purpose,

according to a plan prearranged and predetermined by the Divine Mind

from all eternity. And each world, like each individual, when it

serves the end for which it was called into existence, disappears to

make room for others. This is the grand conception of the universe

which naturally impresses itself on every thoughtful mind that has not

got into confusion about those things called in science the Laws of

Nature.[203]

But the geologist does not pass back from world to world as they stand

related to each other in the order of _succession in time_, as the

astronomer passes from world to world as they stand related to each

other in the order of _coexistence in space_. The researches of the

geologist, moreover, are not only confined to one world, but it is only

a portion of the history of that one world that can come under his

observation. The oldest of existing formations, so far as is yet known,

the Laurentian Gneiss, is made up of the waste of previously existing

rocks, and it, again, has probably been derived from the degradation

of rocks belonging to some still older period. Regarding what succeeds

these old Laurentian rocks geology tells us much; but of the formations

that preceded, we know nothing whatever. For anything that geology

shows to the contrary, the time which may have elapsed from the

solidifying of the earth’s crust to the deposition of the Laurentian

strata—an absolute blank—may have been as great as the time that has

since intervened.

_Probable Date of the Eocene and Miocene Periods._—If we take into

consideration the limit which physical science assigns to the age of

our globe, and the rapid rate at which, as we have seen, denudation

takes place, it becomes evident that the enormous period of 3 millions

of years comprehended in the foregoing tables must stretch far back

into the Tertiary age. Supposing that the mean rate of denudation

during that period was not greater than the present rate of denudation,

still we should have no less than 500 feet of rock worn off the face of

the country and carried into the sea during these 3 millions of years.

This fact shows how totally different the appearance and configuration

of the country in all probability was at the commencement of this

period from what it is at the present day. If it be correct that the

glacial epoch resulted from the causes which we have already discussed,

those tables ought to aid us in our endeavour to ascertain _how_ much

of the Tertiary period may be comprehended within these 3 millions of

years.

We have already seen (Chapter XVIII.) that there is evidence of a

glacial condition of climate at two different periods during the

Tertiary age, namely, about the middle of the Miocene and Eocene

periods respectively. As has already been shown, the more severe a

glacial epoch is, the more marked ought to be the character of its warm

inter-glacial periods; the greater the extension of the ice during the

cold periods of a glacial epoch the further should that ice disappear

in arctic regions during the corresponding warm periods. Thus the

severity of a glacial epoch may in this case be indirectly inferred

from the character of the warm periods and the extent to which the

ice may have disappeared from arctic regions. Judged by this test, we

have every reason to believe that the Miocene glacial epoch was one of

extreme severity.

The Eocene conglomerate, devoid of all organic remains, and containing

numerous enormous ice-transported blocks, is, as we have seen,

immediately associated with nummulitic strata charged with fossils

characteristic of a warm climate. Referring to this Sir Charles Lyell

says, “To imagine icebergs carrying such huge fragments of stone in so

southern a latitude, and at a period immediately preceded and followed

by the signs of a warm climate, is one of the most perplexing enigmas

which the geologist has yet been called upon to solve.”[204]

It is perfectly true that, according to the generally received theories

of the cause of a glacial climate the whole is a perplexing enigma, but

if we adopt the Secular theory of change of climate, every difficulty

disappears. According to this theory the very fact of the conglomerate

being formed at a period immediately preceded and succeeded by warm

conditions of climate, is of itself strong presumptive evidence of the

conglomerate being a glacial formation. But this is not all, the very

highness of the temperature of the preceding and succeeding periods

bears testimony to the severity of the intervening glacial period.

Despite the deficiency of direct evidence regarding the character of

the Miocene and Eocene glacial periods, we are not warranted, for

reasons which have been stated in Chapter XVII., to conclude that these

periods were less severe than the one which happened in Quaternary

times. Judging from indirect evidence, we have some grounds for

concluding that the Miocene glacial epoch at least was even more severe

and protracted than our recent glacial epoch.

By referring to Table I., or the accompanying diagram, it will be seen

that prior to the period which I have assigned as that of the glacial

epoch, there are two periods when the eccentricity almost attained

its superior limit. The first period occurred 2,500,000 years ago,

when it reached 0·0721, and the second period 850,000 years ago, when

it attained a still higher value, viz., 0·0747, being within 0·0028

of the superior limit. To the first of these periods I am disposed

to assign the glacial epoch of Eocene times, and to the second that

of the Miocene age. With the view of determining the character of

these periods Tables II. and III. have been computed. They give the

eccentricity and longitude of perihelion at intervals of 10,000 years.

It will be seen from Table II. that the Eocene period extends from

about 2,620,000 to about 2,460,000 years ago; and from Table III. it

will be gathered that the Miocene period lasted from about 980,000 to

about 720,000 years ago.

In order to find whether the eccentricity attained a higher value about

850,000 years ago than 0·0747, I computed the values for one or two

periods immediately before and after that period, and satisfied myself

that the value stated was indeed the highest, as will be seen from the

subjoined table:—

851,000 0·07454

850,000 0·074664

849,500 0·07466

849,000 0·07466

How totally different must have been the condition of the earth’s

climate at that period from what it is at present! Taking the mean

distance of the sun to be 91,400,000 miles, his present distance at

midwinter is 89,864,480 miles; but at the period in question, when the

winter solstice was in perihelion, his distance at midwinter would be

no less than 98,224,289 miles. But this is not all; our winters are at

present shorter than our summers by 7·8 days, but at that period they

would be longer than the summers by 34·7 days.

At present the difference between the perihelion and aphelion distance

of the sun amounts to only 3,069,580 miles, but at the period under

consideration it would amount to no less than 13,648,579 miles!

CHAPTER XXII.

A METHOD OF DETERMINING THE MEAN THICKNESS OF THE SEDIMENTARY

ROCKS OF THE GLOBE.

Prevailing Methods defective.—Maximum Thickness of British

Rocks.—Three Elements in the Question.—Professor Huxley

on the Rate of Deposition.—Thickness of Sedimentary Rocks

enormously over-estimated.—Observed Thickness no Measure of

mean Thickness.—Deposition of Sediment principally along

Sea-margin.—Mistaken Inference regarding the Absence of a

Formation.—Immense Antiquity of existing Oceans.

Various attempts have been made to measure the positive length of

geological periods. Some geologists have sought to determine, roughly,

the age of the stratified rocks by calculations based upon their

probable thickness and the rate at which they may have been deposited.

This method, however, is worthless, because the rates which have been

adopted are purely arbitrary. One geologist will take the rate of

deposit at a foot in a hundred years, while another will assume it

to be a foot in a thousand or perhaps ten thousand years; and, for

any reasons that have been assigned, the one rate is just as likely

to be correct as the other: for if we examine what is taking place

in the ocean-bed at the present day, we shall find in some places a

foot of sediment laid down in a year, while in other places a foot

may not be deposited in a thousand years. The stratified rocks were

evidently formed at all possible rates. When we speak of the rate of

their formation, we must of course refer to the _mean rate_; and it is

perfectly true that if we knew the thickness of these rocks and the

mean rate at which they were deposited, we should have a ready means

of determining their positive age. But there appears to be nearly as

great uncertainty regarding the thickness of the sedimentary rocks as

regarding the rate at which they were formed. No doubt we can roughly

estimate their probable maximum thickness; for instance, Professor

Ramsay has found from actual measurement, that the sedimentary

formations of Great Britain have a maximum thickness of upwards of

72,000 feet; but all such measurements give us no idea of their mean

thickness. What is the mean thickness of the sedimentary rocks of

the globe? On this point geology does not afford a definite answer.

Whatever the present mean thickness of the sedimentary rocks of our

globe may be, it must be small in comparison to the mean thickness

of all the sedimentary rocks which have been formed. This is obvious

from the fact that the sedimentary rocks of one age are partly formed

from the destruction of the sedimentary rocks of former ages. From the

Laurentian age down to the present day, the stratified rocks have been

undergoing constant denudation.

Unless we take into consideration the quantity of rock removed during

past ages by denudation, we cannot—even though we knew the actual mean

thickness of the existing sedimentary rocks of the globe, and the rate

at which they were formed—arrive at an estimate regarding the length of

time represented by these rocks. For if we are to determine the age of

the stratified rocks from the rate at which they were formed, we must

have, not the present quantity of sedimentary rocks, but the present

plus the quantity which has been denuded during past ages. In other

words, we must have the absolute quantity formed. In many places the

missing beds must have been of enormous thickness. The time represented

by beds which have disappeared is, doubtless, as already remarked,

much greater than that represented by the beds which now remain. The

greater mass of the sedimentary rocks has been formed out of previously

existing sedimentary rocks, and these again out of sedimentary rocks

still older. As the materials composing our stratified beds may have

passed through many cycles of destruction and re-formation, the time

required to have deposited at a given rate the present existing mass

of sedimentary rocks may be but a fraction of the time required to

have deposited at the same rate the total mass that has actually been

formed. To measure the age of the sedimentary rocks by the present

existing rocks, assumed to be formed at some given rate, even supposing

the rate to be correct, is a method wholly fallacious.

“The aggregate of sedimentary strata in the earth’s crust,” says Sir

Charles Lyell, “can never exceed in volume the amount of solid matter

which has been ground down and washed away by rivers, waves, and

currents. How vast, then, must be the spaces which this abstraction

of matter has left vacant! How far exceeding in dimensions all the

valleys, however numerous, and the hollows, however vast, which we can

prove to have been cleared out by aqueous erosion!”[205]

I presume there are few geologists who would not admit that if all the

rocks which have in past ages been removed by denudation were restored,

the mean thickness of the sedimentary rocks of the globe would be at

least equal to their present maximum thickness, which we may take at

72,000 feet.

There are three elements in the question; of which if two are known,

the third is known in terms of the other two. If we have the mean

thickness of all the sedimentary rocks which have been formed and the

mean rate of formation, then we have the time which elapsed during the

formation; or having the thickness and the time, we have the rate; or,

having the rate and the time, we have the thickness.

One of these three, namely, the rate, can, however, be determined with

tolerable accuracy if we are simply allowed to assume—what is very

probable, as has already been shown—that the present rate at which the

sedimentary deposits are being formed may be taken as the mean rate

for past ages. If we know the rate at which the land is being denuded,

then we know with perfect accuracy the rate at which the sedimentary

deposits are being formed in the ocean. This is obvious, because all

the materials denuded from the land are deposited in the sea; and

what is deposited in the sea is just what comes off the land, with the

exception of the small proportion of calcareous matter which may not

have been derived from the land, and which in our rough estimate may be

left out of account.

Now the mean rate of subaërial denudation, we have seen, is about one

foot in 6,000 years. Taking the proportion of land to that of water

at 576 to 1,390, then one foot taken off the land and spread over the

sea-bottom would form a layer 5 inches thick. Consequently, if one foot

in 6,000 years represents the mean rate at which the land is being

denuded, one foot in 14,400 years represents the mean rate at which the

sedimentary rocks are being formed.

Assuming, as before, that 72,000 feet would represent the mean

thickness of all the sedimentary rocks which have ever been formed,

this, at the rate of one foot in 14,400 years, gives 1,036,800,000

years as the age of the stratified rocks.

Professor Huxley, in his endeavour to show that 100,000,000 years is

a period sufficiently long for all the demands of geologists, takes

the thickness of the stratified rocks at 100,000 feet, and the rate

of deposit at a foot in 1,000 years. One foot of rock per 1,000 years

gives, it is true, 100,000 feet in 100,000,000 years. But what about

the rocks which have disappeared? If it takes a hundred millions of

years to produce a mass of rock equal to that which now exists, how

many hundreds of millions of years will it require to produce a mass

equal to what has actually been produced?

Professor Huxley adds, “I do not know that any one is prepared to

maintain that the stratified rocks may not have been formed on the

average at the rate of 1/83rd of an inch per annum.” When the rate,

however, is accurately determined, it is found to be, not 1/83rd of

an inch per annum, but only 1/1200th of an inch, so that the 100,000

feet of rock must have taken 1,440,000,000 years in its formation,—a

conclusion which, according to the results of modern physics, is wholly

inadmissible.

Either the thickness of the sedimentary rocks has been over-estimated,

or the rate of their formation has been under-estimated, or both.

If it be maintained that a foot in 14,400 years is too slow a rate

of deposit, then it must be maintained that the land must have been

denuded at a greater rate than one foot in 6,000 years. But most

geologists probably felt surprised when the announcement was first

made, that at this rate of denudation the whole existing land of the

globe would be brought under the ocean in 6,000,000 of years.

The error, no doubt, consists in over-estimating the thickness of the

sedimentary rocks. Assuming, for physical reasons already stated, that

100,000,000 years limits the age of the stratified rocks, and that the

proportion of land to water and the rate of denudation have been on the

average the same as at present, the mean thickness of sedimentary rocks

formed in the 100,000,000 years amounts to only 7,000 feet.

But be it observed that this is the mean thickness on an area equal

to that of the ocean. Over the area of the globe it amounts to only

5,000 feet; and this, let it be observed also, is the total mean

thickness formed, without taking into account what has been removed

by denudation. If we wish to ascertain what is actually the present

mean thickness, we must deduct from this 5,000 feet an amount of rock

equal to all the sedimentary rocks which have been denuded during

the 100,000,000 years; for the 5,000 feet is not the present mean

thickness, but the total mean thickness formed during the whole of the

100,000,000 years. If we assume, what no doubt most geologists would be

willing to grant, that the quantity of sedimentary rocks now remaining

is not over one-half of what has been actually deposited during the

history of the globe, then the actual mean thickness of the stratified

rocks of the globe is not over 2,500 feet. This startling result would

almost necessitate us to suspect that the rate of subaërial denudation

is probably greater than one foot in 6,000 years. But, be this as it

may, we are apt, in estimating the mean thickness of the stratified

rocks of the globe from their ascertained maximum thickness, to arrive

at erroneous conclusions. There are considerations which show that

the mean thickness of these rocks must be small in proportion to their

maximum thickness. The stratified rocks are formed from the sediment

carried down by rivers and streamlets and deposited in the sea. It is

obvious that the greater quantity of this sediment is deposited near

the mouths of rivers, and along a narrow margin extending to no great

distance from the land. Did the land consist of numerous small islands

equally distributed over the globe, the sediment carried off from these

islands would be spread pretty equally over the sea-bottom. But the

greater part of the land-surface consists of two immense continents.

Consequently, the materials removed by denudation are not spread

over the ocean-bottom, but on a narrow fringe surrounding those two

continents. Were the materials spread over the entire ocean-bed, a foot

removed off the general surface of the land would form a layer of rock

only five inches thick. But in the way in which the materials are at

present deposited, the foot removed from the land would form a layer

of rock many feet in thickness. The greater part of the sediment is

deposited within a few miles of the shore.

The entire coast-line of the globe is about 116,500 miles. I should

think that the quantity of sediment deposited beyond, say, 100 miles

from this coast-line is not very great. No doubt several of the large

rivers carry sediment to a much greater distance from their mouths than

100 miles, and ocean currents may in some cases carry mud and other

materials also to great distances. But it must be borne in mind that

at many places within the 100 miles of this immense coast-line little

or no sediment is deposited, so that the actual area over which the

sediment carried off the land is deposited is probably not greater than

the area of this belt—116,500 miles long and 100 miles broad. This

area on which the sediment is deposited, on the above supposition, is

therefore equal to about 11,650,000 square miles. The amount of land on

the globe is about 57,600,000 square miles. Consequently, one foot of

rock, denuded from the surface of the land and deposited on this belt,

would make a stratum of rock 5 feet in thickness; but were the sediment

spread over the entire bed of the ocean, it would form, as has already

been stated, a stratum of rock of only 5 inches in thickness.

Suppose that no subsidence of the land should take place for a period

of, say, 3,000,000 of years. During that period 500 feet would be

removed by denudation, on an average, off the land. This would make a

formation 2,500 feet thick, which some future geologist might call the

Post-tertiary formation. But this, be it observed, would be only the

mean thickness of the formation on this area; its maximum thickness

would evidently be much greater, perhaps twice, thrice, or even four

times that thickness. A geologist in the future, measuring the actual

thickness of the formation, might find it in some places 10,000 feet

in thickness, or perhaps far more. But had the materials been spread

over the entire ocean-bed, the formation would have a mean thickness

of little more than 200 feet; and spread over the entire surface of

the globe, would form a stratum of scarcely 150 feet in thickness.

Therefore, in estimating the mean thickness of the stratified rocks of

the globe, a formation with a maximum thickness of 10,000 feet may not

represent more than 150 feet. A formation with a _mean_ thickness of

10,000 feet represents only 600 feet.

Climate and Time in Their Geological Relations A Theory of Secular Changes of the Earth's Climate · 第 36 / 41 章

The recent discovery of the Cretaceous formation in Greenland shows