of which bears the stamp of time, are
good to show how slowly the mass has been accumulated.
↑| 1 blocks not present in 1859 1872; present in 1860 1861 1866 1869 | | In the Cordillera I estimated one pile
of conglomerate
at ten thousand feet
in thickness.
|
| him 1859 | | the observer 1860 1861 1866 |
| remark, 1859 | | remark 1860 1861 1866 |
|
countries! Professor Ramsay has given me the maximum thickness, in most cases from actual measurement, in a few cases from estimate, of each formation in different parts of Great Britain; and this is the result:—
Feet. Palæozoic strata (not including igneous beds) .. 57,154
Secondary strata .. .. .. .. .. .. .. 13,190
Tertiary strata .. .. .. .. .. .. .. .. 2,240
1859 |
|
countries! Professor Ramsay has given me the maximum thickness, in most cases from actual measurement, in a few cases from estimate, of each formation in different parts of Great Britain; and this is the result:—
Feet.
Palæozoic strata (not including igneous beds) .. 57,154
Secondary strata .. .. .. .. .. .. .. 13,190
Tertiary strata .. .. .. .. .. .. .. 2,240
1860 |
|
countries! Professor Ramsay has given me the maximum thickness, in most cases from actual measurement, in a few cases from estimate, of each formation in different parts of Great Britain; and this is the result:—
Feet.
Palæozoic strata (not including igneous beds) .. 57,154
Secondary strata .. .. .. .. .. .. .. 13,190
Tertiary strata .. .. .. .. .. .. .. 2,240
1861 |
|
countries! Professor Ramsay has given me the maximum thickness, in most cases from actual measurement, in a few cases from estimate, of each formation in
different
parts of Great Britain; and this is the result:—
Feet.
Palæozoic strata (not including igneous beds) .. 57,154
Secondary strata .. .. .. .. .. .. .. 13,190
Tertiary strata .. .. .. .. .. .. .. 2,240
1866 |
| 1 blocks not present in 1859 1860 1861 1866; present in 1869 1872 | | Professor Ramsay has given me the maximum thickness, in most cases from actual measurement, in a few cases from estimate, of each formation in different parts of Great Britain; and this is the result:—
Feet.
Palæozoic strata (not including igneous beds) ..
57,154
Secondary strata .. .. .. .. .. .. .. 13,190
Tertiary strata .. .. .. .. .. .. .. .. 2,240
|
| these 1859 | | the 1860 1861 1866 1869 1872 |
| Britain, 1859 1860 1861 | | Britain 1866 1869 1872 |
| 1 blocks not present in 1859 1860 1861 1866 1872; present in 1869 | | The consideration of these various facts impresses the mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity.
|
| may be quite erroneous; 1859 |
| has no pretension to strict exactness; 1860 1861 1866 |
| in 1859 1860 | | over 1861 1866 |
| area 1859 1860 | | extensive area 1861 1866 |
| 14 blocks not present in 1859 1860 1861 1866; present in 1869 1872 | | Nevertheless this impression is partly false.
Mr. Croll, in a most
interesting paper, remarks that we do not err "in forming too great a conception of the length of geological periods," but in estimating them by years.
When geologists look at large and complicated phenomena, and then at the figures representing several million years, the two produce a totally different effect on the mind, and the figures are at once pronounced to be
too small.
But in
regard to denudation,
Mr. Croll shows, by calculating the known amount of sediment annually brought down by certain rivers, relatively to the
areas of drainage, that 1000 feet of rock, disintegrated through subaerial agencies,
would thus be removed from the mean level of the whole area in the course of six million years.
This seems an astonishing result, and some considerations lead to the suspicion that it may be much
too large, but even if halved or quartered it is still very surprising.
Few of us, however, know what a million really means: Mr. Croll gives the following illustration: take
a narrow strip of paper, 83 feet 4 inches in length, and stretch it along the wall of a large hall; then mark off at one end the tenth of an inch.
This tenth of an inch will represent one hundred years, and the entire strip a million years.
But let it be borne in mind, in relation to the subject of this work, what a hundred years implies, represented as it is by a measure utterly insignificant in a hall of the above dimensions.
Several eminent breeders, during a single lifetime, have so largely modified some of the higher animals, which propagate their kind much more slowly than most of the lower animals, that they have formed what well deserve
to be called new
sub-breeds.
Few men have attended with due care to any one strain for more than half a century, so that a hundred years represents the work of two breeders in succession.
It is not to be supposed
that species in a state of nature ever change so quickly as domestic animals under the guidance of methodical selection.
The comparison would be in every way fairer with the results
which follow from unconscious selection, that is the preservation of the most useful or beautiful animals, with no intention of modifying the breed; but by this process of unconscious selection, various breeds have been sensibly changed in the course of two or three centuries.
Species, however, probably change much more slowly, and within the same country only a few change at the same time.
This slowness follows from all the inhabitants of the same country being already so well adapted to each other
that new places in the polity of nature do not occur until after long intervals, when
changes of some kind
in the physical conditions
or through immigration have occurred; and individual differences or variations of the right nature, by which some of the inhabitants might be better fitted to their new places under the altered circumstances, might not at once occur.
|
| 2 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | | Moreover variations or individual differences of the right nature, by which some of the inhabitants might be better fitted to their new places under the altered circumstances, would not always occur at once.
Unfortunately we have no means of determining, according to the standard of years, how long a period it takes to modify a species; but to the subject of time we must return.
|
| 5 blocks not present in 1859 1860 1861 1866 1872; present in 1869 | | According to the standard of years we have no means of determining how long a period it takes to modify a species.
Mr. Croll judging from the amount of heat-energy in the sun and from the date which he assigns to the last glacial epoch, estimates that only sixty million years have elapsed since the deposition of the first Cambrian formation.
This appears a very short period for so many and such great mutations in the forms of life, as have certainly since occurred.
It is admitted that many of the elements in the calculation are more or less doubtful, and Sir W. Thomson gives a wide margin to the possible age of the habitable world.
But as we have seen, we cannot comprehend what the figures 60,000,000 really imply; and during this, or perhaps a longer roll of years, the land and the waters have everywhere teemed with living creatures, all exposed to the struggle for life and undergoing change.
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