We seem to think that the earth has sown all her wild oats, that her
riotous youth is far behind her, and that she is now passing into a
serene old age. Had we lived during any of the great periods of the
past, we might have had the same impression, so tranquil, for the
most part, has been the earth's history, so slow and rhythmical have
been the beats of the great clock of time. We see this in the
homogeneity of the stratified rocks, layer upon layer for thousands
of feet as uniform in texture and quality as the goods a modern
factory turns out, every yard of it like every other yard. No hitch
or break anywhere. The bedding-planes of many kinds of rock occur at
as regular intervals as if they had been determined by some kind of
machinery. Here, on the formation where I live, there are alternate
layers of slate and sandstone, three or four inches thick, for
thousands of feet in extent; they succeed each other as regularly as
the bricks and mortar in a brick wall, and are quite as homogeneous.
What does this mean but that for an incalculable period the
processes of erosion and deposition went on as tranquilly as a
summer day? There was no strike among the workmen, and no change in
the plan of the building, or in the material.
The Silurian limestone, the old red sandstone, the Hamilton flag,
the Oneida conglomerate, where I have known them, are as homogeneous
as a snowbank, or as the ice on a mountain lake; grain upon grain,
all from the same source in each case, and sifted and sorted by the
same agents, and the finished product as uniform in color and
quality as the output of some great mill.
Then, after a vast interval, there comes a break: something like an
end and a new beginning, as if one day of creation were finished and
a new one begun. The different formations lie unconformably upon
each other, which means revolution of some sort. There has been a
strike or a riot in the great mill, or it has lain idle for a long
period, and when it has resumed, a different product is the result.
Something happened between each two layers. What?
Though in remote geological ages the earth-building and
earth-shaping forces were undoubtedly more active than they are now,
and periods of deformation and upheaval were more frequent, yet had
we lived in any of those periods we should probably have found the
course of nature, certainly when measured by human generations, as
even and tranquil as we find it to-day. The great movements are so
slow and gentle, for the most part, that we should not have been
aware of them had we been on the spot. Once in a million or a
half-million years there may have been terrific earthquakes and
volcanic eruptions, such as seem to have taken place in Tertiary
time, and at the end of the Palaeozoic period. Yet the vast
stretches of time between were evidently times of tranquillity.
It is probable that the great glacial winter of Pleistocene times
came on as gradually as our own winter, or through a long period of
slowly falling temperature, and as it seems to have been many
hundred thousand times as long, this preceding period, or great
fall, was probably equally long--so long that the whole of recorded
human history would form but a small fraction of it. It may easily
be, I think, that we are now living in the spring of the great cycle
of geologic seasons. The great ice-sheet has withdrawn into the Far
North like snowbanks that linger in our wood in late spring, where
it still covers Greenland as it once covered this country. When the
season of summer is reached, some hundreds of thousands of years
hence, it may be that tropical life, both animal and vegetable, will
again flourish on the shores of the Arctic Ocean, as it did in
Tertiary times. And all this change will come about so quietly and
so slowly that nobody will suspect it.
That the crust of the earth is becoming more and more stable seems a
natural conclusion, but that all folding and shearing and disruption
of the strata are at an end, is a conclusion we cannot reach in the
face of the theory that the earth is shrinking as it cools.
The earth cools and contracts with almost infinite slowness, and the
great crustal changes that take place go on, for the most part, so
quietly and gently that we should not suspect them were we present
on the spot, and long generations would not suspect them. Elevations
have taken place across the beds of rivers without deflecting the
course of the river; the process was so slow that the river sawed
down through the rock as fast as it came up. Nearly all the great
cosmic and terrestrial changes and revolutions are veiled from us by
this immeasurable lapse of time.
Any prediction about the permanence of the land as we know it, or as
the race has known it, or of our immunity from earthquakes or
volcanic eruptions, or of a change of climate, or of any cosmic
catastrophe, based on human experience, is vain and worthless. What
is or has been in man's time is no criterion as to what will be in
God's time. The periods of great upheaval and deformation in the
earth's crust appear to be separated by millions of years. Away back
in pre-Cambrian times, there appear to have been immense periods
during which the peace and repose of the globe were as profound as
in our own time. Then at the end of Palaeozoic time--how many
millions of years is only conjectural--the truce of aeons was
broken, and the dogs of war let loose; it was a period of revolution
which resulted in the making of one of our greatest
mountain-systems, the Appalachian, and in an unprecedented
extinction of species. Later eras have witnessed similar
revolutions. Why may they not come again? The shrinking of the
cooling globe must still go on, and this shrinking must give rise to
surface disturbances and dislocations, maybe in the uplift of new
mountain-ranges from the sea-bottom, now undreamed of, and in
volcanic eruptions as great as any in the past. Such a shrinkage and
eruption made the Hawaiian Islands, probably in Tertiary times; such
a shrinkage may make other islands and other continents before
another period of equal time has elapsed.
Of course the periods and eras into which the geologists divide
geologic time are as arbitrary as the months and seasons into which
we divide our year, and they fade out into each other in much the
same way; but they are really as marked as our seasonal divisions.
Not in their climates--for the climate of the globe seems to have
been uniformly warm from pole to pole, without climatic zones,
throughout the vast stretch of Palaeozoic and Mesozoic times--but in
the succession of animal and vegetable life which they show. The
rocks are the cemeteries of the different forms of life that have
appeared upon the globe, and here the geologist reads their
succession in time, and assigns them to his geologic horizons
accordingly. The same or allied forms appeared upon all parts of the
earth at approximately the same time, so that he can trace his
different formations around the world by the fossils they hold. Each
period had its dominant forms. The Silurian was the great age of
trilobites; the Devonian, the age of fishes; Mesozoic times swarm
with the gigantic reptiles; and in Tertiary times the mammals are
dominant. Each period and era has its root in that which preceded
it. There were rude, half-defined fishes in the Silurian, and
probably the beginning of amphibians in the Devonian, and some small
mammalian forms in the Mesozoic time, and doubtless rude studies of
the genus Homo in Tertiary times. Nature works up her higher forms
Jike a human inventor from rude beginnings. Her first models barely
suggest her later achievements.
In the vegetable world it has been the same; from the first simple
algae in the Cambrian seas up to the forests of our own times, the
gradation is easily traced. Step by step has vegetable life mounted.
The great majority of the plants and animals of one period fail to
pass over into the next, just as our spring flowers fail to pass
over into summer, and our summer flowers into fall. But the law of
evolution is at work, and life always rises on stepping-stones of
its dead self to higher things.
V