there appear some marks of sedges and other vegetation, and these are
inclined to the west, as if the plants had been bent by an east wind.
Just above the height to which the imprints of the vegetation extend,
ripple marks begin to appear, running on a northeast-southwest
course. They were made by a southeast wind, for their northwest
slopes are the steeper. A little above this height some peculiar
small elevations appear on one of the bedding planes, and slightly
raised ridges run for a short distance to the northeast from each
elevation, vanishing in the same direction (Fig. 6). A southwesterly
current was unmistakably obstructed by the little elevations, and
left the small trails of dust in their lee. Six inches higher up the
wind comes more from the south, and for the next foot the ripples
continue to gradually turn still more in the same direction so as to
at last record a due south wind. At this point it suddenly changed
and set in squarely from the west, for the ripples are turned north
and south, with the steeper slopes to the east. This direction seems
to have prevailed as long as the dust kept on falling. It appears
to me that these successive changes are best explained as attendant
upon the passage of a cyclone, or of what our daily weather maps call
a "low area." Going by from west to east, on the north, it would at
first cause an east wind. This would then gradually be turned to the
south and then to the west. One such rotation of the wind generally
lasts a day or two. The shower must then have kept on at least for
the same length of time, if not longer (Fig. 7).
[Illustration: FIG. 7.--CHANGES IN THE WIND AS RECORDED BY THE RIPPLE
MARKS.]
There is reason to believe that this catastrophe occurred in summer.
No crayfish would be out making tracks during the cold months, and
the fossil vegetation could hardly have left such plain marks if it
had been buried by the dust during the winter. The most conspicuous
of these marks are some triangular and Y-shaped molds of the stems
and leaves of sedges. Siliceous skeletons of _chara_ and filamentous
algæ were also found upon a close examination in some of these molds.
It is really difficult to appreciate the change such a shower must
have produced in the appearance of the landscape, and the effect it
must have had on animal and plant life. So far away from the volcanic
source, the wind can not have laid down a layer of this dust several
feet in thickness without scattering it far and wide all around. It
must have covered tens of thousands of square miles. Just imagine,
if you can, a whole State, clad in the verdure of summer, suddenly,
in two or three days, covered over by a blanket of white volcanic
ash! Many species of plants must have found it impossible to grow
in such a soil. And what disaster it must have caused in the animal
world! Grazing herds had their sustenance buried from their sight,
and could save their lives only by traveling long distances in this
loose dust. Many a creature must have had its lungs or its gills
clogged with the glassy flakes floating in the water and in the air.
The sudden disappearance of several mammal species near the beginning
of the Quaternary age has been noted by paleontologists. Does it seem
unlikely that an event like this, especially if repeated, may have
hastened the extermination of some species of land animals? That many
individuals must have perished there can be no doubt. Not very far
away from that outcrop of the dust which I have described, one of the
early settlers in this part of the State once made a deep well that
penetrated the ash. Above the deposit, and some sixty feet below the
surface of the prairie, he found what he described as "an old bone
yard." In digging other wells in this vicinity mammal bones have been
taken up by the settlers from about the same horizon. It is to be
regretted that, with one exception, none of these fossils have been
preserved for study, for it is likely that they were the remains of
animals which were killed in the dust shower.
In the absence of fossils definitely known to be connected with the
ash, its exact age seems yet uncertain. In McPherson County it is
underlaid by clay, gravel, and sand, which contain remains of the
horse, of a megalonyx, and of bivalve mollusks of modern aspect. In
the bluffs of the Missouri River near Omaha pockets of a similar
ash rest on glacial clay under the loess. At the latter place it
must belong to the Pleistocene age, and at the former it can not be
older than the late Pleiocene. These two deposits may not belong
to the same shower, but it appears, at any rate, that the volcanic
disturbances which produced them occurred near the beginning of the
Pleistocene age.
In comparison with the slow and even tenor of the routine of
geological history, the event here sketched appears so unique and
so striking that it may well be called a geological romance. Modern
science has taught us that the geological forces are slow and largely
uniform in their work, and that most of the earth's features must be
explained without taking recourse to theories involving any violent
revolutions or general terrestrial cataclysms. While the making of
this dust is not any real exception to the law of uniformity, we are
here reminded that Nature is quite independent in her ways, and that
even in her sameness there is room for considerable diversity.
* * * * *
Mr. William Ogilvie, of the Topographical Survey of Canada,
estimates that there are more than 3,200 miles of fair navigation
in the system of the Yukon River, of which Canada owns nearly
forty-two per cent. A remarkable feature of the river, with its
Lewes branch, is that it drains the Peninsula of Alaska and nearly
cuts it in two, starting as it does less than fourteen miles, "as
the crow flies," from the waters of the Pacific Ocean, at the
extreme head of the Lewes branch, whence it flows 2,100 miles into
the same ocean, or Bering Sea, which is a part of it. The drainage
basin of the river occupies about 388,000 square miles, of which
Canada owns 149,000 square miles, or nearly half, but that half
is claimed to be the most important. As for the origin of the
name Yukon, the Indians along the middle stretches of the river
all speak the same language, and call the river the Yukonah; in
English, "the great river" or "the river." The Canadian Indians
in the vicinity of Forty Mile call it "Thetuh," a name of which
Mr. Ogilvie could not learn the meaning. The correct Indian name
of the Klondike is _Troandik_, meaning Hammer Creek, and refers
to the barriers the Indians used to erect across the mouth of the
stream to catch salmon, by hammering sticks into the ground.
FOOTNOTE:
[31] Dr. Samuel Aughey, Physical Geography of Nebraska, 1880. Prof.
J. E. Todd, Science, April 23, 1886, and January 8, 1897. E. H.
Barbour, Publication No. V, Nebraska Academy of Sciences. J. A.
Udden, The American Geologist, June, 1891, and April, 1893. R. D.
Salisbury, Science, December 4, 1896. G. P. Merril, Proceedings of
the United States National Museum, 1885.
THE SEASON OF THE YEAR.
BY GRANT ALLEN.
A year is, roughly speaking, the period which it takes the earth
to perform one complete revolution round the sun. I say "roughly
speaking" with due humility, having the fear of the expert ever
before my eyes, because I know that if I do not sing small, that
inconvenient person, the astronomical critic, will come down upon
me at once like a wolf on the fold, with minute distinctions about
the mean, the tropical, and the sidereal year; matters of immense
importance at Greenwich Observatory, no doubt, but elsewhere of very
little interest indeed, seeing that they differ from one another
by so many minutes only. Let us leave the astronomers their own
problems. The year with which I am going to deal humbly here is a
much more commonplace, ordinary, and comprehensible year--the visible
year of vegetation, of plant and animal life, of the four seasons;
the year as roughly known to children and savages, and to the weeds,
the flowers, the bees, and the squirrels.
It has often struck me as curious that people took this complex
concept of the year so much for granted--inquired so little into its
origin and discovery. Yet it is by no means everywhere obvious. How
did men first come to notice, in the tropics especially, that there
was such a thing as the year at all? How did they first observe, save
in our frozen north, any fixed sequence or order in the succession of
Nature? How did they learn, even here, that spring would infallibly
follow winter, and summer be succeeded in due course by autumn?
And, to go a step farther back, how did the plants and animals, in
all parts of the world alike, come originally to discover and adapt
themselves to all these things? How did the bee know that she must
"gather honey all the day from every opening flower," the summer
through, in order to use it up as bodily fuel in winter? How did the
plants learn when to blossom and produce seed? In one word, how did
the seasons come to be automatically recognized?
That they _are_ automatically recognized, even by plants, quite
apart from the stimulus of heat or cold, drought or rain, a single
fact (out of many like it) will sufficiently prove. Trees brought
from Australia to England, where the seasons are reversed, try for
two or three years to put forth leaves and flowers in October or
November--the southern spring. It takes them several autumns before
they learn that the year has been turned upside down--that June is
now summer and December winter. This shows that life moves in regular
cycles, adapted to the seasons, but not directly dependent upon
them. The rhythm of the world has set up an organic rhythm which now
spontaneously and automatically follows it.
At first sight, to the dweller in the temperate zone at the present
day, the questions I have put above may seem needless, not to say
childish. But that is perhaps because we have all too much the
habit of taking it for granted that what is true here and now has
also been true everywhere and always. A first visit to the tropics
often enough rudely disturbs this uninquiring attitude of mind. For
in the tropics, and especially in the equatorial region, there is
no winter and no summer, no spring and no autumn. The world wags
wearily through an unending display of monotonous greenery. As far as
temperature goes, the year is pretty much alike in all its months.
Yet not only do equatorial men recognize the existence of the year as
a natural epoch quite as much as other men--not only do equatorial
savages celebrate annual feasts, count ages by years, and perform
certain rites in certain months only--but also animal and vegetable
nature recognizes the year; trees have their month for blossoming and
fruiting, birds their month for assuming the plumage of courtship,
for nesting and hatching, almost as markedly as elsewhere. The
recognition of the year both by man and by Nature is not therefore
entirely dependent upon the difference of summer and winter, as such.
We must go deeper, and I think, when we come to consider geological
time, much deeper, if we wish to understand the true character of
yearliness--a word which I venture here to coin to express this
meaning.
Have you ever quite realized what the tropical year is like? Suppose
you are living on or near the equator, then in December the sun is
south of you and at its greatest distance away; you have, so to
speak, a relative winter. But in March the sun is overhead; it is
now full midsummer. By the end of June the sun has gone north, and
is once more on a tropic; you have a second winter; not much of
a winter, I admit, but still, a relative winter. By September he
has returned overhead again, and you are enduring a second summer.
In December he has once more retreated to the southern tropic
(Capricorn), and it is comparative winter. Thus the equatorial year
consists of four distinct seasons, in two of which the sun stands
directly overhead, while in two he is at his northern or southern
limit. I may add that the effect is always curious when, as you face
the sun, you see that he is moving in his diurnal path, not from
left to right ("the way of the sun," as we say), but from right to
left (or "widdershins"). You are never till then aware how natural
and inevitable has seemed the opposite direction: when you find it
reversed the effect is surprising.
Now, the distance to which the sun travels north or south of you, if
you live on the equator--I use ordinary terms instead of astronomical
ones for simplicity's sake--is so comparatively small that within the
tropics themselves you never notice much difference as to the amount
of heat between one period of the year and another. In equatorial
countries the day and night temperature is much the same all the year
round: if the country be plain, it is always hot; if mountainous,
like the district about Bogotá, it is "a perpetual spring"; one day
is always much the same as the one that went before and the one that
comes after it. Even on the actual tropics, again, the difference
is too slight to make any marked change in the temperature; people
living on the northern tropic (Cancer), for example, have the sun
vertical to them on June 21st, and some forty-three degrees south of
them on December 21st. Nevertheless, the sun is still as near them
and as powerful as he is at Milan or Venice in the height of summer;
and the consequence is that, as a matter of fact, the thermometer
within the tropics and at sea level seldom descends below 75° or
80°, even at midnight in the relative winters. For the heating power
of the sun depends, of course, upon the directness of his rays, and
lessens with their obliquity; in Venice and Milan they are strong
enough to make the ground very hot in July and August, though it has
been cooled before by a northern winter; much more than in Jamaica or
Madagascar, which have never been cooled, does the accumulated heat
keep everything warm even when the sun is most oblique--and he never
reaches the same obliquity as in an English summer. The ground is
hot, the houses are hot, wood and stone are hot, and they have all
been hot from time immemorial.
Yet tropical and equatorial trees and plants have their definite
seasons to flower and fruit, just the same as elsewhere. This seems
surprising at first when one visits the tropics. You can not see
why everything should not flower and fruit the whole year round.
And yet, at one time pineapples are "in," at another mangoes. And
these seasons differ in the northern and southern hemispheres; what
is mango winter in the one being mango summer in the other. I do
not say the seasons anywhere in the tropics differ markedly; still,
they do differ; the tropical year is divided into times and months
for agriculture just as much as any other. Thus there are regular
dates in each hemisphere for planting, tending, and cutting the sugar
cane. Now, what is the reason of these changes in vegetation, when
temperature remains so constant? Why do not trees and shrubs of each
kind flower up and down throughout the year irregularly--now one
individual and now another? Why are there seasons for things at all
in the tropics?
The answer is, because the same causes which produce summer and
winter in temperate climate produce other changes of other sorts in
the tropical region. The temperature, it is true, remains the same,
or approximately the same; but the meteorological conditions vary.
Even with ourselves, summer is not only hotter but also drier than
winter; winter is marked by rain and snow as well as by lowered
temperature. In the tropics, on the other hand, it is rather the
summer or summers that are wet, for there is a certain moving zone
of equatorial calms in which it practically keeps on raining always.
But this zone is not fixed; it flits with the sun. When the sun goes
northward for the northern summer the rainy zone goes with him; when
he turns southward again the zone shifts after him. Thus places on
or near the two tropics have one rainy season a year, while places
on the equator have usually two. The intervening dry seasons are
often very dry and parched, indeed; and where this is markedly the
case, the rainy season acts just as spring does in the north, or as
the inundation does in Egypt; it is the beginning of vegetation.
The plants that were dry and dormant during the arid months wake up
into fresh life; the branches put forth new leaves; the brown seeds
germinate; the flowers appear; and in due time the fruit ripens.
Everything in these cases depends upon the recurrence of the rainy
season, just as everything in India depends upon the bursting of the
monsoons, and everything in Egypt on the rising of the Nile. I have
seen a dry plain in Jamaica bare and brown one day, and covered six
or eight inches high with fresh green waving guinea-grass the day
but one after. The rains had come meanwhile, and Nature had awaked
with more than springlike awakening. In those hot climates everything
grows by magic as soon as it gets the needed water.
Indeed, we may say that in half the world the seasons, organically
speaking--I mean, the seasons of plant and animal life--depend upon
heat and cold, summer and winter, snow or sunshine; but in the other
half they depend almost entirely upon drought and rainfall. Even as
near home and as far north as Algeria, the summer is far too dry
and dusty for agriculture; the autumn rains set in about October
or November; they are immediately followed by the plowing; and the
winter becomes for most purposes the practical summer. Fruits and
vegetables are at their best in January and February; the fields
are full of flowers up to March or April; in June, July, and August
the country is an arid and weary desert. But the seasons for dates
are almost reversed; they ripen in autumn. In Egypt again, where
everything depends upon the inundation, the seasons are still more
complicated; the inundation begins to subside in October; in Upper
Egypt the winter season which follows is far the most important
for agriculture, and crops sown as the water subsides are reaped
from four to seven months after. But in the Delta, rice, cotton,
and indigo are sown in the spring (March or April) and harvested in
October, November, and December. Here, irrigation and temperature
come in as disturbing elements, for the Delta feels something of the
cold of winter.
I could give many other instances, but these will suffice. As a
general rule, we may say that in the temperate and frigid zones the
seasons for plants and animals are ruled by heat and cold, but that
in tropical and even in subtropical climates, rainfall and drought,
themselves largely due to the same circumstances, are the ruling
factors.
Again, everybody knows that winter and summer, and the other
phenomena which simulate or accompany them, such as wet and
dry seasons, depend upon the fact that the earth's axis is not
perpendicular to the plane in which the earth moves round the sun,
but slightly inclined to it. Now, a year in itself, viewed as a
measure of time, is merely the period which it takes the earth to
perform one such complete revolution. During one half of each such
revolution the north pole is turned at a considerable angle toward
the sun, and during the other half, the south pole. When the north
pole is so turned we call it summer in the northern hemisphere;
when the south pole is being favored, and the north is receiving
less light and heat, we call it winter. Let us suppose for a moment
that the earth had not got this twist or kink in its axis; that
the equator was always presented exactly toward the sun; what then
would happen? Obviously, there would be no change of seasons. The
day and night would have fixed lengths which never varied; climate
would in each place be uniform and, barring accidents of elevation
or distribution of land and water, the climate of each place would
also depend entirely the whole year round on its distance from the
equator. Roughly speaking, the temperature of a district would be
the temperature it now possesses in March and September, only not
quite so cold as March nor so warm as September, owing to the absence
of accumulated heat from summer or of reserves of ice and snow from
winter. In one word, under such conditions there would have been
climates--marked belts of climate; but there would not have been
seasons.
Seasons, however, depend in great part, as Mr. Alfred Russel Wallace
has ingeniously shown, on a great many things besides this mere
inclination of one end or the other of the earth toward the sun in
June and January. Much must be laid to the count of accumulated
stores of heat or cold; and though accumulated cold is physically a
misnomer, still for all practical purposes we may apply the words
fairly enough to the ice caps of the pole and the glaciers of
mountain systems. And here we come face to face with the very core
of our problem: for the odd part of it is that seasons (at least as
we know them) seem to be quite a recent and exceptional phenomenon
in the history of our planet. So far as we can judge, geologically
speaking, the earth during all its earlier life enjoyed, over all
its surface, what we should now consider tropical or subtropical
conditions. England--or rather the land that occupied the part of
the earth's crust where England now stands--had a vegetation of huge
tree ferns and palms and cycads during the Primary period; as late
even as the middle Tertiaries it had a vegetation like that of South
Carolina or Upper India. Greenland itself, in quite recent times,
flourished like a green bay tree, and did not belie its odd modern
name. The world as a whole enjoyed perpetual summer. In one word,
except in something like the equatorial sense, there were practically
no seasons. The sun went north and south, no doubt, as now, but the
temperature, even in the relative winter, seems to have remained
perennially mild and genial.
It is true, occasional slight traces of glacial epochs, earlier than
the great and well-known Glacial epoch, break here and there the
almost continuous geological record of palmy and balmy world-wide
summers; yet, taking the geological monuments as a whole, they show
us few or no signs of anything worth calling a serious winter till
quite recent periods. Large-leaved evergreens are still, in the
day-before-yesterday of geology, the order of the day; magnolias
and liquidambars, cinnamons and holly oaks, vines and rotang palms
formed the forests even of Miocene Britain. The animals during
all the Tertiary period were of what we now regard as tropical or
subtropical types--lions, rhinoceroses, hippopotamuses, monkeys, or
more antique races, equally southern in aspect. There could have
been little change of winter and summer during this long warm spell;
the variations can have been scarcely more than those of dry and
rainy seasons. The trees never lost their leaves; the fruits and
flowers never ceased to follow one another; no interruption of the
food supply drove insects to hibernate in their silken cocoons, or
squirrels and bears to lay by stores of food or fat for the cold and
hungry winter.
Nevertheless, taking the world round as it stands, we must believe
that the distinction of seasons grew up, both for plants and animals,
and for man or his ancestors, during this age of relatively unmarked
summers and winters. For the tropics more than anywhere else preserve
for us to-day the general features and aspect of this earlier time;
they have never had the continuity of their stream of life rudely
interrupted by the enormous changes of the Glacial epoch. Yet, even
in the tropics, things, as we saw, have seasons. There are annuals
and perennials there, as elsewhere. Each kind has its month for
sprouting, for flowering, for fruiting, for shedding its seed; and
men in the tropics, some of them long isolated in oceanic islands,
or in great insulated regions like Australia or New Guinea, from the
rest of their kind in the temperate regions, nevertheless know and
observe the year, and perform all their functions, agricultural or
religious, by yearly cycles. For example, there is among them all
an annual feast for the dead, and widows mourn their husbands for
one year from their burial. Observation of the year, therefore, both
automatically by organisms at large and consciously by man, antedates
and is independent of observation of the existence of summer or
winter.