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Conclusion

Causes Of Mild Geological Climates

Some Problems Of Glacial Periods

Hypotheses Of Climatic Change

The Climatic Stress Of The Fourteenth Century

The Variability Of Climate

The Climate Of History

The Uniformity Of Climate

The Solar Cyclonic Hypothesis

Glaciation According To The Solar-cyclonic Hypothesis[38]



Least Viewed

The Sun's Journey Through Space

Terrestrial Causes Of Climatic Changes

The Changing Composition Of Oceans And Atmosphere

The Earth's Crust And The Sun

The Effect Of Other Bodies On The Sun

Post-glacial Crustal Movements And Climatic Changes

The Origin Of Loess

Glaciation According To The Solar-cyclonic Hypothesis[38]

The Solar Cyclonic Hypothesis

The Uniformity Of Climate






Causes Of Mild Geological Climates








In discussions of climate, as of most subjects, a peculiar psychological
phenomenon is observable. Everyone sees the necessity of explaining
conditions different from those that now exist, but few realize that
present conditions may be abnormal, and that they need explanation just
as much as do others. Because of this tendency glaciation has been
discussed with the greatest fullness, while there has been much neglect
not only of the periods when the climate of the earth resembled that of
the present, but also of the vastly longer periods when it was even
milder than now.

How important the periods of mild climate have been in geological times
may be judged from the relative length of glacial compared with
inter-glacial epochs, and still more from the far greater relative
length of the mild parts of periods and eras when compared with the
severe parts. Recent estimates by R. T. Chamberlin[58] indicate that
according to the consensus of opinion among geologists the average
inter-glacial epoch during the Pleistocene was about five times as long
as the average glacial epoch, while the whole of a given glacial epoch
averaged five times as long as the period when the ice was at a maximum.
Climatic periods far milder, longer, and more monotonous than any
inter-glacial epoch appear repeatedly during the course of geological
history. Our task in this chapter is to explain them.

Knowlton[59] has done geology a great service by collecting the evidence
as to the mild type of climate which has again and again prevailed in
the past. He lays special stress on botanical evidence since that
pertains to the variable atmosphere of the lands, and hence furnishes a
better guide than does the evidence of animals that lived in the
relatively unchanging water of the oceans. The nature of the evidence
has already been indicated in various parts of this book. It includes
palms, tree ferns, and a host of other plants which once grew in regions
which are now much too cold to support them. With this must be placed
the abundant reef-building corals and other warmth-loving marine
creatures in latitudes now much too cold for them. Of a piece with this
are the conditions of inter-glacial epochs in Europe, for example, when
elephants and hippopotamuses, as well as many species of plants from low
latitudes, were abundant. These conditions indicate not only that the
climate was warmer than now, but that the contrast from season to season
was much less. Indeed, Knowlton goes so far as to say that "relative
uniformity, mildness, and comparative equability of climate, accompanied
by high humidity, have prevailed over the greater part of the earth,
extending to, or into, polar circles, during the greater part of
geologic time--since, at least, the Middle Paleozoic. This is the
regular, the ordinary, the normal condition." ... "By many it is thought
that one of the strongest arguments against a gradually cooling globe
and a humid, non-zonally disposed climate in the ages before the
Pleistocene is the discovery of evidences of glacial action practically
throughout the entire geologic column. Hardly less than a dozen of these
are now known, ranging in age from Huronian to Eocene. It seems to be a
very general assumption by those who hold this view that these evidences
of glacial activities are to be classed as ice ages, largely comparable
in effect and extent to the Pleistocene refrigeration, but as a matter
of fact only three are apparently of a magnitude to warrant such
designation. These are the Huronian glaciation, that of the
'Permo-Carboniferous,' and that of the Pleistocene. The others, so far
as available data go, appear to be explainable as more or less local
manifestations that had no widespread effect on, for instance, ocean
temperatures, distribution of life, et cetera. They might well have been
of the type of ordinary mountain glaciers, due entirely to local
elevation and precipitation." ... "If the sun had been the principal
source of heat in pre-Pleistocene time, terrestrial temperatures would
of necessity have been disposed in zones, whereas the whole trend of
this paper has been the presentation of proof that these temperatures
were distinctly non-zonal. Therefore it seems to follow that the sun--at
least the present small-angle sun--could not have been the sole or even
the principal source of heat that warmed the early oceans."

Knowlton is so strongly impressed by the widespread fossil floras that
usually occur in the middle parts of the geological periods, that as
Schuchert[3] puts it, he neglects the evidence of other kinds. In the
middle of the periods and eras the expansion of the warm oceans over the
continents was greatest, while the lands were small and hence had more
or less insular climates of the oceanic type. At such times, the marine
fauna agrees with the flora in indicating a mild climate. Large
colony-forming foraminifera, stony corals, shelled cephalopods,
gastropods and thick-shelled bivalves, generally the cemented forms,
were common in the Far North and even in the Arctic. This occurred in
the Silurian, Devonian, Pennsylvanian, and Jurassic periods, yet at
other times, such as the Cretaceous and Eocene, such forms were very
greatly reduced in variety in the northern regions or else wholly
absent. These things, as Schuchert[60] says, can only mean that Knowlton
is right when he states that "climatic zoning such as we have had since
the beginning of the Pleistocene did not obtain in the geologic ages
prior to the Pleistocene." It does not mean, however, that there was a
"non-zonal arrangement" and that the temperature of the oceans was
everywhere the same and "without widespread effect on the distribution
of life."

Students of paleontology hold that as far back as we can go in the study
of plants, there are evidences of seasons and of relatively cool
climates in high latitudes. The cycads, for instance, are one of the
types most often used as evidence of a warm climate. Yet Wieland,[61]
who has made a lifelong study of these plants, says that many of them
"might well grow in temperate to cool climates. Until far more is
learned about them they should at least be held as valueless as indices
of tropic climates." The inference is "that either they or their close
relatives had the capacity to live in every clime. There is also a
suspicion that study of the associated ferns may compel revision of the
long-accepted view of the universality of tropic climates throughout the
Mesozoic." Nathorst is quoted by Wieland as saying, "I think ... that
during the time when the Gingkophytes and Cycadophytes dominated, many
of them must have adapted themselves for living in cold climates also.
Of this I have not the least doubt."

Another important line of evidence which Knowlton and others have cited
as a proof of the non-zonal arrangement of climate in the past, is the
vast red beds which are found in the Proterozoic, late Silurian,
Devonian, Permian, and Triassic, and in some Tertiary formations. These
are believed to resemble laterite, a red and highly oxidized soil which
is found in great abundance in equatorial regions. Knowlton does not
attempt to show that the red beds present equatorial characteristics in
other respects, but bases his conclusion on the statement that "red beds
are not being formed at the present time in any desert region." This is
certainly an error. As has already been said, in both the Transcaspian
and Takla Makan deserts, the color of the sand regularly changes from
brown on the borders to pale red far out in the desert. Kuzzil Kum, or
Red Sand, is the native name. The sands in the center of the desert
apparently were originally washed down from the same mountains as those
on the borders, and time has turned them red. Since the same condition
is reported from the Arabian Desert, it seems that redness is
characteristic of some of the world's greatest deserts. Moreover, beds
of salt and gypsum are regularly found in red beds, and they can
scarcely originate except in deserts, or in shallow almost landlocked
bays on the coasts of deserts, as appears to have happened in the
Silurian where marine fossils are found interbedded with gypsum.

Again, Knowlton says that red beds cannot indicate deserts because the
plants found in them are not "pinched or depauperate, nor do they
indicate xerophytic adaptations. Moreover, very considerable deposits of
coal are found in red beds in many parts of the world, which implies the
presence of swamps but little above sea-level."

Students of desert botany are likely to doubt the force of these
considerations. As MacDougal[62] has shown, the variety of plants in
deserts is greater than in moist regions. Not only do xerophytic desert
species prevail, but halophytes are present in the salty areas, and
hygrophytes in the wet swampy areas, while ordinary mesophytes prevail
along the water courses and are washed down from the mountains. The
ordinary plants, not the xerophytes, are the ones that are chiefly
preserved since they occur in most abundance near streams where
deposition is taking place. So far as swamps are concerned, few are of
larger size than those of Seistan in Persia, Lop Nor in Chinese
Turkestan, and certain others in the midst of the Asiatic deserts.
Streams flowing from the mountains into deserts are almost sure to form
large swamps, such as those along the Tarim River in central Asia. Lake
Chad in Africa is another example. In it, too, reeds are very numerous.

Putting together the evidence on both sides in this disputed question,
it appears that throughout most of geological time there is some
evidence of a zonal arrangement of climate. The evidence takes the form
of traces of cool climates, of seasons, and of deserts. Nevertheless,
there is also strong evidence that these conditions were in general less
intense than at present and that times of relatively warm, moist climate
without great seasonal extremes have prevailed very widely during
periods much longer than those when a zonal arrangement as marked as
that of today prevailed. As Schuchert[63] puts it: "Today the variation
on land between the tropics and the poles is roughly between 110 deg. and
-60 deg.F., in the oceans between 85 deg. and 31 deg.F. In the geologic past the
temperature of the oceans for the greater parts of the periods probably
was most often between 85 deg. and 55 deg.F., while on land it may have varied
between 90 deg. and 0 deg.F. At rare intervals the extremes were undoubtedly as
great as they are today. The conclusion is therefore that at all times
the earth had temperature zones, varying between the present-day
intensity and times which were almost without such belts, and at these
latter times the greater part of the earth had an almost uniformly mild
climate, without winters."

It is these mild climates which we must now attempt to explain. This
leads us to inquire what would happen to the climate of the earth as a
whole if the conditions which now prevail at times of few sunspots were
to become intensified. That they could become greatly intensified seems
highly probable, for there is good reason to think that aside from the
sunspot cycle the sun's atmosphere is in a disturbed condition. The
prominences which sometimes shoot out hundreds of thousands of miles
seem to be good evidence of this. Suppose that the sun's atmosphere
should become very quiet. This would apparently mean that cyclonic
storms would be much less numerous and less severe than during the
present times of sunspot minima. The storms would also apparently follow
paths in middle latitudes somewhat as they do now when sunspots are
fewest. The first effect of such a condition, if we can judge from what
happens at present, would be a rise in the general temperature of the
earth, because less heat would be carried aloft by storms. Today, as is
shown in Earth and Sun, a difference of perhaps 10 per cent in the
average storminess during periods of sunspot maxima and minima is
correlated with a difference of 3 deg.C. in the temperature at the earth's
surface. This includes not only an actual lowering of 0.6 deg.C. at times of
sunspot maxima, but the overcoming of the effect of increased insolation
at such times, an effect which Abbot calculates as about 2.5 deg.C. If the
storminess were to be reduced to one-half or one-quarter its present
amount at sunspot minima, not only would the loss of heat by upward
convection in storms be diminished, but the area covered by clouds would
diminish so that the sun would have more chance to warm the lower air.
Hence the average rise of temperature might amount to as much at 5 deg. or
10 deg.C.

Another effect of the decrease in storminess would be to make the
so-called westerly winds, which are chiefly southwesterly in the
northern hemisphere and northwesterly in the southern hemisphere, more
strong and steady than at present. They would not continually suffer
interruption by cyclonic winds from other directions, as is now the
case, and would have a regularity like that of the trades. This
conclusion is strongly reenforced in a paper by Clayton[64] which came
to hand after this chapter had been completed. From his studies of the
solar constant and the temperature of the earth which are described in
Earth and Sun, he reaches the following conclusion: "The results of
these researches have led me to believe: 1. That if there were no
variation in solar radiation the atmospheric motions would establish a
stable system with exchanges of air between equator and pole and between
ocean and land, in which the only variations would be daily and annual
changes set in operation by the relative motions of the earth and sun.
2. The existing abnormal changes, which we call weather, have their
origins chiefly, if not entirely, in the variations of solar radiation."

If cyclonic storms and "weather" were largely eliminated and if the
planetary system of winds with its steady trades and southwesterlies
became everywhere dominant, the regularity and volume of the
poleward-flowing currents, such as the Gulf Stream and the Atlantic
Drift in one ocean, and the Japanese Current in another, would be
greatly increased. How important this is may be judged from the work of
Helland-Hansen and Nansen.[65] These authors find that with the passage
of each cyclonic storm there is a change in the temperature of the
surface water of the Atlantic Ocean. Winds at right angles to the course
of the Drift drive the water first in one direction and then in the
other but do not advance it in its course. Winds with an easterly
component, on the other hand, not only check the Drift but reverse it,
driving the warm water back toward the southwest and allowing cold water
to well up in its stead. The driving force in the Atlantic Drift is
merely the excess of the winds with a westerly component over those with
an easterly component.

Suppose that the numbers in Fig. 8 represent the strength of the winds
in a certain part of the North Atlantic or North Pacific, that is, the
total number of miles moved by the air per year. In quadrant A of the
left-hand part all the winds move from a more or less southwesterly
direction and produce a total movement of the air amounting to thirty
units per year. Those coming from points between north and west move
twenty-five units; those between north and east, twenty units; and those
between east and south, twenty-five units. Since the movement of the
winds in quadrants B and D is the same, these winds have no effect in
producing currents. They merely move the water back and forth, and thus
give it time to lose whatever heat it has brought from more southerly
latitudes. On the other hand, since the easterly winds in quadrant C do
not wholly check the currents caused by the westerly winds of quadrant
A, the effective force of the westerly winds amounts to ten, or the
difference between a force of thirty in quadrant A and of twenty in
quadrant C. Hence the water is moved forward toward the northeast, as
shown by the thick part of arrow A.


water.]

Now suppose that cyclonic storms should be greatly reduced in number so
that in the zone of prevailing westerlies they were scarcely more
numerous than tropical hurricanes now are in the trade-wind belt. Then
the more or less southwesterly winds in quadrant A' in the right-hand
part of Fig. 8 would not only become more frequent but would be stronger
than at present. The total movement from that quarter might rise to
sixty units, as indicated in the figure. In quadrants B' and D' the
movement would fall to fifteen and in quadrant C' to ten. B' and D'
would balance one another as before. The movement in A', however, would
exceed that in C' by fifty instead of ten. In other words, the
current-making force would become five times as great as now. The actual
effect would be increased still more, for the winds from the southwest
would be stronger as well as steadier if there were no storms. A strong
wind which causes whitecaps has much more power to drive the water
forward than a weaker wind which does not cause whitecaps. In a wave
without a whitecap the water returns to practically the original point
after completing a circle beneath the surface. In a wave with a
whitecap, however, the cap moves forward. Any increase in velocity
beyond the rate at which whitecaps are formed has a great influence upon
the amount of water which is blown forward. Several times as much water
is drifted forward by a persistent wind of twenty miles an hour as by a
ten-mile wind.[66]

In this connection a suggestion which is elaborated in Chapter XIII may
be mentioned. At present the salinity of the oceans checks the general
deep-sea circulation and thereby increases the contrasts from zone to
zone. In the past, however, the ocean must have been fresher than now.
Hence the circulation was presumably less impeded, and the transfer of
heat from low latitudes to high was facilitated.

Consider now the magnitude of the probable effect of a diminution in
storms. Today off the coast of Norway in latitude 65 deg.N. and longitude
10 deg.E., the mean temperature in January is 2 deg.C. and in July 12 deg.C. This
represents a plus anomaly of about 22 deg. in January and 2 deg. in July; that
is, the Norwegian coast is warmer than the normal for its latitude by
these amounts. Suppose that in some past time the present distribution
of lands and seas prevailed, but Norway was a lowland where extensive
deposits could accumulate in great flood plains. Suppose, also, that the
sun's atmosphere was so inactive that few cyclonic storms occurred,
steady winds from the west-southwest prevailed, and strong,
uninterrupted ocean currents brought from the Caribbean Sea and Gulf of
Mexico much greater supplies of warm water than at present. The
Norwegian winters would then be warmer than now not only because of the
general increase in temperature which the earth regularly experiences at
sunspot minima, but because the currents would accentuate this
condition. In summer similar conditions would prevail except that the
warming effect of the winds and currents would presumably be less than
in winter, but this might be more than balanced by the increased heat of
the sun during the long summer days, for storms and clouds would be
rare.

If such conditions raised the winter temperature only 8 deg.C. and the
summer temperature 4 deg.C., the climate would be as warm as that of the
northern island of New Zealand (latitude 35 deg.-43 deg.S.). The flora of that
part of New Zealand is subtropical and includes not only pines and
beeches, but palms and tree ferns. A climate scarcely warmer than that
of New Zealand would foster a flora like that which existed in far
northern latitudes during some of the milder geological periods. If,
however, the general temperature of the earth's surface were raised 5 deg.
because of the scarcity of storms, if the currents were strong enough so
that they increased the present anomaly by 50 per cent, and if more
persistent sunshine in summer raised the temperature at that season
about 4 deg.C., the January temperature would be 18 deg.C. and the July
temperature 22 deg.C. These figures perhaps make summer and winter more
nearly alike than was ever really the case in such latitudes.
Nevertheless, they show that a diminution of storms and a consequent
strengthening and steadying of the southwesterlies might easily raise
the temperature of the Norwegian coast so high that corals could
flourish within the Arctic Circle.

Another factor would cooeperate in producing mild temperatures in high
latitudes during the winter, namely, the fogs which would presumably
accumulate. It is well known that when saturated air from a warm ocean
is blown over the lands in winter, as happens so often in the British
Islands and around the North Sea, fog is formed. The effect of such a
fog is indeed to shut out the sun's radiation, but in high latitudes
during the winter when the sun is low, this is of little importance.
Another effect is to retain the heat of the earth itself. When a
constant supply of warm water is being brought from low latitudes this
blanketing of the heat by the fog becomes of great importance. In the
past, whenever cyclonic storms were weak and westerly winds were
correspondingly strong, winter fogs in high latitudes must have been
much more widespread and persistent than now.

The bearing of fogs on vegetation is another interesting point. If a
region in high latitudes is constantly protected by fog in winter, it
can support types of vegetation characteristic of fairly low latitudes,
for plants are oftener killed by dry cold than by moist cold. Indeed,
excessive evaporation from the plant induced by dry cold when the
evaporated water cannot be rapidly replaced by the movement of sap is a
chief reason why large plants are winterkilled. The growing of
transplanted palms on the coast of southwestern Ireland, in spite of its
location in latitude 50 deg.N., is possible only because of the great
fogginess in winter due to the marine climate. The fogs prevent the
escape of heat and ward off killing frosts. The tree ferns in latitude
46 deg.S. in New Zealand, already referred to, are often similarly protected
in winter. Therefore, the relative frequency of fogs in high latitudes
when storms were at a minimum would apparently tend not merely to
produce mild winters but to promote tropical vegetation.

The strong steady trades and southwesterlies which would prevail at
times of slight solar activity, according to our hypothesis, would have
a pronounced effect on the water of the deep seas as well as upon that
of the surface. In the first place, the deep-sea circulation would be
hastened. For convenience let us speak of the northern hemisphere. In
the past, whenever the southwesterly winds were steadier than now, as
was probably the case when cyclonic storms were relatively rare, more
surface water than at present was presumably driven from low latitudes
and carried to high latitudes. This, of course, means that a greater
volume of water had to flow back toward the equator in the lower parts
of the ocean, or else as a cool surface current. The steady
southwesterly winds, however, would interfere with south-flowing surface
currents, thus compelling the polar waters to find their way equatorward
beneath the surface. In low latitudes the polar waters would rise and
their tendency would be to lower the temperature. Hence steadier
westerlies would make for lessened latitudinal contrasts in climate not
only by driving more warm water poleward but by causing more polar water
to reach low latitudes.

At this point a second important consideration must be faced. Not only
would the deep-sea circulation be hastened, but the ocean depths might
be warmed. The deep parts of the ocean are today cold because they
receive their water from high latitudes where it sinks because of low
temperature. Suppose, however, that a diminution in storminess combined
with other conditions should permit corals to grow in latitude 70 deg.N. The
ocean temperature would then have to average scarcely lower than 20 deg.C.
and even in the coldest month the water could scarcely fall below about
15 deg.C. Under such conditions, if the polar ocean were freely connected
with the rest of the oceans, no part of it would probably have a
temperature much below 10 deg.C., for there would be no such thing as ice
caps and snowfields to reflect the scanty sunlight and radiate into
space what little heat there was. On the contrary, during the winter an
almost constant state of dense fogginess would prevail. So great would
be the blanketing effect of this that a minimum monthly temperature of
10 deg.C. for the coldest part of the ocean may perhaps be too low for a
time when corals thrived in latitude 70 deg..

The temperature of the ocean depths cannot permanently remain lower than
that of the coldest parts of the surface. Temporarily this might indeed
happen when a solar change first reduced the storminess and strengthened
the westerlies and the surface currents. Gradually, however, the
persistent deep-sea circulation would bring up the colder water in low
latitudes and carry downward the water of medium temperature at the
coldest part of the surface. Thus in time the whole body of the ocean
would become warm. The heat which at present is carried away from the
earth's surface in storms would slowly accumulate in the oceans. As the
process went on, all parts of the ocean's surface would become warmer,
for equatorial latitudes would be less and less cooled by cold water
from below, while the water blown from low latitudes to high would be
correspondingly warmer. The warming of the ocean would come to an end
only with the attainment of a state of equilibrium in which the loss of
heat by radiation and evaporation from the ocean's surface equaled the
loss which under other circumstances would arise from the rise of warm
air in cyclonic storms. When once the oceans were warmed, they would
form an extremely strong conservative force tending to preserve an
equable climate in all latitudes and at all seasons. According to the
solar cyclonic hypothesis such conditions ought to have prevailed
throughout most of geological time. Only after a strong and prolonged
solar disturbance with its consequent storminess would conditions like
those of today be expected.

In this connection another possibility may be mentioned. It is commonly
assumed that the earth's axis is held steadily in one direction by the
fact that the rotating earth is a great gyroscope. Having been tilted to
a certain position, perhaps by some extraneous force, the axis is
supposed to maintain that position until some other force intervenes.
Cordeiro,[67] however, maintains that this is true only of an absolutely
rigid gyroscope. He believes that it is mathematically demonstrable that
if an elastic gyroscope be gradually tilted by some extraneous force,
and if that force then ceases to act, the gyroscope as a whole will
oscillate back and forth. The earth appears to be slightly elastic.
Cordeiro therefore applies his formulae to it, on the following
assumptions: (1) That the original position of the axis was nearly
vertical to the plane of the ecliptic in which the earth revolves around
the sun; (2) that at certain times the inclination has been even greater
than now; and (3) that the position of the axis with reference to the
earth has not changed to any great extent, that is, the earth's poles
have remained essentially stationary with reference to the earth,
although the whole earth has been gyroscopically tilted back and forth
repeatedly.

With a vertical axis the daylight and darkness in all parts of the earth
would be of equal duration, being always twelve hours. There would be no
seasons, and the climate would approach the average condition now
experienced at the two equinoxes. On the whole the climate of high
latitudes would give the impression of being milder than now, for there
would be less opportunity for the accumulation of snow and ice with
their strong cooling effect. On the other hand, if the axis were tilted
more than now, the winter nights would be longer and the winters more
severe than at present, and there would be a tendency toward glaciation.
Thus Cordeiro accounts for alternating mild and glacial epochs. The
entire swing from the vertical position to the maximum inclination and
back to the vertical may last millions of years depending on the earth's
degree of elasticity. The swing beyond the vertical position in the

other direction would be equally prolonged. Since the axis is now
supposed to be much nearer its maximum than its minimum degree of
tilting, the duration of epochs having a climate more severe than that
of the present would be relatively short, while the mild epochs would be
long.

Cordeiro's hypothesis has been almost completely ignored. One reason is
that his treatment of geological facts, and especially his method of
riding rough-shod over widely accepted conclusions, has not commended
his work to geologists. Therefore they have not deemed it worth while to
urge mathematicians to test the assumptions and methods by which he
reached his results. It is perhaps unfair to test Cordeiro by geology,
for he lays no claim to being a geologist. In mathematics he labors
under the disadvantage of having worked outside the usual professional
channels, so that his work does not seem to have been subjected to
sufficiently critical analysis.

Without expressing any opinion as to the value of Cordeiro's results we
feel that the subject of the earth's gyroscopic motion and of a possible
secular change in the direction of the axis deserves investigation for
two chief reasons. In the first place, evidences of seasonal changes and
of seasonal uniformity seem to occur more or less alternately in the
geological record. Second, the remarkable discoveries of Garner and
Allard[68] show that the duration of daylight has a pronounced effect
upon the reproduction of plants. We have referred repeatedly to the tree
ferns, corals, and other forms of life which now live in relatively low
latitudes and which cannot endure strong seasonal contrasts, but which
once lived far to the north. On the other hand, Sayles,[69] for example,
finds that microscopical examination of the banding of ancient shales
and slates indicates distinct seasonal banding like that of recent
Pleistocene clays or of the Squantum slate formed during or near the
Permian glacial period. Such seasonal banding is found in rocks of
various ages: (a) Huronian, in cobalt shales previously reported by
Coleman; (b) late Proterozoic or early Cambrian in Hiwassee slate; (c)
lower Cambrian, in Georgian slates of Vermont; (d) lower Ordovician, in
Georgia (Rockmart slate), Tennessee (Athens shale), Vermont (slates),
and Quebec (Beekmantown formation); and (e) Permian in Massachusetts
(Squantum slate). How far the periods during which such evidence of
seasons was recorded really alternated with mild periods, when tropical
species lived in high latitudes and the contrast of seasons was almost
or wholly lacking, we have as yet no means of knowing. If periods
characterized by marked seasonal changes should be found to have
alternated with those when the seasons were of little importance, the
fact would be of great geological significance.

The discoveries of Garner and Allard as to the effect of light on
reproduction began with a peculiar tobacco plant which appeared in some
experiments at Washington. The plant grew to unusual size, and seemed to
promise a valuable new variety. It formed no seeds, however, before the
approach of cold weather. It was therefore removed to a greenhouse where
it flowered and produced seed. In succeeding years the flowering was
likewise delayed till early winter, but finally it was discovered that
if small plants were started in the greenhouse in the early fall they
flowered at the same time as the large ones. Experiments soon
demonstrated that the time of flowering depends largely upon the length
of the daily period when the plants are exposed to light. The same is
true of many other plants, and there is great variety in the conditions
which lead to flowering. Some plants, such as witch hazel, appear to be
stimulated to bloom by very short days, while others, such as evening
primrose, appear to require relatively long days. So sensitive are
plants in this respect that Garner and Allard, by changing the length of
the period of light, have caused a flowerbud in its early stages not
only to stop developing but to return once more to a vegetative shoot.

Common iris, which flowers in May and June, will not blossom under
ordinary conditions when grown in the greenhouse in winter, even
under the same temperature conditions that prevail in early summer.
Again, one variety of soy beans will regularly begin to flower in
June of each year, a second variety in July, and a third in August,
when all are planted on the same date. There are no temperature
differences during the summer months which could explain these
differences in time of flowering; and, since "internal causes" alone
cannot be accepted as furnishing a satisfactory explanation, some
external factor other than temperature must be responsible.

The ordinary varieties of cosmos regularly flower in the fall in
northern latitudes if they are planted in the spring or summer. If
grown in a warm greenhouse during the winter months the plants also
flower readily, so that the cooler weather of fall is not a
necessary condition. If successive plantings of cosmos are made in
the greenhouse during the late winter and early spring months,
maintaining a uniform temperature throughout, the plantings made
after a certain date will fail to blossom promptly, but, on the
contrary, will continue to grow till the following fall, thus
flowering at the usual season for this species. This curious
reversal of behavior with advance of the season cannot be attributed
to change in temperature. Some other factor is responsible for the
failure of cosmos to blossom during the summer months. In this
respect the behavior of cosmos is just the opposite of that observed
in iris.

Certain varieties of soy beans change their behavior in a peculiar
manner with advance of the summer season. The variety known as
Biloxi, for example, when planted early in the spring in the
latitude of Washington, D. C., continues to grow throughout the
summer, flowering in September. The plants maintain growth without
flowering for fifteen to eighteen weeks, attaining a height of five
feet or more. As the dates of successive plantings are moved forward
through the months of June and July, however there is a marked
tendency for the plants to cut short the period of growth which
precedes flowering. This means, of course, that there is a tendency
to flower at approximately the same time of year regardless of the
date of planting. As a necessary consequence, the size of the plants
at the time of flowering is reduced in proportion to the delay in
planting.

The bearing of this on geological problems lies in a query which it
raises as to the ability of a genus or family of plants to adapt itself
to days of very different length from those to which it is wonted. Could
tree ferns, ginkgos, cycads, and other plants whose usual range of
location never subjects them to daylight for more than perhaps fourteen
hours or less than ten, thrive and reproduce themselves if subjected to
periods of daylight ranging all the way from nothing up to about
twenty-four hours? No answer to this is yet possible, but the question
raises most interesting opportunities of investigation. If Cordeiro is
right as to the earth's elastic gyroscopic motion, there may have been
certain periods when a vertical or almost vertical axis permitted the
days to be of almost equal length at all seasons in all latitudes. If
such an absence of seasons occurred when the lands were low, when the
oceans were extensive and widely open toward the poles, and when storms
were relatively inactive, the result might be great mildness of climate
such as appears sometimes to have prevailed in the middle of geological
eras. Suppose on the other hand that the axis should be tilted more than
now, and that the lands should be widely emergent and the storm belt
highly active in low latitudes, perhaps because of the activity of the
sun. The conditions might be favorable for glaciation at latitudes as
low as those where the Permo-Carboniferous ice sheets appear to have
centered. The possibilities thus suggested by Cordeiro's hypothesis are
so interesting that the gyroscopic motion of the earth ought to be
investigated more thoroughly. Even if no such gyroscopic motion takes
place, however, the other causes of mild climate discussed in this
chapter may be enough to explain all the observed phenomena.

Many important biological consequences might be drawn from this study of
mild geological climates, but this book is not the place for them. In
the first chapter we saw that one of the most remarkable features of the
climate of the earth is its wonderful uniformity through hundreds of
millions of years. As we come down through the vista of years the mild
geological periods appear to represent a return as nearly as possible to
this standard condition of uniformity. Certain changes of the earth
itself, as we shall see in the next chapter, may in the long run tend
slightly to change the exact conditions of this climatic standard, as we
might perhaps call it. Yet they act so slowly that their effect during
hundreds of millions of years is still open to question. At most they
seem merely to have produced a slight increase in diversity from season
to season and from zone to zone. The normal climate appears still to be
of a milder type than that which happens to prevail at present. Some
solar condition, whose possible nature will be discussed later, seems
even now to cause the number of cyclonic storms to be greater than
normal. Hence the earth's climate still shows something of the great
diversity of seasons and of zones which is so marked a characteristic of
glacial epochs.

FOOTNOTES:

[Footnote 58: Rollin T. Chamberlin: Personal Communication.]

[Footnote 59: F. H. Knowlton: Evolution of Geologic Climates; Bull.
Geol. Soc. Am., Vol. 30, 1919, pp. 499-566.]

[Footnote 60: Chas. Schuchert: Review of Knowlton's Evolution of
Geological Climates, in Am. Jour. Sci., 1921.]

[Footnote 61: G. R. Wieland: Distribution and Relationships of the
Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.]

[Footnote 62: D. T. MacDougal: Botanical Features of North American
Deserts; Carnegie Instit. of Wash., No. 99, 1908.]

[Footnote 63: Loc. cit.]

[Footnote 64: H. H. Clayton: Variation in Solar Radiation and the
Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Washington, 1920.]

[Footnote 65: B. Helland Hansen and F. Nansen: Temperature Variations in
the North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths.
Inst., Vol. 70, No. 4, Washington, 1920.]

[Footnote 66: The climatic significance of ocean currents is well
discussed in Croll's Climate and Time, 1875, and his Climate and
Cosmogony, 1889.]

[Footnote 67: F. J. B. Cordeiro: The Gyroscope, 1913.]

[Footnote 68: W. W. Garner and H. A. Allard: Flowering and Fruition of
Plants as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp.
377-400.]

[Footnote 69: Report of Committee on Sedimentation, National Research
Council, April, 1922.]





Next: Terrestrial Causes Of Climatic Changes

Previous: The Origin Of Loess



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