violent destructive shocks and give a warning. Unfortunately this
is not always so, and no warning was given by the earthquakes which
destroyed Lisbon in 1755 and Caracas in 1812, nor by those which
devastated Agram in 1880, nor, finally, in the case of the San
Francisco disaster. A not very severe earthquake without feebler
precursors befell Ischia in 1881, while the violent catastrophe
which devastated this magnificent island in 1883 was heralded by
several warnings. As in San Francisco and Chili in 1906, less violent
concussions generally succeed the destructive shocks. Earthquakes like
that of Lisbon in 1755, consisting of a single shock, are very rare.
The violent concussions often produce large fissures in the ground.
Such were noticed in several places at San Francisco. One of the
largest fissures known, that of Midori, in Japan, was caused by the
earthquake of October 20, 1891. It left a displacement of the ground
ranging up to 6 m. (20 ft.) in the vertical and 4 m. (13 ft.) in the
horizontal direction. This crack had a length of not less than 65 km.
(40 miles). Extensive fissures were also formed by the earthquakes of
Calabria, in 1783, at Monte San Angelo, and in the sandstones of the
Bálpakrám Plateau in India, in 1897. In mountainous districts falls
of rock are a frequent consequence of the formation of fissures and
earthquakes. A large number of rocks fell in the neighborhood of Delphi
during the Phokian earthquake. On January 25, 1348, an earthquake
sent down a large portion of Mount Dobratsch (in the Alps of Villach,
in Carinthia, which is now much frequented by tourists) and buried
two towns and seventeen villages. The earthquake of April 18, 1906,
in California started from a crack which extends from the mouth of
Alder Creek, near Point Arena, running parallel with the coast-line
mostly inland, then entering the sea near San Francisco, and turning
again inland between Santa Cruz and San José, finally proceeding _via_
Chittenden up to Mount Pinos, a distance of about 600 km. (400 miles),
in the direction of N. 35° W. to S. 35° E. Along this crack the two
masses of the earth have been displaced so that the ground situated
to the southwest of the fissure has been moved by about 3 m. (10
ft.), and in some spots even by 6 m. (20 ft.) towards the northwest.
In some localities in Sonoma and Mendocino counties the southwestern
part has been raised, but nowhere by more than 1.2 m. (4 ft.). This is
the longest crack which has ever been noticed in connection with an
earthquake.
[Illustration: Fig. 11.—Clefts in Valentia Street, San
Francisco, after the earthquake of 1906]
The earthquake over, the ground does not always return to its
original position, but remains in a more or less wavy condition. This
can most easily be observed in districts where streets or railways
cross the ground. It is reported, for instance, that the track of
the tramway-lines in Market Street, the chief thoroughfare of San
Francisco, formed large wavelike curves after the earthquake.
As a consequence of the displacements in the interior of the earth
and of the formation of fissures, river courses are changed, springs
become exhausted, and new springs arise. That was the case, for
instance, in California in 1906. The ground water often rushes out
with considerable violence, tearing with it sand and mud and stones,
and piling them up, occasionally forming little craters (Fig. 12).
Extensive floods may also be caused on such occasions. By such a flood
the ancient Olympia was submerged under a layer of river sand which for
some time preserved from destruction the ancient Greek masterpieces
of art—among them the famous statue of Hermes. The floods afterwards
receded, and the treasures of ancient Olympia could be excavated.
Like the natural water channels and arteries in the interior of the
earth, water mains are displaced by the concussions. The direct damage
caused by the floods is often less important than the damage due to the
impossibility of extinguishing the fires which follow the destruction
of the buildings. It was the fires that did most of the enormous
material damage in the destruction of San Francisco.
Still greater devastation is wrought by the ocean waves thrown up by
earthquakes. We have already referred to the flood of Lisbon in 1755,
which was felt on the western coast of Norway and Sweden. Another
wave, in 1510, devoured 109 mosques and 1070 houses in Constantinople.
Another wave, again, invaded Kamaïshi, in Japan, on June 15, 1896,
swept away 7600 houses and killed 27,000 people.
We have repeatedly alluded to the disastrous flood-wave of Krakatoa of
1883. This wave traversed the whole of the Indian Ocean, passing to the
Cape of Good Hope and Cape Horn, and travelled round half the globe
afterwards. Even more remarkable was the aerial wave, which spread like
an explosion wave.
[Illustration: Fig. 12.—Sand craters and fissures, produced
by the Corinth earthquake of 1861. In the water, branches of
flooded trees]
While the most violent cannonades are rarely heard for more than
150 km. (95 miles)—in a single case at a distance of 270 km. (170
miles)—the eruption of Krakatoa was heard at Alice Springs, at a
distance of 3600 kilometres, and on the island of Rodriguez, at almost
4800 km. (3000 miles). The barographs of the meteorological stations
first marked a sudden rise and then a decided sinking of the air
pressure, succeeded by a few smaller fluctuations. These air pulses
were repeated in some places as many as seven times. We may therefore
assume that the aerial wave passed these places three times in the one
direction, and three times in the other, travelling round the earth.
The velocity of propagation of this wave was 314.2 m. (1030 ft.) per
second, corresponding to a temperature of -27° Cent. (17° F.) which
prevails at an altitude of about 8 km. (5 miles) above the earth’s
surface, at which altitude this wave may have travelled.
Within the last decade a peculiar phenomenon (leading to what is
designated variation of latitudes) has been studied. The poles of
the axis of the earth appear to move in a very irregular curve about
their mean axis. The movement is exceedingly small. The deviation of
the North Pole from its mean position does not amount to more than
10 m. (about 33 ft.). It has been believed that these motions of
the North Pole are subject to sudden fluctuations after unusually
violent earthquakes, especially when such concussions follow at rapid
intervals. That would give us, perhaps more than any other observation,
an idea of the force of earthquakes, since they would appear to be able
to disturb the equilibrium of the whole mass of our globe.
A severely felt effect of earthquakes, though most people perhaps
pay little attention to it, is the destruction of submarine cables.
The gutta-percha sheaths of cables are frequently found in a fused
condition, suggesting volcanic eruptions under the bottom of the sea.
We take care now to avoid earthquake centres in laying telegraphic
cables. Their positions have been ascertained by the most modern
investigations (see Fig. 10).
People have always been inclined to look for a connection between
earthquakes and volcanic eruptions. The connection is unquestionable in
a large number of violent earthquakes. In order to establish it, the
above-mentioned committee of the British Association has compiled the
following table of the history of the earthquakes of the Antilles:
1692.—Port Royal, Jamaica, destroyed by an earthquake; land
sinking into the sea. Eruption on St. Kitts.
1718.—Terrible earthquake on St. Vincent, followed by an
eruption.
1766-67.—Great shocks in northeastern South America, in Cuba,
Jamaica, and the Antilles. Eruption on Santa Lucia.
1797.—Earthquake in Quito, loss of 40,000 lives. Concussions
in the Antilles, eruption on Guadeloupe.
1802.—Violent shocks in Antigua. Eruption on Guadeloupe.
1812.—Caracas, capital of Venezuela, totally destroyed by
earthquake. Violent shocks in the Southern States of North
America, commencing on November 11, 1811. Eruptions on St.
Vincent and Guadeloupe.
1835-36.—Violent concussions in Chili and Central America.
Eruption on Guadeloupe.
1902.—April 19. Violent shocks, destroying many towns of
Central America. Mont Pelée, on Martinique, in activity.
Eruption on May 3. Submarine cables break, sea recedes. Renewed
violent movements of the sea on May 8, 19, 20. Eruption on St.
Vincent, cable destroyed on May 7. Violent eruption of Mont
Pelée on May 8. Destruction of St. Pierre. Numerous smaller
earthquakes.
This table distinctly marks the restless state of affairs in that part
of the earth, and how quiet and safe matters are comparatively in old
Europe, especially in the north. Some parts of Central America are so
persistently visited by earthquakes that one of them, Salvador, has
been christened “Schaukelmatte.” It is not saying too much to assert
that the earth is there incessantly trembling. Other districts which
are very frequently visited are the Kuriles and Japan, as well as the
East Indian islands. In all these countries the crust of the earth has
been broken and folded within comparatively recent epochs (chiefly in
the Tertiary age) by numerous fissures, and their compression is still
going on.
[Illustration: Fig. 13.—Earthquake lines in lower Austria]
The smaller earthquakes, of which not less than 30,000 are counted in
the course of a year, do not stand in any closer relation to volcanic
eruptions. This is also the case for a number of large earthquakes,
among which we have to count the San Francisco earthquake.
It is assured with good reason that earthquakes are often produced
at the bottom of the sea, where there is a strong slope, by slips of
sedimentary strata which have been washed down from the land into the
sea in the course of centuries. Milne believes that the seaquake of
Kamaïshi of June 15, 1896, was of this character. Concussions may even
be promoted by the different loading of the earth resulting from the
fluctuations in the pressure of the air above it.
Smaller, though occasionally rather violent, earthquakes are not
infrequent in the neighborhood of Vienna. On the map (Fig. 13) we see
three lines. The line A B is called the thermal line, because along it
a number of hot springs, the thermæ of Meidling, Baden, Vöslau, etc.,
are located, which are highly valued; the other line B C is called the
Kamp line, because it is traversed by the river Kamp; and the third B F
is called the Mürz line, after the river Mürz. The main railway-track
between Vienna and Bruck follows the valleys of A B and E F.
These lines, which probably correspond to large fissures in the
earth-crust, are known as sources of numerous earthquakes. The district
about Wiener Neustadt, where the three lines intersect, is often shaken
by violent earthquakes; some of their dates have been marked on the map.
The curve which is indicated by the letters X X on the map marks the
outlines of an earthquake which started on January 3, 1873, from both
sides of the Kamp line. It is striking to see how the earthquake spread
in the loose ground of the plain between St. Pölten and Tulln, while
the masses of rock situated to the northwest and southeast formed
obstacles to the propagation of the earthquake waves.
[Illustration: Fig. 14.—Library building of Leland Stanford
Junior University, in California, after the earthquake of 1906.
The photograph shows the great strength of iron structures in
comparison to the strength of brickwork. The effect of the
earthquake on wooden structures can be seen in Fig. 11]
Similar conclusions have been deduced from the study of the spreading
of the waves which destroyed Charleston, South Carolina, in 1886.
Twenty-seven lives were destroyed by this shock. It was the most
terrible earthquake that ever visited the United States before the year
1906. In the Charleston concussion the Alleghany Mountains proved a
powerful bar against the further propagation of the shocks, which all
the more easily travelled in the loose soil of the Mississippi Valley.
In San Francisco, likewise, the worst devastation fell upon those parts
of the town which had been built upon the loose, partly made ground
in the neighborhood of the harbor, while the buildings erected on the
famous mountain ridges of San Francisco suffered comparatively little
damage, in so far as they were not reached by the destructive fires. As
regards the destructive effects of the earthquake in San Francisco, the
building-ground of that city has been divided into four classes (the
first is the safest, the last the most unsafe)—namely: 1. Rocky soil.
2. Valleys situated between rocks and filled up by nature in the course
of time. 3. Sand-dunes. 4. Soil created by artificial filling up. This
latter soil “behaved like a semiliquid jelly in a dish,” according to
the report of the Earthquake Commission.
For similar reasons the sky-scrapers, constructed of steel on deep
foundations, stood firmest. After them came brick houses, with
well-joined and cemented walls on deep foundations. The weakness of
wooden houses proved mainly due to the poor connection of the beams,
a defect which might easily be remedied. The superiority of the steel
structure will be apparent from the illustrations (Figs. 11 and 14).
The spots situated just over the crack, of which we spoke on page 25,
suffered the most serious damage. Next to them, devastation befell
especially localities which, like Santa Rosa, San José, and Palo Alto
with Leland Stanford Junior University, are situated on the loose soil
of the valley, whose deepest portions are covered by the bay of San
Francisco. The splendidly endowed California University, in Berkeley,
and the famous Lick Observatory, both erected on rocky ground,
fortunately escaped without any notable damage.
The map sketch (Fig. 15) by Suess represents the earthquake lines of
Sicily and Calabria. These districts have, as mentioned before, been
devastated by severe earthquakes, of which the most terrible occurred
in the year 1783, and again in 1905 and 1907. They have also been the
scene of many smaller concussions.
[Illustration: Fig. 15.—Earthquake lines in the Tyrrhenian depression]
The bottom of the Tyrrhenian Sea—between Italy, Sicily, and
Sardinia—has been lowered in rather recent ages and is still sinking.
We notice on the map five dotted lines, corresponding to cracks in
the crust of the earth. These lines would intersect in the volcanic
district of the Lipari Islands. We further see a dotted circular arc
corresponding to a fissure which is regarded as the source of the
Calabrian earthquakes of 1783, 1905, and 1907. The earth-crust behaved
somewhat after the manner of a windowpane which was burst by a heavy
impact from a point corresponding to the Island of Lipari. From this
point radiate lies of fracture, and fragments have been broken off from
the earth-crust by arc-shaped cracks. The volcano Etna is situated on
the intersection of the radial and circular fissures.
[Illustration: Fig. 16.—Seismogram recorded at Shide, Isle of Wight,
on August 31, 1898]
In recognition of the high practical importance of earthquake
observations, seismological stations have in recent days been erected
in many localities. At these observatories the earthquakes are recorded
by pendulums whose styles draw lines on tapes of paper moved by
clock-work. As long as the earth is quiet the drawn line is straight.
When earthquakes set in, the line passes into a wavy curve. As long
as the movement of the paper is slow, the curve merely looks like a
widened straight line. The subjoined illustration (Fig. 16) represents
a seismogram taken at the station of Shide, on the Isle of Wight, on
August 31, 1898. The earthquake recorded originated in the Centre G,
in the Indian Ocean. The origin has been deduced from the moments of
arrival of the different waves at different stations. We notice on the
seismogram a faint widening of the straight line at 20 hrs. 5 min. 2
sec. (8 hrs. 5 min. 2 sec. P.M.). The amplitude of the oscillations
then began to widen, and the heaviest concussions were noticed at 20
hrs. 36 min. 25 sec., and 20 hrs. 42 min. 49 sec., after which the
amplitudes slowly decreased with smaller shocks. The first shock of
20 hrs. 5 min. 2 sec. is called the preliminary tremor. This tremor
passes through the interior of the earth at a velocity of propagation
of 9.2 km. (5-3/4 miles) per second. It would require twenty-three
minutes to pass through the earth along a diameter. The tremor is
very feeble, which is ascribed to the extraordinarily great friction
characteristic of the strongly heated gases which are confined in the
interior of the earth. The principal violent shock at 20 hrs. 36 min.
25 sec. was caused by a wave travelling through the solid crust of the
earth. The intensity of this shock is much less impaired than that of
the just-mentioned tremor, and it travels with the smaller velocity of
about 3.4 km. (2.1 miles) along the earth’s surface.
The velocity of propagation of concussion pulses has been calculated
for a mountain of quartz, in which it would be 3.6 km. (2.2 miles) per
second, very nearly the same as the last-mentioned figure. We should
expect this, since the firm crust of the earth consists essentially
of solid silicates—_i.e._, compounds of quartz endowed with similar
properties.
Measured at small distances from the origin, the velocity of
propagation of the wave appears smaller, and the first preliminary
tremor is frequently not observed. The velocity may be diminished to
2 km. (1-1/4 miles) per second. The reason is that the pulse partly
describes a curve in the more solid portions of the crust, and partly
passes through looser strata, through which the wave travels at a much
slower rate than in firm ground; for instance, at 1.2 km. through loose
sandstones, at 1.4 km. through the water of the ocean, and at 0.3
km. through loose sand. We recognize that it should be possible to
calculate the distance between the point of observation and the origin
of the earthquake from the data relating to the arrivals of the first
preliminary tremor and of the principal shock of maximum amplitude. The
violent shock is sometimes repeated after a certain time, though with
decreased intensity. It has often been observed that this secondary,
less violent, shock seems to have travelled all round the earth _via_
the longest road between the origin and the point of observation,
just like one portion of the aerial waves in the eruption of Krakatoa
(compare page 27). The velocity of propagation of this secondary shock
is the same as that of the principal shock.
Milne has deduced from his observations that, when the line joining the
origin of the earthquake and the point of observation does not at its
lowest level descend deeper than 50 km. below the surface of the earth,
the pulse will travel undivided through the solid crust of the earth.
For this reason we estimate the thickness of the solid crust at 50 km.
The value is in almost perfect agreement with the one which we had (on
page 16) derived from the increase of temperature with greater depths.
It should further be mentioned, perhaps, that the density of the earth
in the vicinity has been determined from pendulum observation, and that
this density seems to be rather variable down to the depths of 50 or 60
km., but to become more uniform at greater depths. These 50 or 60 km.
(31 or 37 miles) would belong to the solid crust of the earth.
The movement of earthquake shocks through the earth thus teaches us
that the solid earth-crust cannot be very thick, and that the core of
the earth is probably gaseous. The similar conclusions, to which these
various considerations had led us, may therefore come very near the
truth. A careful study of seismograms may, we hope, help us to learn
more about the central portions of the earth, which at first sight
appear to be absolutely inaccessible to scientific research.
II
THE CELESTIAL BODIES, IN PARTICULAR THE EARTH, AS ABODES OF ORGANISMS
There is no more elevating spectacle than to contemplate the sky with
its thousands of stars on a clear night. When we send our thoughts
to those lights glittering in infinite distance, the question forces
itself upon us, whether there are not out there planets like our own
that will sustain organic life. How little interest do we take in a
barren island of the Arctic Circle, on which not a single plant will
grow, compared to an island in the tropics which is teeming with life
in its most wonderful variety! The unknown worlds occupy our minds much
more when we may fancy them inhabited than when we have to regard them
as dead masses floating about in space.
We have to ask ourselves similar questions with regard to our own
little planet, the earth. Was it always covered with verdure, or was
it once sterile and barren? And if that be so, what are the conditions
under which the earth can fulfil its actual part of harboring
organic life? That “the earth was without form” in the beginning is
unquestionable. It does not matter whether we assume that it was once
all through an incandescent liquid, which may be the most probable
assumption, or that it was, as Lockyer and Moulton think, formed by the
accumulation of meteoric stones which became incandescent when arrested
in their motion.
We have seen that the earth probably consists of a mass of gas encased
within a shell which is solid on the outside and remains a viscid
liquid on the inner side. We presume with good reason that the earth
was originally a mass of gas separated from the sun, which is still in
the same state. By radiation into cold space the sphere of gas which,
on the whole, would behave as our sun does now, would gradually lose
its high temperature, and finally a solid crust could form on its
surface. Lord Kelvin has calculated that it would not require more than
one hundred years before the temperature of this crust would sink to
100°. Supposing, even, that Kelvin’s calculations should not quite be
confirmed, we may yet maintain that not many thousands of years would
have elapsed from the time when the earth assumed its first crust at
about 1000° till the age when this temperature had fallen below 100°
(212° F.). Living beings certainly could not exist so long, since
the albumen of the cells would at once coagulate at the temperature
of boiling water, like the white of an egg. Yet it has been reported
that some of the hot springs of New Zealand contain algæ, although at
a temperature of over 80°. When I went to Yellowstone Park to inquire
into the correctness of this statement, I found that the algæ existed
only at the edge of the hot springs, where the temperature did not
exceed 60° (140° F.). The famous American physiologist Loeb states that
we do not meet with algæ in hot springs at temperatures above 55°.
Since, now, the temperature of the earth-crust would much more quickly
sink from 100° to 55° than it had fallen from 1000° to 100°, we may
imagine that only a few thousands of years may have intervened between
the formation of the first crust of the earth and the cooling down
to a temperature such as would sustain life. Since that time the
temperature has probably never been so low that the larger portion
of the earth’s surface would not have been able to support organisms,
although there have been several glacial ages in which the arctic
districts inaccessible to life must have extended much farther than
at present. The ocean will also have been free of ice over much the
greatest portion of its surface at all times, and may therefore have
been inhabited by organisms in all ages. The interior of the earth
cools continually, though slowly, because heat passes from the inner,
warmer portions to the other, cooler portions through the crust of the
earth.