"Briefly then to recapitulate the train of phenomena by which the destruction of the cities might have been brought about, I would suppose that the River Jordan, prior to that event, continued its course tranquilly through the great longitudinal valley, called El Arabah, into the Gulf of Akaba; that a shower of stones and sand from some neighboring volcano first overwhelmed these places; and that its eruption was followed by a depression of the whole of the region, from some point apparently intermediate between the lake of Tiberius and the mountains of Lebanon, to the watershed in the parallel of 30°, which occurs in the valley of El Arabah above-mentioned. I would thence infer that the waters of the Jordan, pent-up within the valley by a range of mountains to the east and west, and a barrier of elevated table-land to the south, could find no outlet, and consequently by degrees formed a lake in its most depressed portion, which, however, did not occur at once, and therefore is not recorded by Scripture as a part of the catastrophe, though reference is made in another passage of its existence in what was before the valley of Siddim."
As regards the turning of Lot's wife into a pillar of salt, Henderson, who has carefully studied this part of the country, remarks: "How natural is the incrustation of his wife on this hypothesis! Remaining in a lower part of the valley, and looking with a wistful eye towards Sodom, she was surrounded, ere she was aware, by the lava, which rising and swelling, at length reached her, and (whilst the volcanic effluvia deprived her of life) incrusted her where she stood, so that being, as it were, embalmed by the salso-bituminous mass, she became a conspicuous beacon and admonitory example of future generations."
To attempt by the unaided senses a determination of the direction in which earthquake shocks reach any certain spot, the velocity with which they are travelling, their degree of intensity, their general character, whether horizontal or vertical, or any peculiarities which might show them to be exceptional would be futile for more reasons than one. Even a skilled scientific observer, familiar with what has already been discovered and eager to discover more, might in the excitement of an earthquake become so excited himself as to make him unable to take reliable observations.
But human ingenuity has succeeded in devising delicate instruments capable of recording not only the exact time of the arrival of an earthquake shock, but also of measuring the different parts of what may seem to be a single shock, the direction in which the shocks reach the place, as well as the variations of intensity in all the shocks.
Crude instruments to do some of these things have been in use from very early times. According to Mallet among the more important of these early instruments was the following: the instrument of Cacciatore of Palmero. This consisted of a circular wooden dish, about ten inches in diameter, placed horizontally, and filled with mercury to the brim of eight notches at equal distances apart. Beneath each notch was placed a small cup. On the passage of the earthquake waves the vessel, being tilted in a direction dependent on the direction in which the waves were travelling, would cause some of the mercury to spill over into one or more of the cups, thus indicating by its amount the intensity of the wave, and by the particular cup or cups that were filled, the direction in which the waves reached the place.
Somewhat similar contrivances were of a vessel partly filled with molasses, or other sticky liquid; or a cylindrical tub, the sides of which were chalked or whitewashed and filled with some colored liquid. In either of these cases, on the passage of the earthquake waves, the vessels were tilted and showed by the height of the marks the intensity of the waves, and by the position of the marks the direction in which the waves first reached the instrument.
These instruments, though satisfactory for the study of earthquake shocks a long time ago, when an earthquake was regarded as practically consisting of but a single shock, or, at the most, of a very few shocks, would be worthless for the study of earthquakes now, for it is finally known that an earthquake consists of a series of many hundreds of vibrations, differing greatly in their rapidity and intensity, and following one another in a definite order.
The old forms of earthquake instruments were known as seismoscopes. The word seismoscope is a compound word from Greek consisting of the two words, seism and scope. It means literally any instrument capable of seeing, or calling attention to, a seism, or earth-shake. In other words, a seismoscope is any instrument capable of calling attention only to an earth-shake.
Of course, neither of the rude seismoscopes just mentioned would be able to give any valuable indications of the successive shakings to which the vessel containing the viscid liquid had been subjected, since the liquid would simply be splashed a number of times over the same parts of the vessel. In order to get a record of the successive shocks another form of apparatus must be employed, a form known as a seismograph.
Concerning the complex character of the apparently single earthquake shock, Professor Milne makes this highly interesting and picturesque statement:
"An earthquake disturbance at a station far removed from its origin shows that the main movement has two attendants, one which precedes and the other which follows. The first of these by its characteristics indicates what is to follow, whilst the latter, in a very much more pronounced manner, will often repeat at definite intervals, but with decreasing intensity, the prominent features of what has passed. Inasmuch as these latter rhythmical, but decreasing, impulses of the dying earthquake are more likely to result from reflection than from interference, I have provisionally called them Echoes."
There are many different forms of instruments known as seismographs that are capable of recording all of these vibrations, but there is this objection to their use: that the records appear in so tangled a form that it is practically impossible to decipher or untangle them. This fact can be grasped by examining Fig. 51, which represents a record of this kind.
It is necessary, therefore, to employ a modified form of instrument called a seismometer, able not only to record all the different vibrations, but to record them in such a manner that they can be easily recognized. Fig. 52, for example, shows results obtained by the use of a seismometer, in which the different vibrations are separated, and so recorded on a sheet of paper, as to be readily understood. Such a record is called a seismogram, and represents a long distance seismogram. Here the large arrow indicates the beginning of the record. And herein, as can be clearly seen, what would appear to an observer without an instrument only a single shock, lasting but the fraction of a minute, in reality consists of the preliminary shake as represented in ab and bc, the principal shake, as represented at c, d1, d2, and d3, and the final portions of the shake or the "echoes" of Professor Milne, as represented from d3 to e.
Except in a very general way there is for present purposes no need of explaining the construction and operation of the seismometer and seismograph. Suffice it to say, there are many forms of these instruments, any of which are capable of recording the details of a passing shock. The most important thing in either a seismograph or a seismometer is to obtain what is known as a steady point; that is, a point consisting of some object or mass that will remain practically at rest, while everything around it, even the support which holds it, is affected by the earthquake.
It is, of course, not very easy to obtain a steady point, but it can be done; and it will be at once comprehended that if a plate or piece of paper were attached to such a steady point or mass, and a pencil or tracer had one of its ends resting on the plate, and its other end attached to the support that vibrates with the earth, a tracing or record would be drawn on the plate from which the character of the motion of the end of the tracer, and, therefore, of the earth, would be marked on the plate.
Various devices have been employed for the steady points. The most successful consists of a heavy mass of lead.
Fig. 53 represents a form of instrument invented by Professor Vicentini of Italy. Here the steady point consists of a heavy leaden bob, of 200, 400, or even 500 kilograms, suspended by three metallic rods united above by a brass cap, hung on a steel wire to a bracket fixed on the wall. This wire may have a length as great as fifty feet.
Fig. 54 represents the recording instrument. Here a tracer is provided that is capable of multiplying the motion fifty times, or even eighty times. A record is traced on a sheet of paper passing over a roller which imparts a rapid motion to a sheet so as to make sure that the different parts of the shock or vibration will be recorded on separate portions of the paper.
As earthquakes are shakings of the earth's crust in places where it is uncovered by the waters of the ocean, so seaquakes are the shakings of those portions that lie on the bed of the ocean.
Mallet points out that the earthquake wave may start either in the interior of the continent, or on the bed of the ocean; that the latter place is the more common, since on the land vents—rude safety-valves, as it were,—are provided by the craters of the volcanoes; that, when earthquakes start on the ocean bed, the impulses are conveyed in different forms of waves, i. e., those through the solid earth, those through the water, and those through the air, with varying sounds like bellowings and explosions, or like the rolling of wagons over rough roads.
To learn when quakes occur on the sea is a much harder task, since on the land we can use seismoscopes, seismographs, or seismometers to indicate, record, or measure the shakings of the crust, while on the sea, where the water is always in more or less motion and the surface so far from the ocean's bed this is impossible, or, rather shall it be said, has hitherto been found so; for that the mind of man may surmount this obstacle is not impossible to conceive.
To detect the wave produced by the quaking of the bed of the ocean is exceedingly difficult, since those in very deep water are flat or possess but a small height. Indeed, in the deepest parts of the ocean this height is probably to be measured only by inches instead of feet. When, however, the waves advance towards the shore they increase in height, and when they reach the shallows near the coast, they begin to curl over and break, thus creating the enormous waves mentioned so often as attending great earthquakes in the ocean.
During the great earthquake of Simoda in Japan, 1854, the waters of the bay were first greatly agitated, and then retreated, leaving the bottom bare in places where the water was formerly thirty feet deep. A wave, thirty feet high, then rushed in from the bay and, climbing the land, swept away everything in its path, covering the town with water to the tops of the houses. This monster wave then receded, but rushed back five times.
In 1751, an earthquake wave suddenly entered Callao, the port of Lima, Peru, sinking twenty-three vessels and driving a frigate inland, where it was left high and dry. This wave extended across the Pacific to the Hawaiian Islands, a distance of 6,000 miles.
On the 13th of August, 1866, an earthquake wave, that started a short distance from shore, produced a number of earthquake waves sixty feet high that reached the coast of Peru half an hour after the principal earthquake shock. These waves reached Coquimbo, 800 miles distant, in about three hours, and Honolulu, on the Sandwich Islands, 5,520 miles distant, in twelve hours, and the coast of Japan, more than 10,000 miles distant, on the next day. Le Conte remarks that these waves would have encircled the earth, had it not been for the barrier interposed by the Andes.
Another great seaquake, known as the Iquiqui seaquake, during 1868 in the same neighborhood damaged severely the towns of north Chile and southern Peru.
While, however, there is difficulty in readily observing the earthquake waves that form in the deep ocean, yet such is at times the violence of an earthquake that there is no difficulty in detecting its presence, even in deep water. Dr. Rudolph has made a careful study of the evidences of earthquakes produced in the deep sea, from a careful examination of a great number of the logs of ships. Logs, as everybody knows, are books in which the captain or commanding officer makes careful entries of all important happenings to the vessel, conditions of the weather and of the sea. From this source Dr. Rudolph obtained considerable information of much value concerning these phenomena.
I have already called your attention to portion of the Atlantic Ocean lying near the Equator, in the warmest part of the ocean, between Africa and South America, as being a region especially liable to submarine volcanic showers. While, generally speaking, there is nothing in this region to indicate the probability of submarine disturbance, yet suddenly, if a vessel happens to pass directly over the point of origin of the quake, there ensues a great quaking or quivering. Loose objects on the ship begin to shake and clatter. Noises arise from some invisible point deep down in the ocean. The disturbance grows, the noises begin to resemble distant thunder, the ship trembles and staggers as though it had struck rocks, and many believe she is about to go down; when, as suddenly as it began, the commotion ceases, the noises stop, and the ship shapes her course as calmly, and as gallantly, as before.
Rudolph gives two excellent examples of seaquakes in this region, both of which were, doubtless, due to submarine eruptions.
On the 25th of January, 1859, as the ship Florence was in lat. 0° 48' N., long. 29° 16' W., about ten miles N. W. by N. from St. Paul's Rock, the people on board felt a sudden shock that began with a rumbling sound like distant thunder. This lasted only forty seconds. The glass and dishes of the vessel rattled so violently that it was feared they would be broken. The shakings were so strong that several objects on the vessel were thrown down. Everyone believed the ship had struck on rocks. The captain leaned over the taffrail in order to see the position of the reef, but soon saw that the vessel had struck nothing, and informed his crew "it was only an earthquake shock."
Another of the log books examined by Rudolph was that of a ship in the same part of the Atlantic Ocean. This record showed that suddenly on a morning, in 1883, strange noises were heard that soon increased and became not unlike the firing of great guns or the peals of distant thunder. The ship vibrated as if its anchor had been suddenly let go, and at the same time a feeling came over all the crew, as if they had been electrified.
In some cases the vibrations were sufficiently severe to throw heavy objects from the deck, as appears in an account given by a French geologist of a quake in the Mediterranean off the shores of Asia Minor.
"Our ship was over the epicentre,"[5] he says, "and was so severely shaken that at first the Admiral feared the complete destruction of the corvette." He then makes the statement that the shocks which were directly upwards were so strong as to throw heavy objects in the air; for example, a heavy gun and its carriage. While it is possible, as Dutton remarks, that this incident of the heavy gun and carriage was grossly exaggerated, yet it should not be forgotten that in the case of submarine eruptions such as that which resulted in the production of the island of Sabrina, an immense column of water, weighing probably many times more than a gun and its carriage, was observed to be shot high into the air.
Where the seaquake is produced by a strong submarine volcanic eruption, there is a violent commotion of the water itself, so that a vessel passing over such a point may be greatly injured, and, indeed, even destroyed. Such disasters, however, are fortunately exceedingly rare.
Among other common effects of seaquakes is the destruction of fish already mentioned by the sudden blow to the water stunning and killing them, just as the explosion of dynamite or other high explosives does in a lake or pond.
The most marked effect, however, of seaquakes is the starting of the great wave on the coasts of continents and islands.
There are certain parts of the ocean that are especially liable to seaquakes. Some of the more important of these, as shown by Rudolph's researches, are:
The region already referred to in the narrowest parts of the Atlantic Ocean between Africa and South America almost immediately under the equator. Here there are two well marked regions. One is in lat. 1° N., long. 30° W., where there is a submarine ridge, the tops of which form what are known as St. Paul's Rock. The ocean here is very deep, the slopes of the ridge descending rapidly. It is on these slopes that earthquakes are very apt to occur just as they do on the steep slopes of mountain ranges. The other region, called by Rudolph the Equatorial District, lies a little further to the east on both sides of the equator in long. 20° W.
It appears from Rudolph's researches that between 1845 and 1893 no less than thirty-seven seaquakes were reported in the logs of ships in the neighborhood of St. Paul's Rock, and between 1747 and 1890, in the equatorial district, there were forty-nine seaquakes. It must not be supposed, however, that these were all the quakes in the regions during these times, since, of course, many shocks must have happened that were not felt even by vessels in the neighborhood and many more, when this portion of the ocean was free from any craft.
In the North Atlantic there is a portion of the ocean's bed known as the West Indies Deep. Here the bed is marked by great depths and by many irregularities and is, therefore, a region where seaquakes are common.
Still another district is found in the North Atlantic in the neighborhood of the Azores. This is the region in which the Lisbon earthquake is believed to have started.
Another region where seaquakes are common is in the Pacific along the coast of South America from the equator to 45° S. lat. "Here," says Dutton, "especially in the vicinity of the angle where the Peruvian and Chilian coasts meet have they been most numerous and formidable. The harbors of Pisco, Arica, Tacua, Iquiqui, and Pisago have been repeatedly subject to these destructive invasions."
There has been considerable discussion as to the exact manner in which the earthquake waves are set up. Whatever be the cause or causes, the action must be sudden, such as an upheaval of the bottom, or a collapse of a large section of the ocean's bed, with a dropping of a vast body of water. Or, possibly, a submarine volcanic eruption causes the water to lift suddenly under pressure of steam generated by escape of the lava and other hot volcanic products.
Dr. Rudolph attributes earthquake waves to submarine volcanic eruptions alone. It would seem, however, as if each one of the other things above referred to might at times be the direct cause.
Earthquakes may occur at any part of the earth's surface, at any time of the day, or at any season of the year, yet they are more frequent at certain parts, certain hours, certain seasons.
Since some earthquakes are unquestionably connected with volcanic eruptions, a map or chart of the volcanoes of the earth would also, to a certain extent, show the parts of the earth that are likely to be visited by earthquakes. Since, however, by far the most severe earthquakes are not directly connected with volcanoes, but are due to sudden slips of faulted strata, a volcanic chart would necessarily fail to indicate accurately the principal earthquake regions.
In the preparation of a map showing the distribution of earthquakes over the earth's surface, Mallet adopted the plan of colorings or tintings in such a manner that the depth of the colors would represent not only the parts shaken, but also the relative number of times shaken, as well as the intensity of the shocks. In order to determine the depth of tint to be employed, Mallet divided earthquakes into the following classes according to their intensity:
Great earthquakes, or earthquakes of the first class; or those in which the area affected is of great size, in which many cities have been overthrown, and many people killed, and parts of the surface greatly altered.
Intermediate earthquakes, or those in which, although the area affected is great, yet the destruction of buildings, or loss of life, has been comparatively small.
Minor earthquakes, or those which, although capable of producing small fissures in the crust, generally leave but few or no traces of their occurrence.
The greatest distance to which earthquake waves of the first class extend is taken by Mallet as being over a diameter of 1,080 miles; those of the second class over a diameter of about 360 miles, and those of the third class over a diameter of about 120 miles.
According to the Rossi Forel scale already given, earthquake shocks are divided according to their relative intensity into ten separate classes, viz.: I. The micro-seismic; II. The extremely feeble; III. The very feeble; IV. The feeble; V. The moderately intense; VI. The fairly strong; VII. The strong; VIII. The very strong; IX. The extremely strong; X. Shocks of extreme intensity.
An earthquake map prepared according to Mallet's scale would show a greater depth of color or tint in the neighborhood of the volcanic districts of the earth and especially in the neighborhood of the mountain regions, where tectonic quakes are most frequent. Oceanic areas would be left almost untinted, not because earthquakes do not occur on the bed of the ocean, but because of the difficulty of observing such earthquakes at great distances from the land. So far from earthquakes being absent on the bed of the ocean it is most probable that they are more frequent there than elsewhere.
Prepared in this way, Mallet's map would show a preponderance of earthquakes along the borders of the continents, especially along the "Great Circle of Fire" on the borders of the Pacific Ocean.
Dutton as well as some others assert that the "Great Circle of Fire" on the shores of the Pacific has in reality no existence; that, instead of there being a continuous region of volcanoes, there is in reality nothing more than a considerable number of volcanoes arranged in groups along the borders of this ocean, but separated by spaces containing no marked volcanic activity. We do not think this a tenable position, since it is well known that volcanoes lie along great lines of fissures at different points or openings which are kept open by subsequent volcanic activity, while the remaining portions are closed soon afterwards; and, moreover, in parts of these so-called non-volcanic regions, there are probably extended regions of extinct volcanoes.
Since the time of Mallet many maps have been made to show the distribution of earthquakes. Among the best of such is that by M. de Montessus de Ballore.
Some idea of the great amount of work required for the preparation of Montessus' map may be formed when one learns that the catalogue of earthquakes collected by him for this purpose included for the years 1880 to 1900, 131,292 quakes.
De Montessus' earthquake map divides the grand divisions of the earth into numerous sub-divisions, too numerous, indeed, for even brief description in a work of this kind. From the map he thus laboriously prepared De Montessus drew the following general conclusions:
1. The parts of the earth that are most apt to be shaken by earthquakes are those which possess the greatest differences of relief between their highlands and lowlands, and that in such regions the most pronounced earthquakes are found on the steepest slopes.
2. Earthquakes are most common along those parts of the crust that are thrown up in huge wrinkles, or mountain ranges, whether these masses be above the level of the sea or are covered by it.
3. Earthquakes are more common in mountainous districts than in plains. But not all mountains are characterized by earthquakes nor are all plains free from them. Sometimes the plain at the base of the mountain appears to be especially liable to shocks, probably by reason of slips along faults at these points.
The great mountain ranges of the world are generally characterized by unequal slopes, the long gentle slope facing the interior of the continents, and the short, abrupt slopes being turned towards the coast. Now, Montessus points out that volcanoes are the most frequent on the short, abrupt slopes. In some cases, however, where the long slopes are the roughest, it is these slopes that are most frequently shaken.
The beds of the ocean that lie along rapidly descending lines, especially when they lie on the borders of large mountain ranges, are especially liable to earthquakes.
Dr. Charles Davison has made a map of the earthquakes of Japan in which he had adopted the plan of representing the origin or centres of earthquakes by a series of contour lines like those employed on topographical maps. The advantage of this type of map over that employed by Mallet is just this: Davison's earthquake map of Japan in which the active volcanoes are marked by dots, and the earthquakes by contour lines surrounding the points of origin, discloses the interesting fact that here the positions of the volcanoes and the earthquake centres coincide, since the mountainous districts where the active volcanoes are numerous are singularly free from earthquakes. This can be seen from an inspection of Fig. 55.
Earthquakes occurred long before man appeared on earth. It is natural, therefore, that our early ancestors, experiencing these unwelcome phenomena, vaguely endeavored to explain their causes. These early attempts at explanation have in many cases been of an exceedingly fanciful character.
The ancient Mongolians and Hindoos declared that earthquakes are due to our earth resting on a huge frog and that they occur whenever the frog scratches its head.
In Japan, where earthquakes are very common, the ignorant people even at a much later day declared that there exists in the depth of the sea an immense fish which, when angry, dashes its head violently against the coast of the island, thus making the earth tremble. This is, doubtless, the biggest fish-story extant.
Another folk-lore explanation in Japan attributes the cause of the tremblings of the earth to a subterranean monster whose head lies in the north of the island of Hondo, while his tail lies between the two principal cities. The shaking of his tail causes earthquakes.
Fantastic and foolish as these explanations are, it is worthy of note that the first of the Japanese explanations shows no little observation on the part of the people, since it locates the starting-points of earthquakes as being not on the land, but on the bottom of the sea. In point of fact, nearly all the great earthquakes in Japan seem to start somewhere between the coasts of the islands on the sea-bottom that leads down to a very deep part of the Pacific known as the Tuscarora Deep.
Many years ago nearly everyone believed that earthquakes were caused solely by the forces that produce volcanic eruptions; that all earthquakes, whether in the neighborhood of active volcanoes, or at great distances therefrom, were to be regarded solely as volcanic in their origin.
It is now recognized that the most severe and far-reaching earthquakes have no immediate connection with volcanic explosions, but are due to the sudden slippings of the earth's strata over lines of faults; or, in other words, earthquakes are most frequently of the tectonic type.
At the present time there is unfortunately much difference in opinion as to the exact cause of earthquakes. By this is not meant the immediate cause, but the ultimate cause. As to the immediate cause, practically all are agreed that quakes of volcanic origin are to be traced to the same forces that produce volcanic eruptions, while quakes of tectonic origin are due directly to the slipping of the strata along the faults. But when inquiry is instituted as to the nature of the forces that cause the volcanic eruptions, or that produce such an alteration of the strata as permits them afterwards to slip and thus jar the earth, there is much difference of opinion.
As can be seen from a few quotations of well-known authorities, only two kinds of earthquakes exist; namely, volcanic earthquakes and tectonic earthquakes.
Dana, for example, while acknowledging that small earthquakes may be caused by the sudden falling of large rock masses into cavities in the crust of the earth, says:
"But true earthquakes come, for the most part at least, from one or the other of the following sources of disturbance.
"1. Vapors suddenly produced, causing ruptures and friction.
"2. Sudden movements or slips along old or new fractures.
"Earthquakes due to the former of these methods are common about volcanoes, and at the Hawaiian islands shakings that are destructive over the island of Hawaii at the moment of some of the more violent eruptions, do not often affect the island of Oahu, a depth of 500 fathoms of water, the least depth between the two islands, being sufficient to stop off the vibrations....
"Earthquakes of the second mode of origin may occur in all regions, volcanic or not. They have their origin mostly in the vicinity of mountain regions, where old fractures most abound. The vibrations may begin in a slip of a few inches, in fact; but where there has been a succession of slips, up and up from 10,000 feet or more, as in the Appalachian, earthquakes of inconceivable volcanic activity must have resulted."
Dana points out that volcanoes stand on lines of fractures in the openings of which their existence began and that, during geological time, slips up or down these fractures have occurred, producing earthquakes and possibly starting eruptions.
Prestwich, a well-known English geologist, speaks very decidedly concerning the causes of earthquakes:
"For my own part, I am disposed to share the belief expressed by Dana that the tension or pressure, by which the great oscillations or plications of the earth's crust have been produced, have not entirely ceased; and that this is generally the most probable cause of earthquakes. The uplifting of the great continental tracts and mountain ranges must have always left the interior of the crust in a state of unstable equilibrium, and any slight slide or settling along an old fracture, or in highly disturbed and distorted strata, would be attended by an earthquake shock.
"In volcanic areas the removal of the large volumes of molten rock from the interior to the surface must produce settlements and strains which might also result in some of these minor earthquakes to which volcanic districts are so subject. Where we have the two conditions combined, as they are in the Andes in South America, these earthquake phenomena are, as we should expect, developed on the grandest and widest scale."
Geikie, the Scotch geologist, says:
"Various conceivable causes may, at different times and under different conditions, communicate a shock to the subterranean regions. Such as the sudden flashing into steam of water in the spherodial state, the sudden condensation of steam, the explosion of a volcanic outpour, the falling in of the roof of a subterranean cavity, or the sudden snap of deep-seated rocks subjected to prolonged and intense stress."
Sir Charles Lyell, the great English geologist, holds the following views concerning the origin of earthquakes. He speaks as follows in his "Principles of Geology":
"1. The primary cause of the volcanoes and the earthquakes are to a great extent the same, and connected with the development of heat and chemical action at various depths in the interior of the globe.
"2. Volcanic heat has been supposed by many to be the result of the high temperature which belonged to the whole planet when it was in a state of igneous fusion, a temperature which they suppose to have been always diminishing and still to continue to diminish by radiation into space....
"The powerful agency of steam or aqueous vapor in volcanic eruptions leads us to compare its power of propelling lava to the surface with that which it exerts in driving up water in the pipe of an Icelandic geyser. Various gases also, rendered liquid by pressure at great depths, may aid in causing volcanic outbursts and in fissuring and convulsing the rocks during earthquakes."
Major Clarence Edward Dutton, U. S. A., an acknowledged authority on seismology, speaks as follows:
"Thus far, then, we have two causes of earthquakes which are apparently well sustained: (1) the downthrows, which have often been observed to be accompanied by earthquakes, and (2) volcanic action. But neither of them have been shown to be connected with more than a comparatively small number. Much the greater part of the earthquakes still require explanation, and the indications are manifold that some of them are produced by some cause yet to be stated."
He acknowledges, however, this unknown cause may be traceable to volcanic agency. To quote him in full:
"It remains now to refer to the possibility that many quakes whose origin is unknown, or extremely doubtful, may, after all, be volcanic. This must be fully admitted, and, indeed, it is in many cases highly probable. Evidences that volcanic action has taken place in the depths of the earth without visible, permanent results on the surface abound in ancient rock exposures. Formations of great geological age, once deeply buried and brought to daylight by secular denudations, show that lavas have penetrated surrounding rock-masses in many astonishing ways. Sometimes they have intruded between strata, lifting or floating up the overlying beds without any indication of escaping to the surface. Sometimes the lava breaks across a series of strata and finds its way into the partings between higher beds. Or it forces its way into a fissure to form a dike which may never reach the surface. In one place a long arm or sheet of lava has in a most surprising and inexplicable manner thrust itself into the enveloping rock-mass, and in the older or metamorphic rocks these offshoots or apophyses cross each other in great numbers and form a tangled network of intrusive dikes. In other places the intruded lava has formed immense lenticular (lense shaped) masses (laccolites), which have domed up the overlying strata into mountain masses. These intrusions, almost infinitely varied in form and condition, are often, in fact usually, inexplicable as mechanical problems, but their reality is vouched for by the evidence of our senses. What concerns us here is the great energy which they suggest and their adequacy to generate in the rocks those sudden, elastic displacements which are the real initiatory impulses of an earthquake. They assure us that a great deal of volcanic action has transpired in past ages far under ground, which makes no other sign at the surface than those vibrations which we call an earthquake."
Koto, the celebrated Japanese student of earthquakes, and a member of the Earthquake Investigation Committee appointed by the Japanese Government for studying the great Mino-Owaro earthquake, in Japan, 1891, is properly regarded as an authority on earthquakes. Living, as he does, in a country where earthquakes and volcanic eruptions are of almost daily occurrence, he has had abundant opportunity for studying these phenomena, especially in connection with the Seismological Institute of Japan. He speaks as follows:
"To make clear once for all my own standpoint, I may say plainly that the chain of volcanoes and the system of mountains of the non-volcanic earthquake, appear to me to have very intimate and fundamental relations with the so-called tectonic line."
Mallet regards earthquakes that can be directly traceable to volcanic origin as unsuccessful efforts on the part of nature to establish volcanoes. He speaks concerning this matter as follows:
"An earthquake in a non-volcanic region may, in fact, be viewed as an uncompleted effort to establish a volcano. The forces of explosion and impulse are the same in both; they differ only in degree of energy, or in the varying sorts and degrees of resistance opposed to them. There is more than a mere vaguely admitted connection between them, as heretofore commonly acknowledged—one so vague that the earthquake has been often stated to be the cause of the volcano (Johnson, 'Phy. Atlas,' Geology, page 21), and more commonly the volcano the cause of the earthquake, neither view being the expression of the truth of nature. They are not in the relation to each other of cause and effect, but are both unequal manifestations of a common force under different conditions."
Before closing this chapter on the causes of earthquakes it may be well to state briefly the explanations that have been suggested by those who hold that the earth is solid and cold throughout its entire mass, except that in the neighborhood of volcanic districts there are limited areas situated only a comparatively few miles below the surface where the rocks are highly heated.
Professor Mallet suggested that the source of heat for these local areas of melted rocks was to be found in the enormous mechanical force that is developed by the crushing of the strata in the earth's crust. The principal objection to Mallet's theory is to be found in the fact that, for this heat to be available for the melting of rocks, it must be produced rapidly, and not spread out over long periods of time. Moreover, there would appear to be no other way to account for the production of the great force required to effect the crushing of the earth's strata save on the assumption of a highly heated interior still cooling and contracting.
In his "Aspects of the Earth" Shaler has suggested an hypothesis that may be regarded to a certain extent as explaining how heat, slowly generated, might be blanketed, or prevented from escaping and so possibly reaching a temperature sufficiently high to melt the materials in portions of the interior not far below the surface of the earth.
"We thus see that in the water imprisoned in the deposits of the early geological ages and brought to a high temperature by the blanketing action of the more recently deposited beds, we have a sufficient cause for the great generation of steam at high temperatures, and this is the sole essential phenomenon of volcanic eruptions. We see also by this hypothesis why volcanoes do not occur at points remote from the sea, and why they cease to be in action soon after the sea leaves their neighborhood....
"The foregoing considerations make it tolerably clear that volcanoes are fed from deposits of water contained in ancient rocks which have become greatly heated through the blanketing effects of the strata which have been laid down upon them. The gas which is the only invariable element of volcanic eruptions is steam; moreover, it is the steam of sea-water, as is proven by analysis of the ejections. It breaks its way to the surface only on those parts of the earth which are near to where the deposition of strata is lifting the temperature of water contained in rocks by preventing, in fact, the escape of the earth's heat."
Another very common theory is that of chemical action, or the heat produced by the oxidation of various substances inside the earth, such, for example, as iron pyrites, a compound of iron and sulphur.
When Sir Humphrey Davy discovered metallic sodium and it was found that this material, when thrown on water, possessed the power of liberating intense heat, the discovery was welcomed by geologists as affording a possible explanation of the cause of volcanoes and earthquakes.
It may be said generally concerning chemical action as the source of the earth's interior heat, that the chief objection against it is the fact that such heat is liberated too slowly to result in the production of a very high temperature. This objection does not exist in the case of such substances as metallic sodium, since here the heat is rapidly developed and is sufficient in amount to fuse the substances produced. But in the lava produced in such great quantities as it is in volcanic districts there must be liberated at the same time large quantities of gaseous hydrogen. Now, although hydrogen is, as we have already seen, sometimes given off with the gases that escape from volcanic craters, yet the quantity which escapes is so small that this theory of volcanic activity has been practically abandoned.
Quite recently, however, among the various chemical substances that are produced under the extremely high temperatures of the electric furnace have been found, or formed, a number of curious substances such as calcium carbide, calcium silicide, barium silicide, etc., that possess the property of becoming highly heated on coming in contact with water.
Now it is an interesting fact that the hydrogen and other gases which are given off by the action of water on these substances are absorbed in large quantities by the materials themselves, so that the objection of the absence of hydrogen and similar gases in the craters of the volcanoes would not be quite as objectionable as in the case of such substances.
Of course, it is impossible to say whether such substances as calcium carbide, etc., actually exist inside the earth's crust, yet, as has been pointed out, the principal condition necessary for their formation, i. e., a high temperature, existed at times long after the earth, assuming the correctness of the nebular hypothesis, was separated from the nebulous sun.
There still remains to be discussed the most curious of all possible causes that have been suggested for the presence of the local heated areas at comparatively short distances below the earth's crust; namely, radio-activity.
In 1896, Henri Becquerel, a Frenchman, while investigating the power of the X-rays, when passing through certain substances, to produce phosphorescence, or causing the substances to shine in the dark, made the extraordinary discovery that some of the salts of uranium possess the power of emitting a peculiar radiation closely resembling the X-rays, that is able to pass through substances opaque to ordinary light as well as to affect photographic plates. But the most extraordinary part of this discovery was that the salts of uranium apparently possess the power of giving out this radiation continuously without being exposed to the sun's rays.
This peculiar property was called radio-activity, and was shortly afterwards found to be present in many other substances besides uranium, and notably so in two newly discovered elements known as polonium and radium.
Now it has been suggested that if there existed somewhere beneath the earth's crust in these locally heated areas, large quantities of radio-active substances, these regions would at last become highly heated, and in this way likely to produce volcanoes and earthquakes. It would not, however, seem that this is probably their true cause.
From what has just been said it is clear how exceedingly difficult it has become to explain the source of the earth's interior heat when the fact of the earth's original highly heated condition is denied. We are, therefore, disposed with Russell to believe, as stated in the first part of this volume, that the ultimate cause of both volcanoes and earthquakes is to be found in the gradual cooling of an originally highly heated globe, and that the greater part of the interior is still in a highly heated condition, hot enough to be melted but yet in a solid condition by reason of the great pressure to which it is subjected.
There were numerous volcanoes in the geological past; therefore, since volcanic eruptions are generally attended by earthquake shocks, it follows that during that remote past the earth has been violently shaken by earthquakes. Indeed, if we assume, as we believe to be the case, that the cause of earthquakes is correctly to be traced to an originally heated globe which is gradually cooling, it follows that the earth was necessarily subject to great earthquakes almost from the time when it began to cool.
But to establish as a fact the occurrence of an earthquake at so remote a time in the earth's history is far more difficult than to detect the occurrence of a volcano at that time. While the earthquake shocks may produce fissures in the earth's crust, and may be accompanied by great changes of level, yet the great time that has elapsed between such occurrences and the present would permit the various geological agencies that are at work either to cover these fissures completely, or completely to remove by erosion, or in other similar ways, the rocks in which they occurred. It is different in the case of a volcano; for the volcanic craters are in many cases still left standing, and then there are the voluminous sheets of lava that have spread over great areas of the earth, as well as numerous volcanic cones. Besides, there are thousands of square miles of surface that have been covered, often to great depths, by deposits of volcanic dust thrown out at one time or another from the craters of the then active volcanoes.
I am sure you will acknowledge that any force capable of causing great cracks or fissures in the earth's crust, must, while doing this, have produced violent shakings of the earth. Great cracks or fissures are to be found in the rocks of all the geological formations. These are a record of the earthquakes that must have attended these convulsions. And there is plenty of evidence to show that the earth's crust has been torn into these fissures in places deep down below the present surface; for, by the action of water, many of these portions have been uncovered so that these great cracks or fissures which have been afterwards filled with a molten rock that has hardened can be seen in the great dikes that still remain.
But there are still other evidences of the existence of earthquakes during the geological past. There are found in the different strata of the earth's crust fossil remains of the plants and animals that lived on the earth long before the creation of man. By a careful study of these fossils we know positively the kinds of animals and plants that lived on the earth, in its waters, or in its atmosphere, when these strata were being deposited. It is in this way possible for a geologist to trace the life of the earth and its development as it is written on the great book of which the earth's different strata form the separate pages. Now, a careful study of the earth's fauna and flora during the geological past, shows, beyond any question, that occasionally great changes have occurred in the earth; for, here and there, during different times, we find that certain species of animals and plants have completely disappeared, to be followed, after certain intervals, by entirely different species. It is evident, therefore, that changes have occurred that have made it impossible for the animals and plants that formerly lived on the earth to exist under the changed conditions. These occurrences are known to geologists as exterminations, catastrophes, or cataclysms. They are also sometimes called revolutions, for they mark a more or less complete wiping-out of the animals living at the time they occurred.
If you will try to think you will readily understand how great a catastrophe must be, that would be able to wipe out or completely destroy an entire race of animals.
You have doubtless read with astonishment the terrible catastrophe that accompanied the eruption of Krakatoa, especially at the loss of life and property caused by the great waves that were set up in the ocean, but far reaching as these losses were they have nevertheless affected but a limited portion of the earth. The plain truth is even more stupendous, for catastrophes of the geological past appear to have been so far-reaching and powerful as to affect the whole surface of the earth, and to have annihilated entire races of animals and plants as if they had never existed.
Geologists are all practically agreed that there are only two ways in which such exterminations of the earth's life could have been caused, and these are changes in the earth's climate, or the starting of waves in the sea by great earthquakes. In the sea; for it must be borne in mind that in the geological past the greater part of the earth's surface was covered by water, and the land areas were comparatively small and low, so that waves created by earthquakes might easily have overwhelmed the entire land surface.
Of course, it is fair to suppose that in many cases these exterminations may have been caused by sudden changes of climate, such as would naturally have resulted from any change in the direction of hot ocean currents which formerly flowed from the equator to the poles. The appearance of a fairly large mass of land in the central parts of the ocean might readily have turned aside the hot ocean currents that formerly swept over the polar regions, thus greatly lowering the earth's average temperature in these regions.
But it seems probable that the principal cause of the destruction of life in the geological past was produced by earthquake waves in the sea, sweeping over the continents. Let us, therefore, examine two of the earth's principal geological revolutions or cataclysms; namely, that which occurred at the close of an early geological time known as the Palaeozoic, and that which occurred at the end of a geological time intermediate between the Palaeozoic time or the time of ancient life, called the Mesozoic time, and the Cenozoic time, or the time immediately preceding the present time. These two revolutions are called by Dana, the Post-Palaeozoic, or Appalachian Revolution, and the Post-Mesozoic Revolution. Both were characterized by the making of great mountain systems, and were, therefore, especially liable to repetitions of tremendous earthquakes that must have produced enormous waves in the ocean.
"Palaeozoic time," says Dana, "closed with the making of one of the great mountain ranges of North America—the Appalachian, besides ranges in other lands, and in producing one of the most universal and abrupt disappearances of life in geological history. So great an event is properly styled a revolution."
Towards the close of the Palaeozoic time immense disturbances of the earth's crust occurred during the uplifting of the Appalachian Mountain System. One may, perhaps, form some faint idea of the immensity of the forces at work, from the fact that there were great faults produced by the uplifting of the lands attended with displacement amounting to 10,000 or 20,000 feet or more; that in parts of southwestern Virginia there were flexure faults 100 miles in length.
As to the probability of the extensive exterminations that have occurred during these times being produced by earthquake waves, Dana speaks thus: