CHAPTER XXXV

EARTHQUAKES OF THE GEOLOGICAL PAST--CATACLYSMS

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:

"The causes of the extermination are two.... (1) a colder climate.... (2) earthquake waves produced by orogenic movements (movements producing mountain ranges). If North America from the west of the Carolinas to the Mississippi Valley can be shaken in consequence of a little slip along a fracture in times of perfect quiet (the allusion here to the Charleston earthquake, in 1886), and ruin mark its movements, incalculable violence and great surgings of the ocean should have occurred and been often repeated during the progress of flexures, miles in height and space, and slips along newly opened fractures that kept up their interrupted progress through thousands of feet of displacements....

"Under such circumstances the devastation of the sea-border and the low-lying land of the period, the destruction of their animals and plants, would have been a sure result. The survivors within a long distance of the coastline would have been few. The same waves would have swept over European land and seas, and there found coadjutors for new strife in earthquake waves of European origin. These times of catastrophe may have continued in America through half of the following Triassic period; for fully two thirds of the Triassic period are unrepresented by rocks and fossils on the Atlantic border."

Coming now to the Post-Mesozoic revolution this period was marked by the making of the greatest of the North American mountain systems.

Dana points out that this revolution affected the summit region of the Rocky Mountains over a broad belt probably as long as the western side of the continent.

This great belt of mountain-making extended from the Arctic regions through North America, probably paralleled by like work, of equal extent, in South America, but on a more eastern line.

"The disappearance of species," says Dana, "at the close of Mesozoic time was one of the two most noted in all geological history. Probably not a tenth part of the animal species of the world disappeared at the time, and far less of the vegetable life and terrestrial Invertebrates; yet the change was so comprehensive that no Cretaceous species of Vertebrate is yet known to occur in the rocks of the American Tertiary, and not even a marine Invertebrate."

In tracing the causes of these disappearances, Dana points out that, perhaps, the principal cause was a decrease in the temperature of the ocean, since the destructions were limited in large measure to marine life. He regards, however, the other most probable cause as traceable to earthquake waves; for the making of a great mountain range along the entire length of the continent resulted in displacements of the rock formations along lines hundreds of miles in length. Such displacements must have been attended by a succession of earthquakes of unusual violence, causing the destruction by sudden shocks beneath, and resulting, directly and indirectly, in waves sweeping over the continent. Since at this time the land was still low for the greater part, the huge waves must have repeatedly swept over the greater part of the land, leaving only the smaller species of animals and the vegetation.

It is evident, therefore, that during the geological past earthquakes occurred that were probably vastly greater than any that have occurred on the earth during more recent times.