Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

Measured by the destruction which was wrought to the interests of man, earthquakes deserve to be reckoned among the direst calamities of Nature. Since the dawn of history the records show us that the destruction of life which is to be attributed to them is to be counted by the millions. A catalogue of the loss of life in the accidents of this description which have occurred during the Christian era has led the writer to suppose that probably over two million persons have perished from these shocks in the last nineteen centuries. Nevertheless, as compared with other agents of destruction, such as preventable disease, war, or famine, the loss which has been inflicted by earth movements is really trifling, and almost all of it is due to an obstinate carelessness in the construction of buildings without reference to the risks which are known to exist in earthquake-ridden countries.

Although all our exact knowledge concerning the distribution of earthquakes is limited to the imperfect records of two or three thousand years, it is commonly possible to measure in a general way the liability to such accidents which may exist in any country by a careful study of the details of its topography. In almost every large area the process of erosion naturally leaves quantities of rock, either in the form of detached columns or as detrital accumulations deposited on steep slopes. These features are of relatively slow formation, and it is often possible to determine that they have been in their positions for a time which is to be measured by thousands of years. Thus, on inspecting a country such as North America, where the historic records cover but a brief time, we may on inquiry determine which portions of its area have long been exempt from powerful shocks. Where natural obelisks and steep taluses abound—features which would have disappeared if the region had been moved by great shocks—we may be sure that the field under inspection has for a great period been exempt from powerful shaking. Judged by this standard, we may safely say that the region occupied by the Appalachian Mountains has been exempt from serious trouble. So, too, the section of the Cordilleras lying to the east of what is commonly called the Great Basin, between the Rocky Mountains and the Sierra Nevada, has also enjoyed a long reign of peace. In glaciated countries the record is naturally less clear than in those parts of the world which have been subjected to long-continued, slow decay of the rocks. Nevertheless, in those fields boulders are often found poised in position which they could not have maintained if subjected to violent shaking. Judged by this evidence, we may say that a large part of the northern section of this continent, particularly the area about the Great Lakes, has been exempt from considerable shocks since the glacier passed away.

The shores which are subject to the visitations of the great marine waves, caused by earthquake shocks occurring beneath the bottom of the neighbouring ocean, are so swept by those violent inundations that they lose many features which are often found along coasts that have been exempted from such visitations. Thus wherever we find extensive and delicately moulded dunes, poised stones, or slender pinnacled rocks along a coast, we may be sure that since these features were formed the district has not been swept by these great waves.

Fig. 22.—Poised rocks indicating a long exemption from strong earthquakes in the places where such features occur.

Around the northern Atlantic we almost everywhere find the glacial waste here and there accumulated near the margin of the sea in the complicated sculptured outlines which are assumed by kame sands and gravels. From a study of these features just above the level of high tide, the writer has become convinced that the North Atlantic district has long been exempt from the assaults of other waves than those which are produced during heavy storms. At the present time the waves formed by earthquakes appear to be of destructive violence only on the west coast of South America, where they roll in from a region of the Pacific lying to the south of the equator and a few hundred miles from the shore of the continent, which appears to be the seat of exceedingly violent shocks. A similar field occurs in the Atlantic between the Lesser Antilles and the Spanish peninsula, but no great waves have come thence since the time of the Lisbon earthquake. The basin of the Caribbean and the region about Java appear to be also fields where these disturbances may be expected, though in each but one wave of this nature has been recorded. Therefore we may regard these secondary results of a submarine earthquake as seldom phenomena.

Duration of Geological Time.

Although it is beyond the power of man to conceive any such lapses of time as have taken place in the history of this earth, it is interesting, and in certain ways profitable, to determine as near as possible in the measure of years the duration of the events which are recorded in the rocks. Some astronomers, basing their conclusions on the heat-containing power of matter, and on the rate at which energy in this form flows from the sun, have come to the conclusion that our planet could not have been in independent existence for more than about twenty million years. The geologist, however, resting his conclusions on the records which are the subject of his inquiry, comes on many different lines to an opinion which traverses that entertained by some distinguished astronomers. The ways in which the student of the earth arrives at this opinion will now be set forth.

By noting the amount of sediment carried forth to the sea by the rivers, the geologist finds that the lands of the earth—those, at least, which are protected by their natural envelopes of vegetation—are wearing down at a rate which pretty certainly does not exceed one foot in about five thousand years, or two hundred feet in a million years. Discovering at many places on the earth's surface deposits which originally had a thickness of five thousand feet or more, which have been worn down to the depths of thousands of feet in a single rather brief section of geological time, the student readily finds himself prepared to claim that a period of from five to ten million years has often been required for the accomplishment of but a very small part of the changes which he knows to have occurred on this earth.