CHAPTER II.

EARTHQUAKES-THEIR EFFECT IN MODIFYING EARTH'S SURFACE-EARTHQUAKE-WAVES.

"Thou sure and firm-set earth.”—Macbeth.

THE theory which attributes to earthquakes a large share in the conformation of the actual surface of the globe we inhabit is as old as Herodotus, who referred the opening of the gorge, through which the Peneus escapes to the sea, and Thessaly ceased to be a lake, to an earthquake caused by Neptune. This view was shared by Strabo. It was adopted by Hooke and Hutton; and their follower, Sir Charles Lyell, proclaimed the continued action of earthquakes as the undisputed agent of his cosmogony, operating now, as in former ages of the world's existence, only with diminished force. Not less than 100 pages of that fascinating work The Principles are devoted to a history of the best recorded convulsions, and to the description of the office they fill. He sums up with the conclusion that the actual configuration of the earth's surface is due to a long

continued series of moderate shocks. The poets of old, he tells us, were wrong to select the rock as the emblem of firmness, and it has fallen to the geologist of modern times to correct their error.

There can be no doubt that the earthquake, when it comes in contact with buildings, or other work of man, is truly terrible, indeed irresistible. "Temple and tower fall to the ground," whole cities are prostrated like packs of cards, and myriads of human beings are annihilated in the ruin. No wonder then that the description by eye-witnesses of these catastrophes should be tinged with the exaggerations of terror. It is surprising, however, that men of science should have transferred such exaggerations to their writings, and should have applied to the works of Nature what is strictly true only in reference to those of man, more especially since the few scientific eye-witnesses who have written from the spot, and near the time of such convulsions, give information which enables us to correct these fables and reduce them to sober reality. If it should turn out that this part of the fabric of modern geology has been hastily laid on foundations not perfectly secure, it is to be hoped that all lovers of science will encourage an investigation which has truth for its object.

1 Lyell's Principles, ii. p. 179.

Earthquake phenomena, little understood when the Principles of Geology first appeared, have been since subjected to minute mathematical and practical investigation by Mr. Hopkins and Mr. Robert Mallet. The last-named gentleman has studied them not only theoretically but on the scene of operations, having visited South Italy after the earthquake of 1857. His authority is acknowledged by Lyell, who quotes his writings, though he does not avail himself of all the results we are bound to bring forward.

First of all let us invite attention to Mr. Mallet's definition of an earthquake as "a movement like the shaking of a sieve," as "simply the transit of a wave or waves of elastic compression, from vertically upwards to horizontally, through the crust and surface of the earth." An eye-witness compares its motion with that of a carpet lifted on one side and shaken along the ground. Another wrote from Callao, August 13th, 1868, "As far as the eye could see along the narrow street, the very street itself rose and fell in long billowy undulations." 2 After the

1 A translation of the Greek σouós. Admiralty Manual, p. 325.

2

* During an earthquake at Samoa, February 1876, an observer writes, "The thatched roof presented the appearance of waves running rapidly across it from south to north.” -Nature, July 27, 1876.

Mallet

first concussions the earth continues for hours, even for days, in a constant tremor or vibration. adds, p. 329, "Earthquakes must not be confounded with the forces producing permanent elevation of the land." These fundamental determinations of learned observers ought carefully to be borne in mind in the following investigation; they are somewhat at variance with the assumption of the Huttonian school of geology regarding both the nature and the permanent changes effected by these convulsions.

Let us examine, then, the various instances recorded in the pages of the Principles, so as to ascertain what enduring consequences earthquakes are really capable of producing, and what evidences they leave behind, on the face of Nature, of the permanent elevation and fracturing of rocks, and especially of great mountain chains.

The earthquake of Calabria in 1783 was one of the most tremendous on record, and its effects were carefully investigated on the spot immediately after, by the Neapolitan Academy, by Sir William Hamilton, Mons. Dolomieu, and others, who published elaborate reports; and Sir Charles Lyell has given a very full account of it from these sources. and foremost occurred the opening of fissures in the ground, as the earth-wave rolled along, and the swallowing up of men and cattle and buildings by

First

the immediate closing of these gashes.

Some re

mained open, however. Next we are struck by the constant occurrence of landslips, caused by enormous masses of earth being shaken down from the hillsides by the sieve-like motion, so that, in the language of the terror-stricken inhabitants, one hill marched down to meet another. The result wasinnumerable stoppages of the river-courses, the obstruction of the drainage of the district, and the formation2 of 215 stagnant ponds and lakes.

But the reader of Lyell who has admired the curious woodcuts of straight and starred fissures, holes, ravines, and chasms, must not for a moment suppose that these were formed in solid rock, that they lasted any time, or that any one visiting the spot would be likely to find any trace of them at the present day. All the fissures gradually closed up, for they were confined to superficial deposits, alluvium, clay, gravel, and an incoherent tertiary sandstone, according to Dolomieu (Brit. Assoc. Rep.,

The filling up of a deep cove near Dusky Bay, New Zealand-surrounded by steep cliffs-after the earthquakes of 1826-27, was evidently the result of landslips. "Trees were seen under water near the coast, having probably been carried down by landslips."-Lyell's Principles, ii. p. 82.

2

Lyell's Principles, ii. pp. 129, 130, 133.