the size of Great Britain was gravely ravaged; and the thrill of its ultimate vibrations may safely be said to have permeated the entire globe. In the central tracts, the solid ground seemed to lose its consistence, and the earth rose in waves which could be seen to approach like rollers on a seacoast, but with the extraordinary velocity of two miles a second. At Nalbari, the singular spectacle was witnessed of rice-fields undulating rhythmically during the transit of the seismic swell. Even at Calcutta, 255 miles from the centre, the oscillations of the soil caused buildings to rock like ships ploughing the sea. The face of the country near Shillong, over a stretch of some four hundred miles, was conspicuously altered by the catastrophe. Mr. Oldham, of the Indian Geological Survey, who made an exhaustive inquiry into its attendant circumstances, ascertained that hills changed in relative height to the amount of twenty-four feet, and shifted their bases by as much as twelve.* A partial revision of the trigonometrical survey was thus rendered necessary; while, by way of compensation, the subsidence of an intervening and inconveniently high hill facilitated heliographic signalling between two stations of the military police. A railway embankment a mile long was displaced nearly seven feet with its rows of trees all standing; railway lines buckled and crumpled up; fissures opened everywhere, and jets of sand and water spouted from the soil. From certain sections of the Garo and Khasi hills forests and foliage slipped like a cast-off garment, leaving them bare skeletons of sandstone, white under the blazing sun. Pillars were twisted sectionally by complex vorticose movements; thirty lakes added themselves to the permanent features of the landscape. Deep-seated structural modifications were further indicated by the development of several great fault-scarps, the extensive throw of which measured, in some degree, the intensity of the strain relieved by the rending of the strata. The subterranean displacement of material which gave rise to this earthquake was considered by Mr. Oldham to have taken place over an area two hundred miles long by fifty wide. 'We may think,' Dr. Davison says, 'of a slice of rock three or four miles in thickness and large enough to reach from Dover to Exeter in one direction, and from London to Brighton in the other; not slipping intermittently in different places, but giving way almost *Dutton, 'Earthquakes,' p. 59. instantaneously throughout its whole extent: crushing all before it, both solid rock and earthy ground alike; and, whether by the sudden spring of the entire mass or by the jar of its hurtling fragments, shattering the strongest work of human hands as easily as the frailest. Such a thrust might well be sensible over half a continent, and give rise to undulations which, unseen and unfelt, might wend their way around the globe.' (p. 319) Sea-quakes, the correlatives of earthquakes, have often proved no whit less destructive. Not through their direct effects. Ships take no serious harm from their vibrations. But the simultaneous booming of submarine artillery may signalise the start of a tidal wave bearing calamity to every encountered shore. Lisbon, in 1755, suffered thus a double destruction. Overthrown first by the swaying of its foundations, it was, within a few seconds, inundated through the irresistible assault of a mighty ocean-roller, originated, probably, by a collapse of the sea-floor somewhere to the eastward of the Azores. An analogous occurrence beneath the Pacific Ocean gave rise to the terrible inundation of August 13, 1868, when Arica in Bolivia met the same fate that had earlier overtaken the city by the Tagus. The sea, to begin with, retired, Major Dutton informs us,* from the shore, so that ships anchored in seven fathoms of water were 'left high and dry. A few minutes later it was seen returning ' in a great wall or "bore," which caught up the ships in the 'roadstead and swept them inland as if they were mere chips ' of wood. Among them was the U.S. steamer Wateree, which 'was carried inland nearly half a mile and left with little injury on shore by the recession of the wave.' There the hulk remained derelict until May 9, 1877, when, by a fantastic coincidence, the wave engendered by the Iquiqui quake' floated it still further inland. The flood of 1868 was felt on the most remote shores of the Pacific. Hakodate in Japan, at a distance of 7,600 nautical miles, was reached by it in twenty-five hours; and even there the oscillations had an amplitude of ten feet with a period of twenty minutes. Their harmless advent gave scarcely a hint of the violence with which, on June 15, 1896, seismic waves, raised in the vicinity, and unspent by long travel, rushed upon the Japanese shores. Twenty-nine thousand inhabitants of the country perished on that day by drowning. Catastrophes of the sort are due to sudden changes of * Earthquakes, p. 281. level in the sea-bottom, the frequent occurrence of which is vouched for by differences in soundings taken before and after earthquake shocks. Broken telegraph cables are another common result of slips and subsidences. Australia was in this way completely isolated in 1888, and the cause being unknown, the approach of a hostile fleet was apprehended, and the reserves were called out.* It is impossible to ignore, however difficult to define, the relationship between seismic and volcanic action. Both accompany strains in the earth's crust; and though the regions of their respective prevalence do not coincide, they are usually adjacent. The crest of a summit-ridge is the characteristic seat of explosive vents; the tilted tracts at its base are the frequent nurseries of earthquakes. Professor Milne's analysis of 10,000 observations on seismic events in Japan showed that only a small minority had their source quite close to the volcanoes which stud the backbone of the country.† Most of them spread outward from centres along the coast, or beneath the rapidly deepening ocean, where pronounced flexures create lines of weakness. Yet the two orders of phenomena cannot be altogether set apart. Scrope long ago referred earthquakes to the snap and jar' caused by the instantaneous rupture of rocky formations, associated, perhaps, with injections of intumescent molten ' matter.' Nor is it doubtful that such combined events take place. Abortive eruptions have left abundant geological traces. Intruded veins, sheets, and lenticular expansions of lava frequently permeate formations which they did not succeed in penetrating. The igneous activity by which subjacent rocks were being altered and displaced was unapparent at the surface except by mechanical concussions. It would be illogical to deny that these have the significance now that belonged to them in Tertiary or Palæozoic ages. They may frequently, although they need not invariably, betoken explosive uprushes in conjunction with abrupt dislocations. Seismic phenomena cannot then be said to depend solely upon strains and stresses. The restoration of equilibrium is possibly only the pulling of a trigger by which an intricate piece of machinery is set going. The connexion is at times unmistakable. Humboldt relates that, on the day of the awful Riobamba earthquake, * Milne, Ency. Brit. vol. xxvii. + Seismology, p. 31. Volcanoes, p. 294, 2nd ed. February 4, 1797, a lofty column of smoke and ashes sent forth during three previous months by the volcano of Pasto in Quito suddenly ceased to be emitted. The distance is 240 miles. Again, a long series of mixed events, volcanic and seismic, culminated in the destruction of Caraccas, the 'earthquake city,' March 26, 1812. The subterranean disturbance of 1772 in the Caucasus, by which the hill of Metshuka was engulfed, coincided approximately with the memorable explosion of Papandayang in Java. And the year 1783 was marked by catastrophes of each description. On February 5, Calabria was laid waste by one of the most severe earthquakes described in detail and authoritatively. In the following June, Skaptar Yökull, in Iceland, ejected prodigious floods of lava at an unusually high temperature; while on August 1 the Japanese crater of Asama-yama burst into vehement eruption. Every conspicuous event of the kind in the West Indies since 1692 has been apparently related to underground displacements either in one of the neighbouring islands or further off in Central America.† The unparalleled explosion of Mont Pélée, May 8, 1902, made no exception. Subterranean trouble first manifested itself in Mexico and Guatemala. Town after town was ravaged by earthquakes from January 16 to September 23; then, on October 24, the dormant crater of Santa Maria woke up with terrific detonations, and, amid lurid flames or pitchy darkness, laid waste the thriving coffee plantations of El Palmar. Similar disturbances were, indeed, general and world-wide. On April 12, 1902, the country round Lake Baikal was violently shaken; on July 5 hundreds of dwellings were overthrown near Salonika by a concussion, the reverberations of which extended to Birmingham, 1,440 miles away; Kishin Island, in the Persian Gulf, marked the centre of a number of destructive shocks, July 9-10; earthquakes occurred a little later in California, at Skagway, and at Carrara. A sea-quake struck a steamer in mid-Atlantic on July 20; and the calamitous Kashgar earthquake of August 22, in which 3,000 persons lost their lives, with minor disasters at Tiflis, in Ferghana, and the island of Guam, completed an unexampled record of desolation and dismay. Some degree of interdependence between earthquakes and volcanic eruptions must accordingly be admitted. Sub * Views of Nature,' pp. 360-1. + Milne, Nature,' vols. Ixvi. p. 151, lxvii. p. 92. terranean adjustments are highly unstable. Their overthrow at a single point may involve their subversion at others very distant from it. The propagated effects of one fault-slip, or the evisceration of a solitary crater, may disturb the balance upon which are poised the foundations of a continent. Reciprocal action is thus inevitable, albeit varied in its modes and seemingly indeterminate as to its measure. To some small extent the state of the earth's interior responds even to extra-terrestrial influences. Many illusory correspondences have, it is true, been alleged. Sun-spots, meteors, aurora, planetary configurations have no true relations with seismic phenomena. These, however, are affected by meteorological conditions, which again depend upon the sun. Earthquakes, for example, are slightly more frequent in winter than in summer, owing, it is thought, to snowy deposits by which inequalities of strain are increased. And their occurrence is, for a similar reason, favoured by steep barometric gradients. The strata, in other words, are most apt to crack and slide when a gale of wind is blowing. Their rupture is also sensibly promoted by lunar attraction, which tends to lighten the load of pressure they have to bear. And this naturally facilitates the elastic rebound by which equilibrium is restored. Now the lunar orbit pivots completely round once in about nine years; and Mr. Espin † has detected an accordant periodicity in earthquakes and volcanic eruptions, their times of maximum agreeing with the most northerly situation of the perigee, when (it might be argued) the moon's pull against gravity is most effective over the best observed parts of the earth's surface. But the genuineness of the connexion is, as yet, far from being established. The new seismology, as already explained, is distinctively the science of wave-transmission through the earth. Vast stores of energy are radiated outward in all directions from deep-buried regions of rock-collapse; and their radiation is governed by laws easy to enunciate, but extremely difficult to apply. Strict inferences from them must be corrected by experience; and instructive experience depends upon a process of disentanglement practicable only at great distances from the foci of disturbance. The comparative study of seismograms has been organised mainly by Professor * Milne, Earthquakes,' pp. 262-6, 4th ed.; 'Seismology, pp. 208-18. + Wolsingham Observatory Circular, No. 49. |