the severe one of Thursday, April 2nd, but even this, though causing some people to run out of their houses, did no damage to buildings. The Kohala plantation, chimney, and buildings were not injured." At Lahaina, on the island of Maui, about 100 miles distant from the epicentral points, the great shock of April 2nd was observed to shake pictures on the walls and rattle loose objects, while on Oahu, 150 miles away, the only persons who noticed it were a few in the second stories of buildings.'

This was one of the most forcible earthquakes which are assigned with certainty to a volcanic origin. Another almost equal to it was associated with the last historic eruption of Mount Ararat, on June 20, 1840. The shocks on this mountain appear to have been extremely forcible, all the villages near its base being shaken down in ruins, with a loss of life which was great relatively to the scanty population. But at Erivan, less than forty miles to the north-east, and a considerable city, the amount of damage was limited to a few cracked walls. At Kars, Erzroum, and Tiflis, all within 150 miles, the shocks were unnoticed.

The recent eruptions of Mount Pelée, in Martinique, and La Soufrière, in St. Vincent, gave rise to no far-reaching shocks or vibrations except some of a peculiar class to which we may here allude. It has sometimes been observed that during a volcanic eruption of the energetic kind, such as are exhibited frequently in the volcanos of the East Indian Archipelago, or in portions of the Andes, the

Detailed accounts of this earthquake may be found in Amer. Jour. Sci., series ii., vol. xlvi. The writer visited the locality in 1882 and questioned some of the witnesses of this affair who were then living and retained vivid recollections of it.

blowing off of the escaping gases at the vent, or the violent movement of the uprising lavas in the volcanic pipes, produce a sort of sustained chattering or vibrations of very short period, which are perceived both as a tremulous quiver and as a deep, humming sound at very great distances. It may continue for hours with long crescendos and diminuendos, while the vibrations of an earthquake, as ordinarily understood, last, usually, not more than one or two minutes. This phenomenon was quite marked in the recent eruption of the Windward Islands, and the sustained rumbling and chattering was recognised throughout the whole of that island chain. It has been repeatedly noticed in some of the old eruptions of Cotopaxi, and has been observed as far north as Cartagena and as far south as Lake Titicaca. The most remarkable instance of this kind appears to have been the eruption of Coseguina, on the Bay of Fonseca, Nicaragua, in 1835, the rumbling being heard at Kingston, Jamaica, at Caracas, Bogota, and Vera Cruz.

This class of tremors seems to be propagated with more than ordinary facility and to great distances through the deeper, more compact, and therefore more perfectly elastic, strata, and also through the water, and their sustained character tends to make them more easily recognisable than individual vibrations of the same, or even of somewhat greater power.

The eruption of Krakatoa in 1883, in which nearly onehalf of the island and its volcanic mountain were blown up and scattered in small fragments over the Straits of Sunda, seems to have been about as energetic an occurrence as any of which we have detailed records. And yet at Batavia,

only 90 miles away, its vibrations were inconsiderable, and the most pronounced evidence of an earthquake was the seawave which entered the harbour. Similarly upon the adjoining Sumatran coast and upon the Preanger coast of Java the sea-wave was the principal phenomenon of a seismic nature, the vibrations alone causing no destruction. The earthquakes which are associated with the eruptions of Etna, though sometimes violent in the immediate vicinity of that mountain, seem never to have been strongly felt across the straits in Calabria. But the Calabrian quakes have been known to shake the Sicilian coast with great power. In the great Calabrian catastrophe of 1783 all Sicily was shaken, and in the city of Messina Sir William Hamilton' and Dolmieu declare that not a house escaped injury, and most of them were partially or wholly destroyed.

Earthquakes are common around the base of Vesuvius and among the islands around Stromboli. But their tremors are seldom strongly felt in Sicily or on the Italian coast.

The restricted fields in which are manifested the vibrations of quakes resulting clearly from volcanic action may be explained by the comparatively small depth at which they originate and to the expenditure of a much smaller amount of total energy than is involved in the greater and more widely extended quakes.

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,

1 Phil. Trans., 1783.

211 Dissertation on the Calabrian Earthquake" (translated), in Pinkerton's Toyages and Travels, vol. v.

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 denudation, 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 dyke 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 dykes. In other places the intruded lava has formed immense lenticular 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.

CHAPTER IV

DISLOCATION OR TECTONIC QUAKES

Chilian Dislocations and Associated Earthquakes-Similar Phenomenon in Cook's Straits-The Sonora Quake of May 3, 1887-The Dislocation which Caused it-The Great Mino-Owari Quake in Japan, October 28, 1891-The Great Dislocation of the Neo Valley-Its Destructiveness and Great Energy-The Bengal-Assam Quake of June 12, 1897Probably the Most Formidable and Energetic of Record-The Characteristic Features of Tectonic Quakes-Their Vast Energy-The Immense Area throughout which they Are Sensible-Their Numerous After-Shocks-Omori's Study of After-Shocks-Scarcity of Preliminary or Warning Shocks-Tectonic Quakes without Visible Signs of Dislocation-The Andalusian Quake of December 25, 1884-The Charleston Quake of August 31, 1886-The Inyo or Owens Valley Quake of March 26, 1872-Absence of a Distinct Epicentre in Tectonic QuakesElongated Figure of the Meizoseismic Area-Agram Quake of November 9, 1880-The Middle Silesian Quake of June 11, 1885-Tectonic Origin of Great Sea Quakes-Schmidt's Study of Quakes in the Eastern Mediterranean

EARTHQUAKES which have been the accompaniments

of dislocations of the strata have been the most impressive of which we have any accurate knowledge. Sometimes the dislocations are visible to the eye. Sometimes the sea covers them, but leaves them to be inferred from the vast sea-waves they are presumed to have caused. some cases, with less confidence, with considerable uncertainty and hesitation, but not unreasonably, we may