VOLCANOES.) In the case of Earthquakes, the earth-wave or wave of a shock underneath a coun try may traverse a wide region and affect it violently at the time without leaving any trace of its passage. Loose objects, however, are apt to be displaced. Thus blocks of rocks already disengaged from their parent masses may be rolled into valleys. Land slides may be produced, making changes in the courses of streams. Fissures are made in the soil, from the size of tiny crevices to wide chasms. Trees may be thrown down and buried, and the • surface of the region may be radically changed. But in a few years these superficial effects may be effaced by the leveling power of the atmosphere. In New Zealand, in 1848, an earthquake fissure 18 inches wide was traced for 60 m., and in 1855 another WAS made of 90 m. in length. Remarkable -circular cavities are sometimes formed in the ground during the passage of the earth-wave. In many cases these holes serve as funnels for the escape of water. They are believed to be caused by the collapse of subterranean water-channels and the consequent forcible ejection of water to the surface. Springs are affected by earthquake movements, becoming more or less in volume, discolored, or muddy, and increasing or diminishing in temperature; and brooks and rivers are accelerated or stopped. Lakes rise or fall at great distances from the center of disturbance. When the earthquake occurred at Lisbon many of the lakes in central and north-western Europe w were so affected as to maintain a succession of waves two or three feet above their usual level.
In some cases lakes have become dry ground, and dry ground lakes. The great sea wave propagated outward from the center of a suboceanic earthquake, and reaching the land after the earth-wave has arrived there, gives rise to much destruction along the maritime parts of the disturbed region. As it approaches the shore, the littoral waters retreat seaward, sucked up, as it were, by the advancing wall of water, which, reach ing a height of sometimes 60 ft., rushes over the bare beach and sweeps inland, carry ing with it everything which it can dislodge and bear away. Loose blocks of rock are thus lilted to a considerable dislance from their former position, and left at a higher level. Deposits of sand, gravel, and other superficial accumulations are torn up and swept away, while the surface of the country, as far as the limit reached by the wave, is strewn with debris. If the district has been already shattered by the passage of the
earth-wave, the advent of the great sea-wave augments and completes the devastation. It has been observed, after the passage of an earthquake, that the level of the disturbed country has been changed. Thus after the terrible earthquake of Nov. 19, 1822, the coast of Chili for a long distance was found to have risen from 3 to 4 ft., so that along the shore the littoral shells were exposed, adhering still to the rocks, amid multitudes of dead fish. The same coast-line has since been further upraised by subsequent earth quake shocks. On the other band, many instances have been observed where the effect of the earthquake has been to depress permanently the disturbed ground. For example, during the Bengal earthquake of 1762, an area of 60 m. on the coast, near Chittagong, suddenly went down beneath the sea, leaving only the tops of the higher eminences abovevater. The succession of earthquakes, which in the years 1811 and 1812 devastated the basin of the Mississippi, produced wide depressions of the ground, over some of which the river spread so as to form new lakes, with the tops of the trees still standing above the surface of the water.
An earthquake shock has been defined by Mr. Mallet a.s the transit of a wave of elastic compression through the crust and surface of the earth, generated by some sud den impulse within the crust. The passage of such a wave has been imitated experi mentally, and some of its characteristic features have been illustrated by accidental explosions at powder-works. But though die phenomena point to some sudden and vio lent blow inflicted upon the crust, it is impossible to do more than speculate on the probable nature of this blow. In some cases it may arise front the sudden flashing into steam of water in the spheroidal state; from the sudden condensation of steam; from the explosion of a volcanic orifice; from the falling in of the roof of a subterranean cavity: or front the sudden snap of subterranean rocks subjected to prolonged and intense strain. But we are still in ignbrance as to the actual immediate cause of any earthquake in regions remote from active volcanoes. This, at least, is certain, that the shock must arise from some sudden and violent impulse, whereby a wave or undulation is propagated in all directions through the solid substance of the crust.