Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/269

Rh EARrHQUAKEs.] accumulations are torn up and swept away, while the surface of the coun{lry,1::is far apfthehlimcilt reache1:1d bybthe w2lwe,1is strewn wit ( e iris. ' t e istrict as een a reacy shattered by the passage of the earth-wave, the advent of the great sea-wave augments and completes the devasta- tion. 4. Permanent C]t((7l{/es of Level.—It has been 0bse1'ved, after the passage of an earthquake, that the level of the disturbed country has been changed. Thus after the terrible earthquake of 19th November 1822 the coast of Chili for a long distance was found to have risen from 3 to 4 feet, so that along the shore the littoral shells were ex- posed adhering still to the rocks amid multitudes of dead ﬁsh. The same coast-line has since been further upraiscd by subsequent earthquake shocks. On the other hand, many instances have been observed where the effect of the earthquake has been to depress permanently the disturbed ground. For example, by the Bengal earthquake of 1702 an area of 60 square miles on the coast, near Chittagong, suddenly went down beneath the sea, leaving only the top of the higher eminenees above water. The succession of earthquakes which in the years 1811 and 1812 devastated the basin of the Mississippi gave rise to widespread de- pressions 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. 5. l)z'st2'ibz¢tion aml G'eolo_r/ical Ifelations of Earth- qua7ces.—Wliile no large space of the earth’s surface seems to be free from at least some degree of earthquake- movcmcnt, there are regions more especially liable to the visitation. As a rule, earthquakes are most frequent i11 volcanic districts, the explosions of a volcano being very generally preceded or accompanied by tremors of greater or less intensity. In the Old Vorld the great belt of earthquake disturbance stretches in an east and west direction, along that tract of remarkable depressions and elevations lying between the Al )s and the mountains of northern Africa and spreading eastlward so as to enclose the basins of the Mediterranean, Black Sea, Caspian, and Sea of Aral, and to rise into the great mountain-ridges of Central Asia. In this zone lie numerous volcanic vents, both active and extinct or dormant, from the Azores on the west to the basaltic plateaus of India 011 the east. The Paciﬁc Ocean is surrounded with a vast ring of volcanic vents, and its borders are likewise subject to frequent earthquake shocks. Some of the most terrible earthquakes within human ex- perience have been those which have affected the western seaboard of South America. 6. Causes of Earl/aqua/ces.—An earthquake shock has been deﬁned by Mr Mallett as the transit of a wave of elastic compression through the crust and surface of the earth, generated by some sudden 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 the phenomena point to some sudden and violent blow inﬂicted upon the solid crust, it is impossible to do more than speculate on the probable nature of this blow. In soipe cases it may arise from the sudden ﬂashing into steam 0 water in the s )hcroidal state fr sudden condensation of steam, [from the expldsiorcisnofhs volcanic oriﬁce, from the falling in of the roof of a sub- terranean cavity, or from the sudden snap of subterranean rocks subjected to prolonged and intense strain. But we are still in ignorance as to the actual immediate cause of any earthquake in regions remote from active volcanoes. So much, at least, is certain, that the shock must arise from so1ne sudden and violent impulse, whereby a wave or un- dulation is propagated in all directions throucrh the solid substance of the crust. O GEOLOGY 255 Section III.—Secular Upheaval and Depression. Besides the sudden movements due to earthquake-shocks, the crust of the earth undergoes in many places oscillations of an extremely quiet and nniform character, sometimes of an elevatory, sometimes of a subsiding nature. So tranquil are these changes that they produce from day to day no appreciable alteration in the aspect of the ground affected. Only after the lapse of several generations, and by means of careful measurements, can they really be proved. In- deed, in the interior of a country nothing but a series of accurate levellings from some unchanged datum-line might detect the change of level, unless the effects of this terres- trial movement showed themselves in altering the drainage. It is only along the sea-coast that a ready measure is afforded of any such movement. In popular language it is usual to speak of the sea as rising or sinking relatively to the land. But so long as the volume of the ocean remains the same, the general sea-level can neither rise nor fall, un- less by some movement of the solid globe underneath it. And, as we cannot conceive of any possible augmentation of the oceanic waters, nor of any diminution save what may be due to the extremely slow process of abstraction by the hydration of minerals, or absorption into the earth’s in- terior, we are compelled to regard the sea-level as practi- cally a constant datum-line, any deviation from-which, in the apparent heights of sea and land, nmst be due to move- ment of the land and not of the sea. There are indeed certain cosmical causes which may affect the relative levels of sea and land. Thus the accumu- lation of large masses of snow and ice as an ice-cap at one of the poles would, as has been above pointed out (ante, p. 217), tend to displace the earth’s centre of gravity, and as a consequence to raise the level of the ocean in the hemi- sphere so affected, and to diminish it in a corresponding measure elsewhere. The return of the ice into the state of water would produce an opposite effect. Dr Croll has also drawn attention to the fact that, as a consequence of the diminution of centrifugal force owing to the retardation of the earth’s rotation caused by the tidal wave, the sea-level must have a tendency to subside at the equator and rise at the poles. A larger amount of land need not ultimately be laid bare at the equator, for the change of level resulting from this cause would be so slow that the general degrada- tion of the surface of the land might keep pace with it, and diminish the terrestrial area as much as_the retreat of the ocean tended to increase it. Dr Croll has further pointed out that the waste of the equatorial land, and the deposition of the detritus in higher latitudes, must still further counteract the effects of retardation and the consequent change of ocean-level.‘ Such widespread general causes of change must produce equally far-reaching effects. But in examining the changes of level between land and sea, we ﬁnd them to be eminently local and variable in character, pointing to some local and unequally acting cause,—-so that, while admitting these cosmical and widespread influences to be part of the general system of geological change, we must yet hold the sea- level, for all practical purposes, to be invariable, any apparent oscillations of that level upon the land being due to terrestrial movements. § 1. .:l[ovements of Up/aeaval. Various maritime tracts of the land have been ascertained to have undergone in recent times, or to be still undergoing, a gradual elevation above the sea. Thus, the coast of Siberia, for 600 miles to the east of the river Lena, the western tracts of South America, and the Scandinavian peninsula, with the exception of a small area at its southern 382; 1 Croll, Phil. .l[a_r/., 1868, p. Soc. Glasgow, iii. ‘.223. Sir W. Thomson, Trans. (hol.