216 



EARTHQUAKE PHENOMENA, 



[Sect, VII. 



Strange liquid or solid matters, sometimes of dry ashes 

 or dust, are recorded, and occasionally fiery eructations 

 have occurred, especially near volcanic centres, and 

 blasts of steam-vapours, or gases, whose chemical cha- 

 racters should be in all the above cases observed, as 

 far as possible. The dust of overthrown buildings must 

 not be confounded with these. Fissures, often of profound 

 depth, open and remain so, or close again : their directions, 

 dimensions, time and order of production, and closing up 

 and the formations in which they occur, to be noted ; 

 bodies engulphed to be detailed, as future organic remains. 

 Fissures most probably arise from the range of displace- 

 ment of molecules by the passing wave, exceeding the 

 elastic limit of the materials disturbed. Permanent 

 elevations and depressions of the land usually accompany 

 earthquakes, and are of much importance to science. 



5 



ILo modes of observing these rather belong to geology 

 proper. The half-tide level must in all cases be taken 

 the datum-line, and opportunities alor 



as 



wharfs, or inland, along mill-streams or irrigating channels, 



&c., diligently sought for, where evidence may be trust- 

 wortliily collected of changes of depth of water. Occa- 



sionally local but widely-extended permanent elevations 

 or depressions accompany earthquakes, which seem to 

 result from lateral compression, and not from direct 

 elevatory forces. Ihese should be distinguished from the 

 preceding. 



Rivers are stated to have sometimes run dry during 



gam 



to flow after the shock. 



This is presumed to arise either from the transit of an 

 earth-wave along their courses up stream, thus damming 



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