1872.] OLDHAM AND MALLET — CACHAR EARTHQUAKE. 269 



and if water be carried up before it, or separate from the precipi- 

 tating mud itself, or rain fill the mouth of the aperture, ponds or 

 wells of water, putrid or black, may remain more or less perma- 

 nently, as was the case in the great Calabrian earthquake, in the 

 plain of Rosamo, which reposes on a bed of tenacious water-soaked 

 clay. 



That the accidental conditions under which this general play of 

 forces may act may be greatly vai'ied, and so give rise to many 

 strange and interesting phenomena, is obvious ; and that when gases 

 are extricated by the sudden alteration of pressure upon ooze con- 

 taining much organic matter still in course of chemical change, 

 splutterings of mud, sand, water, and gases may be ejected together, 

 and present to the uninstructed eye the appearance of a sort of vol- 

 canic eruption without fu'e, is equally obvious. 



And where the production of these fissures, and the pressing up 

 of muddy liquid, sand, and so forth occurs upon the vast scale of 

 the Cachar shock, and especially in a tropical climate, many very 

 strange meteorological phenomena may present themselves at and 

 above the fissures, deceptive to the uninstructed eye, and the care- 

 ful observation and analysis of which must be of great interest to 

 the physicist ; upon these, however, the writer must not dwell. 



Nor, at the conclusion of a paper perhaps already too long, can 

 he venture to enlarge adequately upon what he believes to be the 

 origin of those recurrent noises, like those of distant artillery firing, 

 mixed with rumbling or rattling noises, which have been observed 

 in this Cachar and many other earthquakes, at shorter or longer in- 

 tervals after the shock. 



The writer believes these to be produced either by repeated falls 

 of rock upon rock in subterranean cavities, temporarily produced or 

 preexistent, or to the sudden giving way by crushing of rock, or to 

 the sudden giving way, by sliding, of one mass upon another under 

 steadily increasing lateral or other pressure, such changes of in- 

 ternal pressure in the more rigid masses of the earth's crust, as well 

 as the less coherent ones, always constituting part of the machinery 

 by which earthquakes are brought aboiit. Such changes of pres- 

 sure beneath the surface taking more or less long time to readjust 

 themselves into normal equilibrium, those sudden movements, ac- 

 companied by the noises of fractures, falls, or grinding of surfaces, 

 may continue, and at more or less regularly recurring intervals, for 

 a considerable time after the shock, or they may precede it for 

 months, as at East Haddam, and in Scotland and Switzerland, or 

 may closely accompany the actiial shock itself. 



Dr. Oldham's remarks as to the curious statements made to him 

 by an educated native, of what he fancied he had observed of the 

 transit of the earthquake across his garden, afford a good lesson to 

 all engaged in the systematic observation of such phenomena, not 

 to trust to statements of supposed facts made by unpractised ob- 

 servers, however desirous these may be of only stating what they 

 believe to be the truth. 



