270 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 10, 



Discussion. 



Sir Henry James inquired whetiier there was any trace of fissuring 

 in the lower beds beneath the slimy ooze. 



Mr. Scott wished to ascertain the anther's opinion as to the pos- 

 sibility of predicting earthquakes on meteorological grounds, as had 

 been done by M. Bulard, of Algiers, several of whose prophecies were 

 said to have been fulfilled (vide ' Comptes Eendus,' March 11, 1872), 



Mr. D. FoKBEs gave some details of the earthquake of Mendoza, a 

 town situated on the vast alluvial plain at the foot of the Andes, in 

 which the phenomena remarkably coincided with those detailed by 

 Dr. Oldham. In that case also he found that the rumours as to fire 

 and smoke having been emitted from fissures were entirely without 

 foundation, the presumed smoke having been nothing but dust. The 

 earthquake was felt over a distance of 1200 miles; and wherever 

 the firm rock came to the surface there was no trace of fissure. But 

 in the plain at the foot of the Andes, consisting of alluvial soil, the 

 deep ground was in parts fissured, and in one place the surface com- 

 pletely furrowed and the turf turned over. He had witnessed nume- 

 rous earthquakes, and in some cases had been in deep mines during 

 their occurrence ; the sound only could be heard, and he could testify 

 to their effiects being confined to the surface. The direction of the 

 fissures was invariably at right angles to the line of shock. In South 

 America all the earthquakes could be traced to volcanic centres. 



The Presipent inquired as to the distinction to be drawn between 

 the primary and secondary eflfects of earthquakes, and whether the 

 author thought that no fissures were attributable to the direct action 

 of earthquakes. As to the cause of the sound, like that of a cart 

 carrying iron bars or of an artillery waggon, he wished for further 

 information. 



Mr. Mallet, in reply, explained that fissures only take place where 

 masses were comparatively free in one direction. They might extend 

 to enormous depths, though they often closed in rapidly. With re- 

 gard to the power of predicting earthquakes, he disbeheved in it 

 wholly, and considered that any fulfilment of such prophecies must 

 be due to accident ; earthquakes are so numerous, that the chances 

 of such fulfilments are great. The blow or impulse originating 

 earthquakes could not be attributed solely to one cause. It arose 

 often from deep subterranean volcanic action ; but it also — espe- 

 cially in the ease of long- continued tremors, like those of Comrie or 

 Pignerol— arose from the breaking up or the grinding over each 

 other of rooky beds at a great depth, through the tangential pres- 

 sures produced in the earth's crust by secular cooling. The arrested 

 impulse of the fall of the Rosberg in Switzerland produced a sensible 

 earthquake. Fissures in hard rock could not be produced directly 

 by the shock, because the velocity of impiilse in such rock greatly 

 exceeded that of the elastic wave-particle. The earth's crust was at 

 present not in a state of tension, but of compression, through secular 

 cooling. 



