Robbs — Features Formed at the Time of Earthquakes.. 255 



As the gas and water rises, it brings up a little gray mud, which, 

 on exposure to the air, dries and hardens while the water evapo- 

 rates, producing first a low crater basin with a dry rim of mud, 

 then a cone with a crater on the top, in the center of which the 

 gas finds vent." 



It will be noted that we have here an evolution of topogra- 

 phic forms identical with, those illustrated by the sand phe- 

 nomena of earthquakes as above outlined. Other petroleum 

 mud cones occur upon the Apennines at Sassuolo and San 

 Venanzio, at Tanan and Baku in the Caucasus, and upon the 

 Island of Trinidad, while Macalnba in Sicily rests upon beds 

 of clay containing gypsum, salt, sulphur, bituminous matter, 

 etc. or much the same mixture as that to be found beneath 

 the gulf mounds. The smell of sulphur which accompanies 

 the eruptions of Macaluba is of interest because it offers a pos- 

 sible explanation of the sulphurous odors with which the air 

 was charged during the great New Madrid earthquake of 

 1811-12. 



The small amount of attention which " mud volcanoes" have 

 attracted we owe perhaps to their name, which classifies them 

 with volcanic phenomena, but with which they have little in 

 common. Generally born at the time of earthquakes, they 

 show a sympathetic response to seismic shocks within their 

 neighborhood, and are properly classed as phenomena conse- 

 quent upon the derangements of the ground water and gas 

 systems by earthquakes. Our knowledge of the brontidi now 

 makes it possible within a seismic province where mud vol- 

 canoes are frequent (Italy, for example), to determine what 

 relation exists between their periods of activity and the per- 

 ception of subterranean rumbling. It seems certain that where 

 mud or sand cones are now forming, orographic blocks are 

 being depressed, which accounts for their common occurrence 

 within delta regions. The study of bradysisms has clearly 

 shown that nearly all coast lines of the continents are today 

 rising, the marked exceptions being the deltas of the great 

 rivers* The whole Mississippi flood plain, with the excep- 

 tion of Lake County, Tenn., and the opposite shore of the 

 river, would appear to be included within the area which in 

 the isostatic adjustment about the Gulf is being depressed. f 



Further light is likely to be shed upon the origin of the 

 Gulf Plain mounds through the careful mapping of them 

 within definite districts. If, as seems likely, they have been 



* Arturo Issel, Le oscillaziani lenti del suelo o bradysismi. Saggio di geo- 

 logia storica. Atti della E. Universita di Genova, vol. v, pp. 417, map, 1885. 



fW. J. McGee, A fossil earthquake, Bull. Geol. Soc. Am., vol. iv, pp. 411- 

 413, 1893. 



