248 Hobhs — Features Formed at the Time of Earthquakes. 



instances, the material ejected was a salty mud, which upon 

 desiccation yielded hard mud cones ranged upon fissure lines 

 (fig. 1). At Chemakha, moreover, a second displacement upon 

 the line of the fissure produced a distinct fault wall a meter in 

 height which cut the dried mud cones. 



In Italy, a notable earthquake country, gas and water yield 

 mud volcanoes in the period between earthquakes, but the 

 action in them is usually either more or less vigorous during 

 seismic shocks in the neighborhood. A series of such mud 

 volcanoes runs in a nearly straight line entirely across Sicily 

 from Siculiana upon the south coast to Patera 6 upon the flanks 

 of JEtna. Mud volcanoes are also numerous in the Apen- 

 nines, though no attempt has yet been made to determine 

 their arrangement or their relation to structural lines. This is, 

 therefore, a most promising field for future studies in struc- 

 tural geology. 



The whole subject of the extensive derangement of the 

 ground water during earthquakes has been difficult of explana- 

 tion by the centrum theory, and has not received the attention 

 which it deserves or which it is likely in the near future to 

 attract. The introduction of the crustal block or compartment 

 theory of earthquakes affords an explanation, both simple and 

 natural, of these derangements. If the elastic waves which 

 we describe as earthquakes arise from the mutual friction upon 

 the edges of earth blocks, which through an abrupt jolting 

 movement seek to readjust themselves to the stress system, 

 looked for consequences are : (1) the squeezing out of water 

 from the trunk channels of circulation within those districts 

 where blocks are depressed, (2) a sucking down of the water 

 of swamps and ponds wherever blocks are elevated, (3) a suck- 

 ing down following upon a squeezing out of water (often 

 actually observed with the formation of cratered sand cones) 

 when the slight elevation of a block succeeds to its depression, 

 and (4) a squeezing out of water following a sudden draining 

 of a district when a slight settling follows the elevation of a 

 block.* Quite analogous appears to be the forcing of fire 

 damp into the galleries of Belgian coal mines, which, from 

 quite recent studies of Van den JBroeck, appears to take place 

 chiefly when the Mistpoeffers are heard and when there are 

 earthquakes in moderately distant districts. f 



It is a fact of much significance that in the "sunk country " 

 of the Lower Mississippi the springs are often surrounded by 

 little cones of sand admixed with lignite, and that the inhabit- 



* Gerland's Beitraege zur Geophysik, vol. viii. Heft 2, 1907. 

 f E. Van den Broeck, 5th Int. Congress of Hydrology, Climatology and 

 Geology, Liege, 1898, pp. 1-18. 



