254 Hobhs — Features Funned at the Time of Earthquakes. 



torn." The gas which is emitted from the lumps is, as Hilgard 

 believes, in volume quite insufficient to alone carry on the 

 action. Fissure springs are best suited to explain all the con- 

 ditions, and the known existence of strong fresh-water springs 

 at numerous off-shore points beneath the Gulf, and especially 

 off the mouth of the Mississippi,"" indicate the continuation 

 beneath the Gulf of the artesian water stratum characteristic 

 of the lower flood plain. 



Moreover, there is other evidence that such fissure springs 

 are in definite alignment. A fact which Lyell considered so 

 important as to print in his rare italics is, " they (the lumps — ed.) 

 were always situated of some one of the mouths of the river. f 

 To this Hilgard has given support by a statement, also in 

 italics, " Mudlump formation is at present the normal mode 

 of progression of the visible delta into the gulfp% It would 

 thus appear that as the mud lumps become clogged, new ones 

 develop along the extension of the same fissure through a 

 steady migration seaward of the process. The peculiarly 

 straight but divergent channels of the unique " birdfoot " por- 

 tion of the delta support this hypothesis. 



While the mud lumps are perhaps not developed at the time 

 of sensible earthquakes, it is a question whether the settlement 

 of the Mississippi delta takes place gradually or per saltum. 

 The tendency at the present time is to look upon such brady- 

 seismic movements as different in degree rather than in kind 

 from those accompanied by earthquakes — a tendency which 

 the increasing knowledge of subterranean sounds (brontidi) is 

 strengthening. 



Many mud volcanoes owe their activity to the high tempera- 

 ture of the subsurface layers of the earth's crust, which sup- 

 plies steam to raise the mud and eject it with violence. § 

 The Minbu mud volcanoes of India are of a different type and 

 have a special interest for the present study because they 

 indicate that the petroleum beneath the mounds of the Gulf 

 Plain may well have played a part in their formation. Accord- 

 ing to Caclel :j| 



"The Minbu salses, for such they are in reality, are due to the 

 escape of carbureted hydrogen from the oil-bearing strata on the 

 top of the anticline, which rises through the clay beds mixed 

 with a little water and oil and slowly bubbles up at certain spots. 



*0. H. Hitchcock, Fresh-water springs in the ocean. Pop. Sci. Month., 

 pp. 682, 683. 



fL. c, p. 446. JL. c, p. 863. 



§See, for example, D. P. Barrows, The Colorado Desert, Nat. Geog. Mag., 

 vol. xi, p. 340, 1900. Also D. T. Macdougal. The delta of the Eio Colorado, 

 Bull. Arn. Geog. Soc, vol. xxxviii, p. 10, 1906. 



I Henry M. Cadel, A Sail down the Irrawaddy, Scot. Geog. Mag., vol. xvii, 

 p. 263, 1901. 



