Ilohhs — Features Formed at the Time of Earthquakes. 258 



rise are about 6 inches in diameter, vertical, and as regular in 

 form as though bored with an auger." A most important con- 

 tribution to our knowledge of these unique phenomena, in 

 which it has clearly been shown that the material of the mud 

 lumps is of a different nature from that surrounding them, has 

 been very recently made by Hilgard.* He has shown that 

 the sticky mud is the " blue delta clay " or " blue clay bottom " 

 derived from a much lower layer of Coast Pliocene, or Port 

 Hudson age, above which the delta deposits are laid down. As 



Fig. 5. 

 Ansrlesea. 



Principal sandstone pipe where weathered ont upon shore, 

 (Photograph by Edward Greenly, Esq.) 



the latter are deposited they imprison a thin overlying stratum 

 of very thin mud which results from the clarifying of the 

 river-water when it meets the sea-water outside the bar. The 

 settlement of the delta region under its increasing burden of 

 sediment is the plausible cause to which Hilgard appeals for 

 the forcing up of the mud lump clay, and its associated thinner 

 mud. The lumps are thus in reality true " mud volcanoes." 

 The very stickiness of this blue clay, upon which the river's 

 scour makes little impression, in the opinion of the writer, 

 makes it necessary to assume that a current either of water or 

 of gas, or both, is forced xx^from beneath the " blue clay bot- 



* E. W. Hilgard, The exceptional nature and genesis of the Mississippi 

 delta, Science, vol. xxiv, pp. 861-866, Dec. 28, 1906. 



