E. W. Hilgard— Geology of the Delta, e 



Art. LXII— On the Geology of the Delta, and the Mudlumps of 



the Passes of the Mississippi; by EUG. W. HiLGAED. 



[Continued from page 368.] 



Origin of the Mudlumps.—The causes which give rise to the 

 formation of mudlumps have been to some extent discussed bj 

 Sidell, Thomassy, and Lyell {he. cit). The former is inchned to 

 ascribe the upheaval chiefly to the pressure of gas formed m the 

 decay of driftwood and the like, buried in the river deposits. 

 Thomassy resorts to the hypothesis of the existence of subterra- 

 neous channels communicating with the river, or with equally 

 hypothetical reservoirs of water, far above ; while Lyell ascribes 

 the bulging of the bottom to the pressure of newly formed de- 

 posits upon a substratum of yielding mud, accompanied, and 

 aided incidentally only, by the evolution of marsh gas in the 

 decay of organic matter. I myself, having become aware of 

 the existence of a strong artesian water pressure m the littoral 

 formations of the Gulf, was inclined to ascribe the origin of the 

 upheaving force to that source ; and my visit to the mouths had 

 for its object mainly, the comparison of the facts with each ot 

 the three admissible hypotheses, that of Thomassy being too 

 fanciful to be seriously entertained. ^ . ^ 



As already stated, I at once found that the evolution of gas 

 in the active vents was too insignificant to be considered as the 

 cause of the rising of the liquid mud, which so greatly exceeded 

 it in bulk, that the ascensional force of the bubbles, especially 

 m so wide a vent-tube, would be utterly inadequate to balance 

 the downward tendencv of so heavy a liquid. It might still be 

 alleged, in favor of the gas-hypothesis, that its pressure raigW 

 be exerted statically upon the surface of the mass of liqmd 

 mud covered by impervious strata; but it is obvious that m 

 such a case, the gc4 itself, necessarily accumulatmg at tne 

 %hest, and therefore weakU points, of f^^ ^'^V^^^^^^^^ 

 niass, would be much more likely to break through by iteel^ 

 promptly exhausting its force and quantity at any one point 

 No suc^ rushing outbreaks of gas have ever been recorded, 

 «ave in the case Sf blowing up of a lump ^}\f''ZltZ:rZt 

 ^ Lyell remarks, this view rendei-s inexplicable the occurrence 

 of lumps exclusively about the mouths of the passea 



The latter objectLn applies equally to the typothe^is ^^^^^^^^^^ 

 artesian origin if mudliliSp force, unless upon the (ujrov^d 

 supposition that the excavation of the river ^^^^^/^l^^l 

 have rendered the outbreak of the artesian wa . - , 



than elsewhere. But instead of excavat^nff, ^be ^^^^^ T^ 

 for a long time past always thrown shallows in advance ot its 

 Am. Jour. Sci.-Third Series, Vol. I, No 6.-June, ISrl. 



