484 E. W. Hilgard— Geology of the Delta, etc. 



a stronger force, less delicately balanced than tlie equilibrium 

 of the bar, is " at the bottom " of the whole phenomenon. 



This force, I think, is to be sought in the constantly increas- 

 ing weight of the alluvial area above the mouths, which, itself 

 possessing a series of mudlump vents at one time, must yet be 

 resting in a great measure upon the still unexhausted mud 

 stratum ; as is proven by the existence of active lumps in the 

 marshes, even though the increased resistance of superincum- 

 bent deposit as well as matted vegetation must render their 

 occurrence there a rarity. There must still be a comnmnica- 

 tion of liquid pressure between the older and newer portions 

 of the modern delta ; and this point is especially strengthened 

 by the fict that a high stage of water in the river, which does 

 not sensibly affect the depth on the bars, yet exerts a decided 

 influence on mudlump activity. The river not only overflows 

 the marshes, but loads them with additional sediment; and 

 doubtless the increased hydrostatic pressure stops many a vent 

 of gas, mud or water, which ordinarily discharges into the 

 river s bed.* 



Morgan's Lump, in the marsh of Southwest Pass, and 

 Marindin's Lump on Passe k I'Outre, are now known to have 

 been in undiminished activity for twenty-five years at least 

 Since that time the bars have moved gulf ward a mile and a 

 half; and one would think that, if the activity of the cones 

 depended upon them alone, a notable difference" ought to have 

 been observed. But if the main force is a vis a iergo, while 

 the bar serves mainly to prevent the escape of the mud to 

 seaward, there is good cause for the secular persistence of vents 

 that have escaped mechanical obstruction. » 



How fiir above the present mouths the head of pressure may 

 extend, I do not pretend to conjecture. The borings at New 

 Orleans seem to indicate that the mud stratum originally 

 existed there also, but it would be extravagant to suppose that 

 such pi-essure as that exerted in the gas wells of that city, could 

 now be felt a hundred miles below. Yet it seems not at all 

 unlikely, that the weight which steadily forces up the hquni 

 mud to the top of Morgan's Lump, seven miles above the 

 mouth, may, in part, be furnished by the enormous mas? ol 

 vegetation which annually develops 'in the marshes, wdlow 

 battures, and perhaps even cypress swamps above. Nor is the 

 effect of gaseous pressure resulting from the constantly pro- 

 gressing decay of organic matter to be overlooked, although 

 doubt that this cause plays, ordinarily, anything more than a 

 very subordinate part. 



* I give on the plate (page 35 

 the "orisrin of mudlumns." 



