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



in many cases prove " the hair that breaks the camel's back. " 

 when acting upon a portion of bottom just about to peld to 

 the upheaving force, and therefore in a state of unstable equi- 

 librium. Future observations, however, will readily settle tiie 



Of authentic observations illustrating ordinary rates of risiiiir, 

 I quote the following remarks of Mr. H. L. Marindin, U. 6. 

 Coast Survey, who in 1867, while engaged in the survey of the 

 mouths, at my request paid considerable attention to the phe- 

 nomena of the mudlumps, and made an interesting report to me 

 on the subject, dated June 20th, and accompanied by speei- 



" Since the first examination of the mudlumps on Soiitl 

 Pass Bar, there have appeared numerous lumps in the vii 

 of the main channel, whose appearance cannot be attribiuc 

 other causes than the upheaval of the bottom by some unkii 

 agency ; since tidal observations made during the month of Apr; .. 

 May and part of June, show no material decrease in the height oi 

 water in the river, other than the daily ebb of the tide. In one 

 instance, also, on this bar, it has been ascertained from surveys 

 made at different times, that where in the month of April a 

 channel with fifteen feet of water existed, there is now a lump. 

 doubtless still forming, with only six feet of water on it at the 

 highest staga" 



Making allowance for a possible accumulation of deposit, the 

 rate of rise cannot, in this instance, be estimated at less than two 

 feet per month ; which, so long as the lump remains under wa- 

 ter, seems to be no unusual amount. When, however, the sur- 

 face of the water is once reached, the rising becomes slower or 

 ceases altogether, owing probably to the greater weight acquired 

 by the "material upon its emergence. At least, I have failed to 

 find much above tide level, any material which did not bear 

 distinct evidence of its having been formed, not by river allu- 

 vion, but by the action of the regular mudsprings ; which, if not 

 now active upon all the lumps, have demonstrably been instru- 

 mental in forming the great majority of the masses now above 



I do not know that any one has ever witnessed the first out- 

 burst of a mudspring on a newly risen lump ; but we find them 

 m all stages of progress, from the islet bearing its first tuit o 

 rushes, to the active cones glistening in the sunshine, and from 

 time to time, when an unusually large gas bubble rises, spatter- 

 ing the liquid mud (which usually flows in a quiet stream) au 

 over the slopes ; then the half-extinct cone, in whose crater a 

 little pool of salt water is at long intervals agitated bv a g^ 

 bubble ; then the extinct and collapsed cone, surrounded by a 

 circular moat and Somma-like ring- wall ; next, the old lump 



