238 THE POPULAR SCIENCE MONTHLY 



The usual shape of a normal delta is a convex protrusion beyond the 

 main shore-line, with usually slight protrusions at the mouths of the 

 distributaries ; as can be seen by an inspection of the maps of the deltas 

 of any of the larger rivers, such as the Nile, Ganges, Brahmaputra, 

 Danube, A^olga, Lena and others. Within the delta-areas of these 

 streams, large and small distributaries form a complex network, fre- 

 quently changing at times of high water. No such changes are shown 

 by the narrow-banked, diverging arms of the lower Mississippi delta, 

 which steadily advance into the Gulf singly, and without any permanent 

 distributaries being formed. The only approach to the form and struc- 

 ture of an ordinary delta occurs about three miles above the Head of 

 the Passes, on the east side, where small and shallow channels connect 

 with the main river through Cubit's Gap, a shallow lateral outlet. 



Notwithstanding these facts, the Mississippi delta is figured in the 

 latest edition of Chamberlin and Salisbury's extended work on geology, 

 apparently as an example of a " normal " delta, and its formation is 

 somewhat elaborately, but unconvincingly explained on the basis of the 

 formation of the ordinary river deltas. The explanations do not, un- 

 fortunately, fit the facts as observable by any one examining the banks 

 of the Mississippi Passes; nor is any mention whatever made of the 

 existence and formation of the " mudlumps," which have proved of such 

 vital importance to the commerce passing through the mouths of the 

 Mississippi, that they have been called the ^' evil geniuses of the Passes." 



Mudlumps 



Considering that these mudlumps have for many years been known 

 to, and discussed by pilots, navigators and United States engineers, and 

 have been somewhat elaborately treated of by Lyell many years ago, it 

 is remarkable that their existence, and the part they have so obviously 

 played in the regime of the Mississippi Passes, should have been wholly 

 ignored by writers on general geolog}', and even in the standard work 

 of Eussell on the " Elvers of the United States." 



As my detailed investigation of the subject, made in 1867, is not 

 even mentioned among the references given by Chamberlin and Salis- 

 bury, it seems proper to recapitulate that investigation in print, espe- 

 cially since recent events in the delta seem to have strikingly confirmed 

 my results. 



The Port Hudson Clay (" Blue Delta Clay " of former Writers) 

 The investigations of Humphreys and Abbott had established the 

 fact that the sands and silts of the true Mississippi delta, at least from 

 Baton Eouge to the mouths, are underlaid at comparatively shallow 

 depths by a stratum of blue clay, 10 to 20 feet in thickness, practically 

 impervious to water, and almost inerodable by water alone. This is 



