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, Volga, Lena and others. Within the delta-areas of these streams, large and small distributaries form a complex network, frequently 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 structure 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, unfortunately, 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."
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 régime of the Mississippi Passes, should have been wholly ignored by writers on general geology, and even in the standard work of Russell on the "Rivers 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 Salisbury, it seems proper to recapitulate that investigation in print, especially 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 Rouge 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