Page:Popular Science Monthly Volume 80.djvu/241

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By Professor E. W. HILGARD



IN 1867 the writer was commissioned by the Smithsonian Institution to determine, if possible, the geological age and mode of formation of the rock salt deposit on Petite Anse Island, Louisiana. This involved, of course, a general examination of the coast formations of Louisiana, and among them, of the Passes of the Mississippi, and of the puzzling phenomena of "mudlump" upheaval in the Passes, which, at times, seriously obstructed commerce, but the origin of which remained a matter of conjecture. It had, to some extent, been investigated by Sir Charles Lyell (1858) and is commented upon in the tenth edition of his "Elements of Geology"; it was also conjecturally discussed by General A. A. Humphreys and other engineers connected with the Mississippi River Commission. My results, so far as the salt deposit is concerned, were published as Memoir No. 248 of the Smithsonian Institution; while the full report of my investigations of the Mississippi mouths and the mudlumps was published in the American Journal of Science in 1871-72.

As this work and its publication dates back so many years, and the latest publications on American geology and hydrography have wholly omitted any mention of it; and since a new phase of the subject has lately arisen confirmative of the views expressed and forecast made by me in 1872, it seems appropriate to recall that work to mind, and direct attention to the unfortunate fulfilment of a former prediction.

The Lower Mississippi Delta not a Normal One

The bird-foot shape of the lower Mississippi delta, with deep embayments in between, is unexampled in any other large river delta in the world. The bays between the delta-fingers ("Passes") are being very slowly shallowed, chiefly by wave and tidal action from the Gulf, carrying in the bar sands; and only subordinately by river overflow. The river in this lower delta region is for 50 miles below Fort Jackson bordered by narrow banks of unyielding gray clay, between which is carried the entire volume of the river through the narrow-banked "Neck," until it reaches a common point of divergence, the "Head of the Passes," whence similarly narrow-banked channels diverge, unbranched, in bird-foot form. (See the map accompanying this paper.)