Page:Popular Science Monthly Volume 80.djvu/243

Rh called the "blue delta clay" by Humphreys and Abbott; it will here be called the Port Hudson Clay, because it is entirely independent of the modern delta formation built up by the river.

In this stratum, when exposed in-shore or in shallow water, there frequently appear stumps of the deciduous cypress, suggesting that in former geological times a cypress swamp extended out gulfward, perhaps to the deep-water line at the edge of the continental shelf. Beneath this Port Hudson clay stratum lie formations materially different, and of such a character, both physical and biological, as clearly proves them to be not river alluvium, but of marine, brackish and paludal origin. But these formations, as well as the Port Hudson Clay, have nothing to do with the present problems of the delta, beyond serving as the floor on which it is built forward. The depth of the sands and silts of the true delta is practically from thirty to forty feet, and rarely reaches above sixty feet. That so great a river should show so small a depth of alluvium, when compared with such rivers as the Nile, Ganges, Hoangho and others, at first appears incredible; but it becomes intelligible when considered in connection with the existence of the underlying Port Hudson clay stratum, and the extraordinarily rapid extension of the mouth of the river towards the Gulf; the advance of the bar at the mouth of the Southwest Pass being, at the time mentioned, about 340 feet per annum. That this advance, however, is not made by the usual process of delta formation, is clearly shown on the accompanying map of the mouths of the Mississippi, and the "normal" delta of the Volga.

As is well known, a continental shelf, covered by a comparatively shallow depth of water, runs out for about thirty miles beyond the present mouths of the Mississippi Elver, then breaks off into the deep waters of the Gulf. The original surface stratum of this shelf is the Port Hudson clay (the "blue delta clay" of Humphreys and Abbott); but it is now coming to be gradually covered with the delta deposits of river sediment; and it would be natural to connect the shallow-lying shelf with the unusually rapid advance of the river mouths.

It is not easy to see at first sight why even the existence of the Port Hudson clay stratum should interfere with the ordinary, merely