Page:Popular Science Monthly Volume 80.djvu/247

Rh A comparison of the mud ejected with that forming the narrow banks of resistant clay bordering the Passes, as above outlined, and also the same resistant clay banks at the Head of the Passes which cause their divergence, shows the materials to be undistinguishable. The conclusion is inevitable that the entire bird-foot delta at least, and doubtless also the narrow Neck in which the main river flows below Forts Jackson and St. Philip, are the outcome of the formation and destruction of mudlumps as the river progressed; and that these clay banks constitute the normal mode of progression of the emerged portion of the lower delta.

The extraordinary resistance of the mudlump clay, when once consolidated in the channel banks, to erosion by water, serves, together with the somewhat similar characteristics of the underlying Port Hudson clay, to explain the exceptional form of the lower delta of the Mississippi.

I should add that a microscopic, physical and chemical examination of the mudlump mud, and of the clay from the banks of the Passes, bear precisely the biological characteristics to be expected under the conditions outlined. There is an intermixture of fresh-water and brackish marine organisms; while the water forming the mud is manifestly sea-water in a condition of considerable dilution, and changed by maceration with the organic débris brought down by the river. As a result of such reductive maceration, the sulphates in sea-water have been largely eliminated in the form of minute crystals of iron pyrites, and the lime as carbonate; while the ratios of the chlorids have suffered little change. The details of this investigation are set forth in my paper in the Journal of Science, already referred to.

In the early 70's of the past century, even the widest of all, the Southwest Pass, had become so obstructed by mudlumps that deep-sea navigation was very difficult to maintain, despite the most active dredging on the part of the government, and the construction of tugs of enormous power, designed to pull deep-drawing vessels through the upheaved mud. Captain James Eads, the builder of the St. Louis bridge, then conceived the idea that, as the South Pass was unobstructed by mudlumps, it might be made the main and permanently navigable channel if sills of willow mattresses were placed across the entrance of the other distributaries (the Southwest Pass and Pass à l'Outre), and if jetties were constructed at its mouth to maintain a current so strong as to carry away the obstacles caused by river deposits of all kinds. Accordingly, a bill was introduced into Congress for the construction of these improvements. When this came to my