Page:The Habitat of the Eurypterida.djvu/86

 beds varying in thickness from a few inches to several feet, and looks more like an ancient stratified series than a modern deposit. The material of the Nile mud is a more or less uniformly fine-grained one, the size of the grains varying from $$\tfrac{1}{13}$$ to $$\tfrac{1}{100}$$ mm., rarely reaching to $$\tfrac{1}{10}$$ mm. in size" (Grabau, 87, 614). The Mississippi delta is spread out in the remarkable bird-foot form and the whole of its lower part is covered with a network of distributaries which often empty into large fresh-water lakes. In these lakes and over all the interstream areas the fine muds are deposited. They contain shells of fresh-water molluscs and much driftwood, which is often united into floating rafts.

In the portions of the delta nearer the sea, fresh-water and marine organisms are both found, not intermingled, however, but in separate layers, depending upon whether beds were deposited in the sea or by the streams above the sea. Thus a bed with fresh-water shells and lignite is often intercalated between beds with marine remains, giving evidence of the shifting conditions of deposition in deltas where streams continually change their channels and where consequently the areas of terrestrial deposition are shifted, while the sea advances in the interfluve areas and a wedge of marine deposits is formed.

Here a few details in regard to the nature of the Indo-Gangetic delta will give a good idea of what types of sediments and organic remains are to be expected. Lyell states that "No substance so coarse as gravel occurs in any part of the delta of the Ganges and Bramapootra, nor nearer the sea than 400 miles" (154, 280). A boring to a depth of 481 feet made near Calcutta showed below the surface soil, at the top of the first 120 feet of the boring, a stiff blue clay succeeded downwards by a sandy clay and this in turn by a peat bed. A nodular limestone, the kankar, of fresh-water origin, was encountered. Below the first 120 feet there were found various beds "consisting of clay, marl, and friable sandstone with kankar here and there intermixed, [while] no organic remains of a decidedly marine origin were met with . . . . The only fossils obtained in a recognizable state were of a fluviatile or terrestrial character. Thus, at the depth of 350 feet the bony shell of a tortoise, or Trionyx, a fresh-water genus, was found in sand, resembling the living species of Bengal . . . . At the depth of 380 feet, clay with fragments of lacustrine shells was incumbent on what appears clearly to have been another "dirt-bed," or stratum of decayed wood . . ..