80 THE HABITAT OF THE EURYPTERIDA 



zontal 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 T V to T |o mm., rarely reaching 

 to yV 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 re- 

 mains are to be expected. Lyell states that "No substance so 

 coarse as gravel occurs in any part of the delta of the Ganges and B ram - 

 apootra, 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 suc- 

 ceeded downwards by a sandy clay and this in turn by a peat bed. 

 A nodular limestone, the kankar, of fresh-water origin, was encoun- 

 tered. 3 Below the first 120 feet there were found various beds "con- 

 sisting 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 recog- 

 nizable 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 . . ... 



3 For description of kankar, see Grabau's Principles of Stratigraphy, 87, pp. 586, 719. 



