ORGANIC REMAINS OF DELTAS 615 



as pitcli, "pech." The bearing of this fact on the origin of some 

 black shales will be considered later. 



Organisuis of the Delta. Marine organisms are not uncommon 

 in sea-coast deltas, but they are, as a rule, distributed in certain 

 layers only. Lyell has explained the occurrences of marine molluscs 

 in extensive beds between the fresh water layers of the delta as due 

 to wave work, which casts masses of shells upon the growing delta 

 surface. In some parts of the Rhone delta marine and fresh water 

 shells alternate in the deposit. This is explained by Lyell as probably 

 due to the alternating occupancy of lagoons and channels on the 

 growing delta by salt or by fresh water, according as the prevailing 

 wind or other causes may ordain. As already noted, the delta of 

 the Po also contains fresh water organisms associated with marine 

 shells, but only in the lower beds, while upward the shell deposits 

 become purely marine. Foraminiferal shells often abound in mod- 

 ern deltas. Thus the Mississippi mud was found to contain an 

 abundance of marine Polygastrica and Phytolitharia as well as 

 fresh water Polythalamia. Phytolitharia also abound in the mud of 

 the Nile. Indeed, the range of foraminiferal shell material in the 

 Nile mud is from 4.6 to 10 per cent., while the Ganges carries as 

 high as 12.4 or even 25 per cent, of foraminiferal material. 



Other animal remains have been found, but are less abundant. 

 Remains of artliropods occur in the J*o delta sometimes in close as- 

 sociation with the lignites. Remains of river animals also abound 

 in modern deltas, as shown by the presence of turtles and croco- 

 dile remains in the delta deposits of the Ganges and the Zambesi. 

 Terrestrial vertebrate remains have likewise been found in these 

 deltas, among them bones of recent antelope, buffalo, lion, hippo- 

 potamus, and other mammals. 



Gaseous Emanations of Deltas. The gradual decomposition 

 of the organisms in the delta deposits gives rise to gaseous emana- 

 tions which either escape through artificial borings in the delta or 

 find natural passageways through the mud, building up craters 

 as in the mud-lumps of the Mississippi delta. The nvmierous bor- 

 ings in the delta of the Po have furnished an abundance of inflam- 

 mable gas, the use of which for illuminating purposes has actually 

 been attempted. Sulphuretted hydrogen is also developed, espe- 

 cially where sea water comes in contact with the decaying vege- 

 table matter, as noted in the case of marine marshes (see ante, 

 page 493). In the mud-lumps of the Mississippi delta, the volume 

 of gas emitted is between 1/20 and 1/30 that of the mud flow 

 from these craters. The gas is probably not instrumental in these 

 mud eruptions, but merely an accompaniment of the same. 



