6i4 PRINCIPLES OF STRATIGRAPHY 



deposits of the Nile. At low water these are visible in the steep 

 banks which then rise 8 to lo meters above water level. The hard- 

 ened Nile mud forms a series of horizontal beds varying in thick- 

 ness from a few inches to several feet, and looks more like an an- 

 cient stratified series than like a modern deposit. The material of 

 the Nile mud is a more or less uniformly line-grained one. the 

 size of the grains varying from 1/13 to i/ioo mm., rarely reaching 

 i/io mm. in size. It is a unique deposit probably not paralleled by 

 any other modern one on the face of the earth. An analysis of 

 the mud (Clarke-i 2 :/(?/) gave: 



SiOa 



AI2O3 



FeaOa 



MgO 



CaO 



K2O 



NaaO 



SO3 



HoO 



100.47 



The remarkable fact about this mud is its high iron and low organic 

 content, though some analysts have found the tinely divided or- 

 ganic matter as high as 5.53 per cent, or even 7.9 per cent. Some 

 analyses show an admixture of barium carbonate over wide areas. 



In other deltas the organic material is pronounced. The abun- 

 dant admixture of leaves and more or less lignitized wood in some 

 deltas has already been noted. In the Po delta it occurs in four 

 successive horizons. In the Ganges delta such deposits are found 

 between 9 and 15 meters in depth, together with carbonized trunks 

 of trees characteristic of the region, such as Heriticra littoralis, 

 which abounds in the lower part of Bengal. In the lower Mississippi 

 delta driftwood is common, the logs being at times united into 

 floating rafts. Not infrecjuently erect trunks are found among 

 these, with their roots spreading in all directions, as if while grow- 

 ing there they were submerged by a subsidence. The Mackenzie 

 River delta likewise contains an abundance of carbonized drift- 

 wood, and this is true of many other deltas. 



More striking, however, in some ways is the abundance of 

 finely divided vegetable matter in some deltas. Thus the mud 

 of the Vistula (Weichsel) loses, according to G. Bischof (9) 23.3 

 per cent, on ignition, most of this being organic material. The 

 clay of the Vistula delta in the Bay of Dantzig is so rich in or- 

 ganic material that it has a deep black color, and is locally known 



