130 The Petrography and Genesis of Sediments 



This general view of sedimentation diagrams affords a sufficient basis 

 for the special consideration of the sediments discussed in this chapter, 

 To turn then to samples 1 and 2. 



No detailed field and laboratory study of delta sediments has been pub- 

 lished, to the writer's knowledge; but from what little can be learned of 

 such deposits it appears that the beds from which samples 1 and 2 are 

 taken show many of the characteristics of delta formations. In their field 

 relations the rapid alternation, the extremes represented, the thin partings 

 of sand, and the abundance of carbonaceous matter support this view. 

 And consideration of the conditions of sedimentation in a delta leads to 

 the same conclusion, for, according to the principle laid down by Johannes 

 Walther, only such facies can succeed each other as can exist side by side. 

 Now, in a delta there is a sharp difference between the channels and the 

 waters lying to the side of them, so that in one there would be deposited 

 relatively coarse sand, while in the other fine sediments would slowly 

 settle. Then sudden changes of channel, such as would be produced by 

 high water in a region with the extremely low relief of a delta, would bring 

 two such facies into vertical succession, producing the type of section seen 

 at this locality. The sandy partings, on the other hand, would result 

 merely from the passing conditions of a single flood without a change of 

 channel. 



The mechanical analyses, also, fall in with this general view. To be 

 sure, A, p. 169 ( = sample 1 ) and E, p. 170 (= a lagoon sediment) show a 

 similarity which amounts almost to identity. But the quiet, open bodies of 

 water in a delta would, in their conditions of sedimentation, be entirely 

 equivalent to a lagoon, like that from which E, p. 170, is derived. In B, 

 p. 169, the upper shaded portions of the five left-hand columns represent 

 the analysis recalculated to a basis of 100 after subtracting the clay and 

 silt, and in this form the similarity of the diagram to a stream sediment 

 like M, p. 170, with the abrupt rise of the curve on the left and the poor 

 sorting, is strikingly brought out. Both these analyses therefore lit in well 

 with the conditions that would exist in a broad delta. 



Formation of pyrite is another characteristic of such deposits. It is 

 due. as noted above ( Sample No. 1, Summary and Conclusions ) . to the ILS 



