128 The Petrography and Genesis of Sediments 



sediments (A) and (B), p. 169, with the diagrams on p. 170, representing 

 various types of deposits. The high percentage of carbonaceous matter is 

 also a characteristic of both samples. Both contain concretionary pyrite 

 grains or small nodules, and in both there are the peculiar clay granules 

 that have been noted. 



Before interpretation of the beds is attempted their manner of occur- 

 rence in the field should be taken into account. This is characterized 

 above all by the rapid and rather extreme alternation of the beds between 

 the two types, sandy and argillaceous, as fairly well represented by these 

 two samples. Carbonaceous matter is conspicuous, also micaceous beds, 

 while thin films of whitish sand between the beds are a characteristic 

 peculiarity. 



The mechanical analyses of sediments, that are represented on p. 170, 

 are not numerous enough nor sufficiently correlated with the exact condi- 

 tions of their formation to justify direct matching of the above analyses 

 with them. They illustrate certain general factors in sedimentation rather 

 than definite types of sediment, and this first discussion of them may 

 therefore be made a general introduction. 



The principal factors in the diagrams are : ( 1 ) The maximum, that is, 

 the predominant portion. Both the extent to which it exceeds the other 

 portions as an indication of the degree of sorting of the sediment, and the 

 size which it represents as indicating the strength of the sorting agent are 

 significant. (2) The sharpness of the " curve" as Mohr calls it, on each 

 side of the maximum, that is, the extent to which the maximum exceeds 

 the portions on both sides of it. (3) The general form of the curve, espe- 

 cially whether it shows more than one maximum. This last feature, how- 

 ever, while theoretically important is evidently very much influenced by 

 the degree and limits of subdivision of the sample. In the diagrams of 

 these Cretaceous sediments the only second maximum is that representing 

 the clay portion, but that this would in most cases probably disappear is 

 indicated by the analyses given by Mohr. 1 He makes many subdivisions 

 of the portion classed as clay in Thoulet's method of analysis, with the 

 result that there is often a steady fall of the curve through these portions. 



'Mohr, E. C. Jul., Ergebnisse mech. analysen, etc. (op. cit). 



