Theory of Formation of Sedimentary Deposits. 117 



contrast, be considered of secondary importance, while the 

 factors which determine the character of the material as 

 to kind (source) and as to condition (processes of erosion 

 and transportation) may be regarded as factors of a third 

 order. It is easily seen that factors of a fourth or even 

 higher order are in constant operation in the actual forma- 

 tion of sedimentary deposits, and that the complete con- 

 sideration of the history of an area involves the considera- 

 tion of diverse and complex factors ; and although the 

 final result of the operation of these factors is apparent, 

 the share to be assigned to each is often indeterminate. 



Considering now the two primary factors, Rate of 

 Depression of the Land and Rate of Supply of detritus, we 

 find that the former may be uniform, while the latter is 

 either uniform or variable, or the rate of depression may 

 be variable, while the rate of supply of detritus is either 

 uniform or variable. In each of the four possible relations 

 one of three results must obtain, according to the inter- 

 relations existing between the two factors. The ocean 

 over the area of deposition may be gradually deepening, 

 may retain a constant depth, or it may be becoming 

 gradually shallower. In the first case, for convenience in 

 discussion, the rate of depression will be said to be greater 

 than the rate of supply of detritus, in the second to equal it, 

 and in the third to be less than it. The first six of this 

 series of twelve possible relationships which may thus 

 exist between these two factors may be tabulated thus : — 



1. The uniform Rate of Depression may be greater 

 than the Rate of Supply of Detritus which is uniform. 



2. The uniform Rate of Depression may be greater 

 than the Rate of Supply of Detritus which is variable. 



3. The uniform Rate of Depression may be equal to 

 the Rate of Supply of Detritus which is uniform. 



4. The uniform Rate of Depression may be equal to 

 the Rate of Supply of Detritus which is variable. 



5. The uniform Rate of Depression may be less than 

 the Rate of Supply of Detritus which is uniform. 



