CONDITIONS OF SEDIMENTARY DEPOSITIONS. 479 



the land determine the deposition of calcareous formations they 

 will do so most efficiently during this topographic phase, and 

 in the absence of mechanical sediments the corresponding de- 

 posits will be limestones or dolomites. As the topographic phase 

 passes to its close and the sloping plains sink to base-level, the 

 power of streams to transport mechanical sediment fails, and 

 rivers finally carry only silt in lessening proportion ; hence the 

 upper portions of a great limestone deposit may be less clayey 

 than the lower. Furthermore, the mantle of residual clays, 

 accumulating upon the extended base-level, will check solution, 

 and thus, in so far as the deposition of limestone is influenced 

 by contributions from the land, will limit the growth of the for- 

 mation ; and with the cessation of both mechanical and chemical 

 supply, terrigenous deposits will cease to form beneath the sea. 

 Then, while these conditions endure geologic ages may pass with- 

 out record in sediments unless there is a marine source of supply. 

 Thus far this statement has tacitly assumed a constant relation 

 of elevation between coast and ocean. Assume that the long 

 quiet, which has been necessary for the reduction of a mountain 

 range to base-level and the deposition of the corresponding sedi- 

 ments, is interrupted by sinking or heaving of the land area. The 

 surface is low, flat and covered by a mantle of residual sand and 

 clay intimately mingled. Moderate subsidence must lead to ex- 

 tensive transgression and the invading sea, margined by tide flats, 

 will spread arenaceous, clayey deposits, bearing the marks of 

 shallow water formations and resting unconformably upon the 

 ancient rocks. If the residual soil be red, the sediments will be 

 of similar color, since the process of deposition on tide flats does 

 not involve much attrition and the ferruginous coating of the 

 grains will remain. 1 The base of the deposit may be a zone of 

 transition, composed of cores of undecomposed rocks, imbedded 

 in more or less re-arranged products of partial decomposition. 2 



'Bull. U. S. G. S. No. 52. I. C. Russell, Subaerial Decay of Rocks and Origin 

 of Red Color of Certain Formations. 



2 R. Pumpelly. The Relation of Secular Rock Disintegration to Certain Transitional 

 Schists. Bull. Geol. Soc. of America. Vol. II., p. 209. 



