802 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



higher rate than that of limestone formations, but these rates of accumula- 

 tion are due to the relative rapidity of crust movements generally implied 

 by the coarser waste as compared to the finer^ not because the individual 

 beds of sandstone or mud stone are individually more rapidly deposited. 



In river deposits the migration of the channel produces scour into the 

 floodplain deposits, but the natural levees are laid down on them. Thus, 

 in alluvial formations there may be both continuous and discontinuous 

 deposition. Shifts in the regimen of the stream due to variations between 

 the supply of waste and tlie amouiit of water are more important. Cli- 

 matic pulsations thus affect the river grades, and. stages of scouring alter- 

 nate with those of upbuilding. The high plains of the western United 

 States have in this manner suffered alternate dissection and partial 

 rebuilding on a large scale recurrently through tbe Pleistocene. This 

 oscillation in fluvial transportation is apparently an important factor lead- 

 ing to the division of clastic formations into members, even where the 

 final deposition of the sediment is on the floor of the epeiric sea. 



S. H. Knight, in his study of the ^''Origin and Age of the Eed Beds of 

 Southeastern Wyoming,^^ ^^ has shown that certain beds have l)een ce- 

 mented; then eroded, and by a return to aggradation, great residua] 

 boulders were reburied in situ in younger beds of enveloping sediments 

 of the same formation, thus giving rise to a peculiar type of interforma- 

 tional conglomerate. 



Farther from the source of sediments, on the deltas of rivers, or in front 

 of the delta on the floor of the shallow sea, the possilnlity of deposition 

 depends more closely on subsidence. The river l)uilds up ({uiekly after 

 each stage of down-sinking, and then, after grade is reached, carries its 

 sediment farther out. It must wait for further subsidence before it can 

 make more than thin deposits on the inland portion of the delta. 



In limestone formations discontinuity of sedimentation is indicated 

 by recurrent exposure to the air, but also by several other features. The 

 cementation of lime muds is fa^'ored l)y supersaturation of the waters; 

 solution, on the other hand, results fj-om fresheniug and u'ndersaturation. 

 The wearing down of shells and the transportation of the lime mud else- 

 where may retard or even stop the local accumulatio]i. The alternate 

 freshening and saturation of the waters tends to carry away the calcium 

 carbonate and concentrate the less soluble magnesium into beds of 

 dolomitic limestone. 4'lius there may be continuous or discoid innous 

 accumulatiou in liuiestones, Ijut the dominant features of most of tliese 

 formations suggest that sedimentation was markedly discontinuous from 

 l)('d to ])ed. 



'^ Presented orally to the Geological Society of America December 29, 191G. 



