GEOLOGICAL IMPORTANCE OF SEDIMENTATION 449 



marked by an abundance of leached argillaceous and ferruginous 

 silts discharged at numerous points along the coast. If from a- dis- 

 tant mountain system, the material will be similarly fine-grained, 

 but will show less decomposition and leaching, and be discharged 

 in great quantity by great rivers at a few widely distant points. Thus 

 there are a variety of types of marine sedimentation, the material 

 becoming more uniform in character and more widely spread over 

 the growing circum-continental shelves as the erosion passes into the 

 stages of maturity. Finally, with old age the amount of mechanical 

 detritus greatly lessens, the conditions for limestone formation 

 approach near to the shores, but greensands and the limy shales are 

 still indicative of the presence of land-waste long after all subaerial 

 detrital deposits have ceased to form. 



Even in old age, however, it is still possible that slight regional 

 uplifts and warpings may result in a temporary renewal of rapid 

 erosion, since under such circumstances the regohth will have formed 

 a deep and voluminous mantle to the continent, readily removed and 

 swept to sea upon the least rejuvenation of the rivers. If the move- 

 ment is accompanied by an adjacent down-warping in a continental 

 interior, the deeply decayed rock mantle may be swept into it and 

 be built up by river aggradation as a continental deposit, which 

 further warping may carry beneath the sea. 



GENERAL APPLICATIONS TO GEOLOGICAL HISTORY 



Having sketched these outlines of the relations of continental and 

 marine sedimentation to the broad earth-movements which have 

 separated and individualized the geological ages, it will be in order 

 to apply them, by way of illustration, to certain typical periods. It 

 will be seen that they suggest, though they do not prove, rather dif- 

 ferent interpretations for certain formations from those which have 

 been ordinarily held. 



Pre-Camhrian aon of continental extension. — It is a matter of 

 familiar knowledge that nearly everywhere the marine deposits of 

 the Cambrian lie upon a far older and unconformable basement. 

 Occasionally the Cambrian appears to have a downward unfossili- 

 ferous extension, as in the southern Appalachians, or sometimes 

 rests unconformably upon older, usually barren sediments, but little 



