898 J. BAERELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



by the sea. But altliough the rate of advance falls off, the outward 

 growth will continue during the progress of maturity in the cycle of 

 erosion and deposition. In old age, however, on account of the ever 

 slackening supply of waste and the larger proportion carried in suspen- 

 sion and solution, the sea at last gains the mastery and begins to plane 

 inland across the low-lying and unconsolidated materials projecting into 

 the sea. Eapid headway is finally made against the weakened river; the 

 territory conquered by the river in its youth is reclaimed and the sea at 

 last will beat once more against the margin of the old land. Thus far 

 the assumption which has underlain the discussion is that a constant sea- 

 level has prevailed. But in so far as the initial land uplift was far ex- 

 tended there will follow a resultant simultaneous erosion of the land and 

 filling of the sea by river waste from many lands. During the progress 

 of the cycle there will result therefore from erosion of the lands, even 

 with a stationary crust, an appreciable rising of the sea, as pointed out 

 by Suess and Chamberlin. This conclusion from deduction is in accord 

 with the observations in regard to the characteristic transgressive over- 

 lapping of marine formations laid down during a period of crustal quiet. 

 Because of the ratio of land to sea the elevation of the sealevel may 

 amount to about one-third of the lowering of the average land level. 

 The elevation of sealevel without diastrophic cause may therefore readily 

 be an amount greater than the depth of wave base. Consequently such a 

 condition should be added to the discussion of the delta cycle with sta- 

 tionary crust. This is illustrated in the figure, where it is seen that not- 

 withstanding the final destruction of the upper beds of the delta by ma- 

 rine planation a certain amount of subaerial beds may be preserved 

 below the wave base. From the factors which control delta growth and 

 the nature of that growth as a warfare between the rivers and the sea, it 

 is seen that in periods of quiet and low-land relief deltas might nearly 

 disappear from the physiography of the earth. At other times, when tlie 

 relief of the continents, especially through mountain-making move- 

 ments, was greatly increased and erosion quickened without elevation of 

 the negative elements of the continent, the building of deltas may have 

 risen to a dominant mode of sedimentation rivaling in volume the ma- 

 rine deposits. Such physiographic conditions characterized especially the 

 Upper Devonian and later periods of the Paleozoic. Tliey are found 

 in connection with the late Cretaceous movements of western Xorth 

 America. In Eurasia they became pronounced in the Oligocene and early 

 Miocene, when mountain-making prevailed and the great regional up- 

 lifts of the Xeocene had not yet begun. Tlie application of the principle 

 of the delta cycle brings into truer perspective many fresh-water deposits 



