790 



J. BAKRELL xMEASTTREMENTS OF GEOLOGIC TIME 



and wave action to fluctuate in level; the extreme conditions of excep- 

 tional years are especially potent in upbuilding or in down-scour. Slow 

 changes in sealevel, hundreds or, more usually, thousands of years in 

 length, form pulsations of a higher order which affect marine and delta 

 deposits. Climatic fluctu^ations of short and long periods cause varia- 

 tions in the supply of sediment and the carrying power of rivers. These 

 oscillatory rhythms are combined witli the discontinuous movements of 



Figure 4. — Diagram postulating a progressive Tilting of the Crust, comhiiied with a 

 rhythmic Elevation and Depression of Baselevel 



This diagram illustrates a variable rate of sedimentation dependent on' baselevel and 

 distance from axis of tilting. R = radius of oscillation. 0, 1, 2, 3, etcetera, represent 

 the position of baselevel at equal time intervals. 



tilting which are not oscillatory, but irregular and progressive in one 

 direction. In studying the stratigraphic consequences of the combination 

 of the two, the tilting may be regarded as progressive — that is, as con- 

 tinuing longer than the other rhythms — and the latter as producing a 

 parallel rising and falling of the baselevel. In such rise and fall tbe 

 motion at the turning points is for most kinds of motions very slow, as 

 in the tidal ebb and flo\A', or may be even arrested until sufficient force 

 has become accumidated. This combination of progressive tilting witli 



