RHYTHMS IN SEDIMENTATION 791 



oscillatoiT rise and fall of baselevel must be very general in its applica- 

 tions and figure 4 is draAvn to illustrate its results. 



In explanation of this figure it will be noted that the tilting or rotation 

 indicated about the axis through results in sedimentation on the right, 

 in erosion on the left. The oscillatory rise and fall, for the sake of sim- 

 plicity, are assumed to be of a harmonic nature, as in the rise and fall 

 of a point on a wheel which revolves on a fixed shaft. The successive 

 stages during one cycle are shown at by the levels 1, 2, 3, etcetera. 

 But on the line B B' the progressive rise in baselevel due to tilting is the 

 same during one oscillation as the circumference of the circle of oscilla- 

 tion. The combined motion gives a cycloid, the path of a point on the 

 L'iicumference of a rolling wheel. The successive levels 1, 2, 3, etcetera, 

 at equal time intervals show no scouring, but a marked slowing up of 

 deposition between stages 4 and and a momentary cessation at 6. 

 Farther from the axis, as at C C\ there is continuous rise of baselevel 

 and accompanying depositiou, but at unequal rates. At a great distance 

 from the axis the rate approaches uniformity. 



Bet^veen B B' and the axis the curve is a trochoid. In this section 

 ("here is downscouring and loss of record through a. part of the cycle of 

 oscillation. At A A', half way iK^ween B B' and the axis 0, there is a 

 sedimentary record of only one-half the time. At deposition and 

 erosion alternate and balance. At M M' erosion dominates, but during 

 one-half of the time at this place there is sedimentation. 



To apply this diagram to the geologic record: Most regions of sedi- 

 mentation have been subject to alternate fill and scour, with a balance in 

 favor of the fill, representing the seietion of the curve between and B B'. 

 Where formations thin out, only occasional thin beds are preserved. 

 Many beds are missing and the record is ^.ery impei-fect. On the~ other 

 hand, on the foreset beds of deltas, on lake bottoms, occasionally on the 

 rapidly subsiding axes of geosynclines, and generally on the slopes of 

 the continental platforms, the conditions approach those shown on C C 

 and a complete sedimentary record is developed. The diagram is valuable 

 in visualizing the incomplete natui:e of the sedimentary column in most 

 regions and in emphasizing the large time values of the breaks in deposi- 

 tion, a part of the lost interval being represented by beds which are later 

 removed. Here only one rhythm is shown. In nature the record will 

 ordinarily be more complicated, minor oscillations being superimposed 

 on larger. During the passage of a single rhythm there would be in 

 effect a to and fro migration of the transition line which separates erosion 

 from deposition betM-pen B B' niid a poini (Hjiially distant on the other 

 side of 0. 



LIX — Bull. Geol. Soc. Am., Vol. 28, 1916 



