RHYTHMS IN SET>rMENTATIOX 785 



by current-base. These controls have l)een subject to rhythmic oscillation 

 in level— complex and superimposed oscillations, of longer and longer 

 periods, years, decades, centuries, millenniums, and geologic periods in 

 duration. 



RATE OF SEDIMENTATION DETERMINED BY SUBSIDENCE 



During the i^ast century^ although the prevalence in the sedimentary 

 rocks of the marks of shallow water ^A^as observed, the importance of 

 continental deposition was unappreciated; neither was there recognition 

 of the control of baselevel over both erosion and deposition. It is very 

 easy to transpose cause and effect where the two are essentially simul- 

 taneous in action and complex in their nature; and thus the older 

 generations of geologists looked on the marks of shallow water, persistent 

 through thousands of feet of strata, as the result of a delicate balance 

 controlled by deposition, and not by depression. Ever}^ foot of sediment 

 was supposed to be the cause, leading, as an effect, to a depression of 

 the basin just one foot, with the result that a sea, always shallow, was 

 supposed to have existed continuously throughout the Paleozoic. E^^en 

 if sujch a marvelously delicate relation of subsidence produced by sedi- 

 mentation were possible, they did not try to explain why the balanced 

 condition should be attained at a level just below the surface of the sea. 

 This necessary corollary is in itself sufficient to indicate that land con- 

 ditions were in places confused with marine, and that the cause, a dowii- 

 warping, was mistaken for the effect. 



As another line of evidence, the writer has discussed elsewhere the 

 features of the deltas of the Nile and Niger, built out on the slopes of 

 the icontinental platform into the ocean basins. The extension of the 

 land, as shown by the form of the deltas, into what was previously deep 

 ocean indicates that the crust has there been able to sustain al)urden 

 equivalent to the weight of several thousand feet of rock extending over 

 tens of thousands of square miles.-^ The stability of the land during 

 baseleveling and the competence of the crust to support volcanoes are 

 further evidence of its degree of stiffness and resistance to yielding under 

 load. Beyond a certain limit, however, isostatic yielding is known to 

 take place. 



Although large rivers show a power to maintain their courses across 

 growing mountain ranges, it is nevertheless true tliat the general drainage 

 of a continent is away from the broadly upwarped areas and toward those 

 which undergo downwarping. A relief map of the Xoi'th American con- 



27 .Toseph Barren : The strength of the earth's crust, part 1. .Tournal of Geology, vol. 

 xxii, 1914, pp. 28-48. 



