244 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



sometimes can allow. If, for instance, a continent has an average 

 elevation of 2,000 feet and is being lowered at the rate of 1,000 feet 

 in a million years, then, if the elevation plotted relative to the time 

 may be represented by a hyperbola, the greater the time the slower 

 the lowering and, instead of being absolutely flat in 2 milUon years, 

 it will be reduced to half the height, and to one-tenth of the height 

 in 10 less 1 ; that is, 9 times 2 milUon years. This is the process known 

 as peneplanation. 



We have said that there was deposition to match the degradation. 

 In general, the finer the sediments, the less the rate of deposition. 

 Here the cycUc annual varves may help us to estimate the yearly rate 

 of deposition. In the fine-grained sediments it is usually estimated, 

 according to the authors cited by Bradley, as something Uke 0.1 milli- 

 meter, or 250 years to the inch, 3,000 to the foot. In recent deepest 

 sea soundings a rate of deposition of 20,000 to 25,000 years to the 

 foot has been suggested. The coarser the sediments, in general the 

 more rapid is their accumulation. Comparison of the thickness of 

 difi'erent rocks in different places formed during the same period gives 

 us some idea of the relative rate of accumulation. 



Very elaborate calculations of deposition and denudation have been 

 made, as summarized by C. Schuchert '^ and H. P. Woodward. Such 

 estimates of time are, however, subject to large possible errors, since 

 it is nearly impossible to find exposures or sections of strata where 

 deposition has not been interrupted. If, during the interruption, the 

 lower strata were tilted or converted into land and deeply eroded, 

 such interruptions may be more easily noted, but the gaps (known 

 as diastems), if due simply to underwater currents, may be almost 

 unrecognizable. It is, however, fair to say that at least 300,000 to 

 400,000 feet of rather fine-grained beds containing characteristic 

 fossils are known, which might easily indicate a time of several 

 hundred miUion years. If that proved to be not enough, unrecog- 

 nized diastems could be assumed. On the other hand, it would not 

 be impossible to knock a cipher or two from the age if other methods 

 of estimating time pointed to such action. 



When estimates of the composition of the average sedimentary rock 

 are compared with that of the average igneous rocks from which they 

 are supposed to be derived, it is foimd that there is a shortage of 

 sodium, even making allowance for all the salt beds known or rea- 

 sonably suspected. It is natural to compare the ocean with a great 

 salt lake, and, noting that all such lakes with no outlet become salt, 

 to suppose that that has also been the case with the ocean. Then, 

 if we can compute the amount of sodium added each year by the 

 rivers, and make allowance for the salt of the salt breezes, for the 

 cyclic sodium derived from the erosion of salt beds and beds contain- 



'• Nat. Res. Counc. Bull. 80, pp. 10-64, 1931. 



