RHYTHMS l^ SEDIMENTATlOiS' 787 



iievertiieless reniaiiiecL .shallow. Such a relation between parallel uplift 

 and depression may be due to two general canses. Tangential pressure 

 aicting throngli a groat do])tli of c-nist, if it bows np an arch on one 

 side will tend to flex down the trough on the other; or the weight of 

 ;i. mountain S3'Stem will tend isostatically to depress the crnst in front 

 of it; for example, the great weight of the Himalaya must tend to de- 

 press the rock floor of the geosyncline of the Indo-CTangetic fioodplain 

 in front of it over which it tends to be thrust. Whatever the cause, 

 whether from liorizontal thrust or from vertical load, there appears to 

 be in times of high relief an accentuation of both uplift and depression. 

 The rate of geanticlinal denudation and the rate of geosynclinal deposi- 

 tion are both at such times accelerated. At times of most pronounced 

 uplift, liowe\Br, the deposition is apt to be continental, the sediments 

 excluding the sea. Elevation thus dominates over local subsidence, the 

 excess of sediment is carried l^eyond, and the geosyncline represents a 

 trap whieh retains only that amount of fdl which is necessary in order 

 that the rivers shall maintain their grade and carrying power across it. 



N'otwithstanding these clear indications pointed out in the preceding 

 •pa rag rap] IS, iiearly all arguments on the measurement of geologic time 

 have rested more or less unconsciously on the assumption that the sedi- 

 ment would be deposited in a limited basin as fast as it was received. 

 It has been conceived to be necessary merely to select the best rate of 

 denudation and the relative areas of erosion fields and catchment basins 

 in order to ascertain the rate of deposition. It is seen that in general 

 the ratio of areas is but one factor of the problem, since usually only a 

 ])art of the sediment has been deposited in the basin. 



There were times, however, especially in the Paleozoic, when arenaceous 

 and argillaceous deposits changed with distance from the shore into 

 limestones. The limestones generally bear the marks of shallows-water 

 origin. The material of the limestones must be looked on as derived 

 from the ocean water and only in part from the same source as the 

 clastic deposits. When the clastic materials change in this transitional 

 manner into organic deposits it appears therefore that, unlike the cases 

 previously discussed, the amount of land waste has been just about suf- 

 ficient to fill the subsidence without a large balance to be carried farther 

 beyond. Here, in the geosyncline, the rate of supply of sediment ap- 

 proaches the rate of deposition and a continuous record is more nearly 

 approached. 



The extreme case in slackness of sedimentation is that even in the 

 geosyncline calcareous material may be the dominant deposit, as hi 

 the great Cambr(>-Ordo^-ician limestones of the eastern United States, 



