786 J. liAREELL MEASUREMENTS OF GEOLOGIC TIME 



tinent shows that the hirger relief is determined at the present time by 

 regional uplift and not by the dissection by river systems. The valleys 

 are relatively shallow details carved in this relief, not the cause of it. 

 This convergence of the larger drainage systems toward downwarping 

 areas leads to the luiilding of deltas selectively in regions of subsidence. 

 If the land waste is moderate in amount, an epeiric sea rather than a 

 delta occupies the downward regioii and is the reservoir of sediments. 

 If, however, the downwarping is \'ery profound, fore-set slopes and 

 bottom-set deposits are built into an unfilled geosyncline or mediterranean 

 deep. 



On the surfaces of deltas or tlic floors of epeiric seas sedimentation 

 records the rate of subsidence, not the rate noi- amount of d.enudation. 

 During stages of no subsidence tliere is no sedimentary record, except 

 in fore-set or bottom-set beds usually on the margins of the ocean basins. 

 During stages of slow subsidence a few feet of sediment may correspond 

 in the top-set beds to a vast length of time. During stages of rapid sub- 

 sidence in geosynclines -the sediment has generally been supplied still 

 more rapidly. As an illustration familiar to the writer, on the axis of 

 the Appalachian geosyncline in Pcnnsylvajria the floor subsided a maxi- 

 mum of 10,000 feet in the Upper Devonian. This did not result in a 

 deepening of the water; but, on the contrary, the subaerial surface of 

 the Catskill delta was actually built out across the axis of maximum 

 subsidence and by the close of the Devonian had converted this whole 

 section of the geosyncline into a land surface.-"' The ^vork ^vas oojitiuued 

 in this region into the later Paleozoic periods, but there were times of 

 interrupted record of unknown duration, as marked especially by the 

 unconformity or disconfonnity at the base of the Pennsylvania!!. It 

 may be readily granted that this load of sediment, acling in the same 

 direction as tlie forces initiating sujjsidence, would tend to continue it 

 and carry it to greater depths; l)ut without the sediments, deep water 

 would almost surely have resulted, such as exists at present in the south- 

 ern part of the Gulf of California, a, geosyncline filled only at its northern 

 end. 



A relation appears to prevail frequently between the rate and heiglit 

 of uplift of a geanticline and rate and depth ot depression of an asso- 

 ciated geosyncline. Wiien Apjnilaoliia supplied coarse and a])iriulnnt 

 waste it was deposited as thick formations in the parallel geosyju-line. 

 In the Ordovician a low rate of erosion is indicated by the dominance 

 of limestones. The rate of su]')]ily of sediment was slow, but the water 



-s .Joseph Barren: The Upper Devonian deUa of the Appalachian geosyncline. Am. 

 Jour. Sci., vol. xxxvi, 1913, pp. 429-472 ; vol. xxxvii, 1914, pp. 87-109, 225-25?,. 



