822 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



terrace, but are revealed by the erosion of compressed and folded troughs, 

 These lay on the sides of ancient mountain systems facing the continental 

 interiors. These geosynclines were great downwarps which were parallel 

 with and related to great upwarps, the geanticlines. The latter supplied 

 the bulk of the sediments which filled up the troughs. The continental 

 interiors, on the whole, added little or nothing to them, since especially 

 in the Paleozoic they were often the seats of shallow seas which received 

 sediment carried across the geosynclines from the uplands. If waste was 

 sometimes washed back into the geosynclines its ultimate source was 

 nevertheless in greater part the mountainous uplands on the other side. 

 Following the views of Suess, the geosynclines are to be looked on as 

 genetically related to the geanticlines; the mountain ranges depress the 

 forelands or foredeeps and the waste from the mountains is washed into 

 these parallel troughs. 



The best present illustrations are found in association with the great 

 Eurasian mountain systems upraised in the Neocene. The valley of the 

 Po with the northern half of the Adriatic show filling by waste from the 

 Alps and Apennines. The plains of Mesopotamia and the Persian Gulf 

 constitute the catchment basin for the associated mountains. The Indo- 

 Gangetic alluvial plains and deltas lie in front of the Himalayas. On 

 examining the maps., of such regions it will be found that the area of 

 deposition instead of being one-tenth of the area of drainage, as assumed 

 by Sollas, is approximately equal to it. The sediments of the geosyn- 

 cline represent a mountain system eroded and inverted. 



In times of low relief, however, the proportion of waste brought in 

 from a distance might increase. Assume, then, that the waste of the geo- 

 syncline was derived on the average from a land of twice its area. Take 

 the mean rate of denudation as between one foot in 8,000 years and one 

 foot in 16,000 years. Using the other conditions regarding the shape of 

 the geosynclinal prism as postulated by Sollas, the rate of accumulation 

 of the maximum thickness becomes one foot in 1,650 to 3,300 years. 

 Introducing an appropriate ratio for the times of non-deposition, it is 

 seen that Paleozoic time may be anywhere from fifteen to thirty times as 

 long as the estimate given by Sollas. 



This rate of accumulation may seem intolerably slow to those used to 

 thinking of sediments as rapidly deposited, as seen by the testimony of 

 their structures. In reconciliation it should be pointed out that the prin- 

 ciples developed under composite rhythms in Part II go to show that 

 individual beds may be deposited witli groat rapidity as determined by 

 tlie fluctuation of atmosplieric and liydrosplieric forces, and yet the for- 

 mation can accumnlate no faster than sediment is supplied by the mean 



