798 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



The growth of glaciers abstracts water from the sea and locally depresses 

 the land, bnt a more general effect through geologic time is dependent 

 on changing river grades. In times of lessened water and increased bur- 

 den, the}^ steepen; mider opposite conditions, they flatten. Where the 

 rivers flow 100 miles or more from mountains to the regions of deposi- 

 tion a change of baselevel amounting even to some hundreds of feet may 

 result. This affects especially the continental deposits, but unbalances 

 also the ratio of supjoly of waste to the carrying and eroding power of 

 the sea.-^*^ Such an oscillation in river grade appears to be an essential 

 factor in the interpretation of ; the upper Paleozoic formations of the Ap- 

 palachian geosyncline and must apply to certain other regions also. 



The curve, to correspond to nature, should be imagined as less regular 

 and with more orders of ]-hythms. For regions wliere the sediment carried 

 past to other localities is large couipared to that which comes to rest at 

 that locality, the record becomes very discontinuous. As a result of in- 

 troducing further rhythms, in those places the black bars in the upper 

 part of the figure which show tlie time intervals recorded by sedimenta- 

 tion should be in reality broken into gridirons by smaller cycles, cutting 

 out by diastems more and more of the record until the whole comes some- 

 A^diat to reseml)le the Fraunhofer lines in the solar spectrum. 



A fundamental conception connected with composite rhytlnns is that 

 a long time interval may be represented l)y a short columuar section, and 

 yet the individual beds may be deposited rapidly. In coal measures, tree 

 trunks may be preserved in erect attitude, showing burial before decay. 

 In marine deposits, sponges and coral colonies are in certain beds found 

 smothered in mud. Ammonite shells were partially or wholly buried 

 before they could be destroyed. Such burials may have heen due to single 

 unusual storms, recurrently scouring sediment from one locality to deposit 

 it in another. At the most, but a few^ years could have beeu occupied in 

 the burial. The^^e features show rates of accumulation so rapid that even 

 the advocates of very short geological time would have to admit for such 

 instances recurrent deposition ,with lost intervals between. But having 

 once admitted the principle, the basis is destroyed for estimating the time 

 of accumulation of the whole formation by means of an assumed rate of 

 continuous denudation and corres]:»onding sedimentation. 



ILLUSTRATIONS OF RHYTHMS IN SEDIMENTATION 



In advocating a view that sedimentation on the continental platforms 

 in either marine or terrestrial deposits is more often discontinuous thau 



"0 Joseph Bai'i-eU : Relations between cUmate and terrestrial deposits. Journal of Geol- 

 ogy, vol. xvi, 1908, pp. 159-190, 2.55-295, 303-384. The Upper Devonian delta of the Ap- 

 palachian geosyncline. American Journal of Science, vol. xxxvii, 1914, pp. 239-243. 



