77U J. BARREL!. MEASUREM.ENTS 01'^ GEOLOGIC.^ TIME 



iiiternal evidence of the sediments themselves and the entombed faunas 

 ajid floras. Schuchert assigns to tlie lowest Trenton sea a flooding of 



57.2 percent of the present land area of North America, the widest inun- 

 d^ation of geologic time. To the Cretaceous sea he gives a maximum 

 spread of 41.3 per cent.^-' Schnchert's maps are of great value because 

 each represents a limited period of time and the shores are drawn as close 

 to tlie limiting outcrops as is permissible. As he recognizes, this tends to 

 naiike the areas of the seas too small, but increase of knowledge will show 

 where and to what extent the Ixmndaries must be extended. These facts 

 must not l)e forgotten in using the maps. Considering the inevitable cut- 

 ting back of outlying outcrops in cycle after CTcle of erosion, it apjDears 

 to the writer that the lowest Trento]i sea probably covered as much as 65 

 or. 70 per cent of North America and the Cretaceous seas and deltas at 

 1 heir" maximum piobabl_y 50 per cent. 



Anothe]' significant feature in interpreting tJie mean attitude of the 

 continents in the past is found in the fact that the mantle of; sedimentary 

 rock, made uji by formatimis of difl^erent periods, is wider than the de- 

 ]K)sits of any one sea, this statement applying to other continents as well 

 as to North America. To what degree is this composite mantle, accumu- 

 lated -through earlier ]>eriods, now subject to destruction by a reversal of 

 the balance from de]X)sition to erosion ? To ans:\A"er this we may compare 

 the areas of ancient marine sediments relative to the areasriow under- 

 going marked denudation. 



Tlie area of the land is (U), 000,000 square miles. Ol' this it is estimated' 



20.3 ])er cent,-" or 12,000,000 square miles, (!\i)ose Precambrian rocks; 

 18,000, 0(t<i square miles a-re of sedimentary rocks or of eru]>tives which 

 mostly oN'crlie sedinu'nts. As to elevation, 18,000,000: square miles, or 30 

 \)(^r cent of the land, lies less than. 600 feet above: the sea anclanay be taken 

 as the loyvland arcM. It is at the present: time subject in pari of it« area 

 to erosio:n:; iii.])art it is.receiving delta: and Imsin deposits. The remainder 

 of 42,000,000 square miles of land is above: 600 feet in elevation. and is 

 sidjject to more 'or: less vigorous (ji'osioii. If, leaning backward in the- 

 argumejit, all theiPrecambriaTi; should l^e assumad as within this upland 

 area, there would still remain 30,000.000 square miles, or one-half of the. 

 contijiental surface, which consists of ancienf sedimentary fomuttiohs. 

 mostlv marine, which in tlie later Tertiary ajid Pleistocene Imve- beeu, 

 u])lifted \ncvv: irlian (ioO fed, in part to several th(3iisand feet, aiiH shbj:ected^ 

 1o rapid: and -ctee}' denudation, ft the present erosion, shall persist :rntd^ 



N^M". Sc'lmclieKt:, J.'a](Oii('ogT!ii)li.v oj: Narth. America. BiiU. (JpoI. Soc. Am., vol. 20. 



]?iio; 1). ow. ' ■ / ' .' . ■ 



r:-!^Von TUh.rVromiites Keivdlli. I'iiris. \M»1. 114. 1S92..pp./24n. 007 



