c;iis«(,Ul.| 470 I March 6, 
Here, howx'vcr, wc arc able to identify llic ("rctaccous |»cn(' 
plain wliero it oincrges on tlie wostorn side of the enibayinent 
and so establish an identity in tlie geographic liistory of a large 
part of the southern United States during the Cretaceous aiul 
later cj'cles. 
An account of the development of the Cretaceous ])eneplain in 
Arkansas has been ])ublislied by the writer^, but the essential 
])oints in its history will be repeated here. In the tirst place it is 
advocated by Winslow^ and the writer^ that the folded Palaeozoic 
strata of Avestern central Arkansas represent a trans-Mississip])ian 
extension of the Appalachian folding, the type of folds, the date 
of folding, the columnar section involved, the source of the 
sediments, and the direction of pressure all corresponding in the 
two cases. Secondly, by comparing the writer's account with 
that of Hayes and Campbell it will be recognized that the 
geographic history has been similar on the two sides of the 
embayment. The post,-Carboniferous folding of the Palaeozoic 
sediments in Arkansas was followed l)y a very long period of 
general erosion. The Cretaceous strata in Arkansas lie upon the 
upturned and eroded edges of the Lower Carboniferous grou}>, 
and this gives evidence that great erosion had taken place before 
the beginning of Cretaceous times, an erosion measured by a 
coluiiinar section perhaps four miles thick (the total of Lower 
Carboniferous and Carboniferous in Arkansas) Avith no allowance 
for increase in. thickness of the rock mass removed given by 
folding which left the strata in vertical position. The Cretaceous 
strata were laid down upon a pretty evenly denuded Palaeozoic 
surface, a peneplain, for the Cretaceous boVder line is straight 
and- no peaks of Palaeozoic rocks project through the Cretaceous 
strata south of the border line ; whereas had the Palaeozoic 
surface been rough the border line would be irregular in outline 
and isolated patches of the older rocks common, since the slope 
of the Palaeozoic surface seems to be a gentle one. From the 
altitude, thickness, and character of the Cretaceous sediments we 
'Aim. rept. Ark. t;eol. siirv., v. ;!, j). 220-222, 1890. 
^The geotectonic and i)hysio<;;rai)hi(' geology of western Arkansas. r.iiU. geol. soc. 
Amer., v. 2, p. 23. 
3 0i)?f.w?7.,p. 212-21.3. 
