308 0. R. Keyes-^Time Values of 



tain its old course. The present uplift, which is due to one 

 general movement, is now apparently divided into two elevated 

 regions separated by a broad valley. 



While in the south there is this prodigious record of the 

 strata, in the north there is no record at all in sediments. The 

 period of erosion is represented by only a thin irregular plane 

 of unconformity from which alone no time value can be deter- 

 mined. In spite of the enormous thickness of the Arkansan, 

 the time element must have been very much less than would 

 ordinarily be inferred from the figures relating to the vertical 

 interval occupied. The period might have been as extended 

 as the Mississippian ; but, all things considered, it would seem 

 much more likely that it was actually considerably more lim- 

 ited than that of the series immediately below. Probably the 

 time ratio would be very nearly the same as for the Des 

 Moines. 



Although the Des Moines series is so thin over such a large 

 part of the area occupied by it, it is actually very much more 

 important as a terrane than its relative thickness would indi- 

 cate. North of the Boston mountains the formation is per- 

 haps nowhere over 500 feet in thickness. The beds composing 

 the series were laid dowu along a shore which was on the 

 whole gradually sinking.* Many oscillations permitted sub- 

 aerial erosion to take place time and again, so that the newly 

 formed sediments were frequently removed or worked over 

 almost as fast as they were formed. It was a period when 

 erosion contended against deposition for supremacy, with a 

 result of one making advances at one time and the other at 

 another. The net result was finally a slight gain for sedimen- 

 tation, f The period was manifestly a long one, for it is 

 believed that in Arkansas no less than 3,500 feet of strata are 

 referable to it. Its time ratio is certainly not very much less 

 than that of the Mississippian. 



The Missourian series is essentially a marine formation inter- 

 calated between two terranes which were laid down in shallow 

 waters. Its limestones are nowhere the coarse-grained breccias 

 such as occur so frequently in the Mississippian. They are 

 fine-grained and often earthy, and are separated from each 

 other by important beds of shale, which are moreover usually 

 much thicker, reaching measurements of often several hun- 

 dreds of feet4 It is not improbable that part of these beds 

 were laid down while some of the nearer shore sediments of 

 the Des Moines series were being carried into still waters. § 



* Iowa Geol. Sur., vol. ii, 1894, p. 113. 

 •f This Journal (3), vol. xh, 1891. p. 273. 

 % American Geologist, vol. xxiii, 1899, p. 298. 

 £ Iowa Geol. Sur., vol. ii, 1891, p. 162. 



