ovrnm ) a EVIDENCES OF EROSION. f>9 



thinned a little; but this loss of thickness in the Jurassic and Tertiary 

 has been abundantly discounted in the estimate given of the mass of 

 strata denuded. As regards the general rule that strata vary greatly 

 in thickness, it may be stated that the Plateau country is a remarkable 

 exception to it. One of the most striking features in its stratigraphy is 

 the wonderful persistency with which its formations maintain their vol- 

 umes and lithological features over great areas. In this respect the 

 province has no parallel, not even in the calm and undisturbed terrains 

 of the Mississippi Valley. 



Further support of this conclusion may be found by reverting to the 

 section (Plate XI). On the eastern side of the swell the section shows 

 a great monoclinal flexure where the strata extending eastward rapidly 

 bend downward and subsequently flex back to horizontality. Before this 

 flexure began to form, the Cretaceous strata had already been deposited. 

 Possibly, also, the Tertiary had been laid down, but of this we are not 

 as yet certain. But we know that it was formed after the Cretaceous 

 age, for the strata abundantly betray it. If we could bend back the 

 strata now inclined upward in that flexure, we should have a wall about 

 8,000 feet high or more, looking down from the east upon the central 

 amphitheater, and in that wall would appear the broken edges of the 

 Permian and Mesozoic beds, though the upper part of the Cretaceous 

 and Tertiary would be wanting at the summit. Thus nearly four-fifths 

 of the denuded strata appear upon the eastern side of the swell, in very 

 close proximity to it, and the remainder make their appearance at vary- 

 ing distances beyond. There is no appreciable loss of volume in the 

 exposed beds of the monocline as compared with the corresponding beds 

 to the westward. 



But we may with advantage pursue the task of restoring the beds to 

 the position they held during the period of their deposition by straight- 

 ening out or bending back the strata in those parts where they have 

 been tilted and flexed since their accumulation. This is readily done 

 here. They were deposited originally in layers which were quite hori- 

 zontal. We know this by reasoning upon the following facts. Prom 

 the summit of the Permian, and I think we may say quite confidently 

 from the summit of the Carboniferous upwards, the whole series was 

 deposited in very shallow waters. The evidence of this is overwhelm- 

 ing. We find proof that the surfaces of deposition throughout Mcsoxoie 

 time oscillated repeatedly a little below and a little above sea-level. 

 The cross-bedded sandstones of the Trias and Jura, the sandy shales 

 wonderfully ripple-marked, the occurrence of bands containing the silic- 

 ieifled remains of forest trees, the occasional recurrence of contacts 

 showing "unconformity by erosion" without any unconformity of dip,* 

 the occurrence of brackish-water types of mollusca in the Jurassic, the 



* There are throughout the scries numerous instances of heds resting upon surfaces 

 slightly eroded and channeled by streams without any discrepancy of dip in the 

 apposed heds. 



