OHA.PTBR II. 



SEDIMENTARY GROUPS OF THE PLATEAU 



PROVINCE. 



We turn now to a consideration of the Plateau Province. Throughout 

 its extent it is -traversed by profound gorges or canons ; high cliffs are 

 found ; long ridges and lone buttes are seen, all presenting escarpments 

 unclad with vegetation where the geological structure is plainly revealed, 

 and it is nowhere concealed to any important extent by subaerial gravels, 

 river deposits, deep soil, or rich vegetation. The whole region has been flexed 

 and faulted on a vast scale ; the flexures are truncated by erosion, and the 

 faults are crossed by canons and lines of cliffs ; and thus by a combination 

 of circumstances the whole region is an open book to the geologist, revealing 

 a wonderfully complicated structure and a grand succession of formations. 

 Accumulations of sediments may be studied of Cenozoic, Mesozoic, and 

 Paleozoic Ages, each represented by formations that are measured by thou- 

 sands of feet. In the hearts of the mountains and depths of the canons 

 Eozoic rocks are found ; on the mesas and elevated valleys sheets of lava 

 have been spread; and naked volcanic cones crown the geological series. A 

 general section of the sedimentary beds alone sums up a total of nearly 

 60,000 feet, and the relations of the groups into which they can be divided 

 can be determined with a certainty rarely attainable in the eastern portion 

 of the United States. When we group these beds in such a manner as the 

 structural geology demands we have a series of groups or succession of 

 formations separated by epochs of change, producing unconformities or 

 resulting in extensive stratigraphic peculiarities, and in constructing a 'gen- 

 eral section of this country this natural series cannot be ignored without 



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