84 EXPLORING EXPEDITION FROM SANTA Pfi 



incognita, and was viewed with eager interest, both as the scene of our future explora- 

 tions ;ind as the possible repository of truth which we might gather and add to the sum 

 total of human knowledge. 



An outline sketch of this portion of the ( lolorado Plateau has been already given in 



Chap. Ill, and but little more need he said to make Its general feat nres clearly compre- 

 hended. The plain which stretches westward from the Sierra de la Plata — indicated on 

 the accompanying map, where it is called the Great Sage-plain— is part of an immense 

 plateau which once stretched continuously tar beyond the course of the( lolorado. I low 

 it has been divided by the canons of the draining streams, and how. by erosion, its 

 plateau character has been so modified as to he locally lost, will fully appear in the 



progress of our geological narrative: yet no one who observes the orderly and un- 

 broken arrangement of its underlying rocks, the perfect correspondence of the sections 

 on opposite sides of the profound canons which cut it, will hesitate to assent to the 



assertion that an unbroken table-land once stretched from the base of the Sierra de la 

 Plata all the way across to the mountain chains west of the Colorado, and that from 

 this plateau, grain bv grain, the sedimentary materials which once filled the broad and 

 deep valleys of the Colorado and San Juan have been removed by the currents of 



these streams. The mind is awestruck in the contemplation of the magnitude of the 

 element of time which enters into the analysis of the process by which these stupendous 



monuments of erosion have been produced; but if the numerical faculty is baffled in 

 the effort to count the years or ages which must have been consumed in the erosion of 

 the valleys and canons to which 1 have referred, even the imagination itself is lost when 



called upon to estimate the cvcles on cvcles during which the much grander features 



of the high table-lands were wrought from a plateau which once overspread most of 



the area of the Colorado Basin, burying the present Sage-plain 2, ('00 feel beneath its 



upper surface. 



As I have before said, the Grreat Sage-plain is everywhere floored by the Lower 



Cretaceous sandstones; massive resistent strata, 300 to 500 feet in thickness. These 



are succeeded in the ascending series, where the upper members of the formation are 



present, by the Middle ( Iretaceous shales, principally soft argillaceous beds, from 1 ,200 



to I, f>0() feet in thickness. Above these belong the Upper ( Yetaceous sandstones and 



marls, occupying an equal vertical space. Now, over the greal plateau, floored and 

 protected by the Lower Cretaceous sandstones, are everywhere scattered mounds of 

 greater or less elevation, composed of the .Middle Cretaceous shales, and in the inter- 

 val between these mounds are scattered thousands and millions of the fossils charac- 

 teristic of these shale-beds, washed, like pebbles, from their soft envelope. Here we 



have conclusive evidence that the Middle ('retaceons shales ouce completely covered 

 the sandstone-floor of the Sasre-nlain from which thev have been nearly removed by 

 aqueous action. As we approach the margin of the sandstone plateau, we find the 

 mounds of overlying shale becoming more numerous and higher, until, reaching the 



edge of the high mesa, they blend in solid escarpments more than 1,000 feet- in 

 altitude, crowned by the massive?, but soft, sandstones which form the third and upper 

 division of the Cretaceous series. 



Such arc some of the facts which may be observed about the base of the Mesa 



