g 4 GRAND CANON DISTRICT. 



minimum. The San Rafael district may be regarded as one of these 

 areas of which the central part is an area of maximuin erosion while its 

 peripheral parts are areas of minimum erosion. The Grand Canon dis- 

 trict is another, and there are still others whirl, we Deed not here specify. 



Before concluding the introductory part of this paper it will be desir- 

 able to recite briefly the succession of geological events which the study 

 of the region has thus far brought to light, selecting only such as will 

 hereafter be of special utility. 



Throughout the great Carboniferous age the entire area ot the I lateau 

 Province was submerged beneath the ocean. Deposition of strata went 

 on continuously. The thickness of the strata accumulated m that age 

 appears to have varied greatly, and the deposits were laid down uncon- 

 formable over the surface of a country which had been ravaged by a 

 great erosion. Such exposures of the Carboniferous as now exist, how- 

 ever, exhibit for the most part a remarkable evenness of stratification. 

 In the interior spaces of the province the beds are either horizontal, or 

 if disturbed, give full evidence that the disturbances took place long 

 after their deposition. The close of this age evidently left a subaqueous 

 surface, which was exceedingly fiat, and, except around the borders of 

 the province, quite free, so far as we now know, from any appreciable 

 inequalities. 



The thickness of the Carboniferous system is from 4,500 to 5,000 feet 

 in the interior of the province, but around its borders, and in the Uinta 

 Mountains, it is sometimes found in far greater volume. Its strata 

 consist of impure limestones, occasionally of enormous thickness in 

 the individual beds, and alternating with fine-grained homogeneous 

 sandstones. Extensive beds of gypsum also occur. 



After the Carboniferous came the Permian age, in which were laid 

 down from 800 to 1,500 feet of sandy shales. The stratification was won 

 derfully even and everywhere horizontal. The Permian beds are often 

 ripple-marked and betray many evidences that they accumulated in shal- 

 low waters. Among these evidences are the appearance at several hori- 

 zons of indications that for a time the sea-bottom was laid bare by the 

 recession of the waters, or by the elevation of the platform itself; for 

 we may discern evidences of slight erosion at the contacts of tho beds. 

 But the horizontally of the beds appears never to have been notably 

 disturbed. 



The same state of affairs continued through the Trias. There, too, 

 we find evidence of alternations of emergence and submergence in the 

 shape of slight unconformities by erosion, and in the occurrence of ex- 

 tensive remains of silicified forests. The Triassic series is composed 

 almost wholly of sandstones, the only calcareous matter being thin seams 

 of gypsum. The sandstone beds are very numerous and often shaly. 

 They are usually of no great thickness individually, but there is one 

 very notable member of which we shall see more when we come to view 

 the Vermilion Cliffs. 



