20 INTRODUCTORY. 



There are five of these concentric hnes of cliifs. In the centre there is an 

 elHptical area about 40 miles long and 12 to 20 broad, its major axis lying 

 north and south, and as completely girt about by rocky walls as the valley 

 of Rasselas. It has received the name of the San Rafael Swell. Its floor 

 is covered with the lowest Triassic strata, and probably in some portions 

 of it the Carboniferous is laid bare, though it has not yet been seen. But, 

 at all events, we' know that the Carboniferous is very thinly covered, even 

 if it be not exposed. 



Thus, as we pass from the summit of the Wasatch Plateau to the floor 

 of the Red Amphitheatre, we cross the outcrops of nearly 10,000 feet of 

 strata. The Tertiary is found only at a distance of 40 miles from it. Yet 

 if we look back to Eocene time we shall find that the whole sti-atigraphic 

 series from the base of the Mesozoic to the summit of the Eocene covered 

 this amphitheatre. One after another, in orderly succession, the vast 

 stratigraphic members have been stripped off", and the edges of the remain- 

 ing portions are seen in the successive cliffs which bound the encircling 

 terraces. 



Still more vast has been the erosion which took place in the vicinity 

 of the Grand Canon of the Colorado. Here the Carboniferous now forms 

 the floor of the country, though a few patches of Trias stiU remain in the 

 vicinity of the river. But the main body of Triassic rocks now stands 50 

 miles north of the river, and beyond them, in a series of terraces, rise the 

 Jura, the Cretaceous, and the Tertiary, the latter usually capped by great 

 masses of volcanic rock. 



We may note here another question which presents itself in connection 

 with the differential movements among the various parts of the province. 

 Those areas which have been uplifted most have suffered the greatest 

 amount of denudation. Is it not possible in some cases and under certain 

 restrictions to invert this statement and say that those regions which have 

 been most denuded have been most uplifted, thereby assuming the removal 

 of the strata as a cause and the uplifting as the effect? May not the 

 removal of such a mighty load as 6,100 to 10,000 feet of strata from an area 

 of 10,000 square miles have disturbed the earth's equilibrium of figure, and 



