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BASIN OF THE VIRGEN. 79 



At the border of the Jurassic the profile drops upon the summit of the 

 Trias, but from the summit of the Markagunt nothing is visible in detail 

 of that formation. The faces of the escarpments are turned away from 

 us and only the crestlines are visible. The view from the Markagunt, 

 however, is memorable because it is characteristic. To study the Trias 

 we must leave the verge of that Plateau and descend the terraces to the 

 southward. 



On our way we may note several things of some importance. We 

 may observe, first, that the strata all have a very slight dip to the north. 

 This dip on the average is less than two degrees, but here and there 

 inclinations as great as four or five degrees may be seen. This dip is 

 very general throughout the terraces. Its effect is to make the altitudes 

 of the higher or more northerly platforms less— or, conversely, to make 

 the altitudes of the lower and more southerly terraces greater — than they 

 would be if the entire series were horizontal. In the entire series of beds 

 which are exposed, the aggregate thickness from the top of the Carbon- 

 iferous to the summit of the local Eocene is not far from 10.000 feet, but 

 the summit of the Eocene at present lies only about 5,000 to 0,000 feet 

 above the Carboniferous platform of the Grand Canon District. Thus, if 

 the strata were horizontal, we should in ascending the terraces go up 

 10,000 feet, but the dip to the northward gradually carries down the 

 horizons so that in crossing the edges of 10,000 feet of strata we only 

 gain 5,000 to 0,000 feet in altitude. We find this same northward dip 

 prevailing in the Carboniferous to the southward, and it is a feature of 

 great moment in the studies which are to follow. 



Another point to benoted is that the strata slowly diminish in thick- 

 ness from west to east. The attenuation, however, is ordinarily very 

 slow and gradual, and the observer would have to travel manj miles 

 along the escarpments exposing the edges of the strata before he be- 

 came aware of it. It is most noticeable in the Trias, and in the sequel 

 this will be more fully discussed. The meaning of this attenuation of 

 the strata towards the east is as follows. 



It is a common fact that the greatest thickness of a group of strata 

 is usually found near the shorelines of the mainlands from which their 

 materials came. As we recede from these ancient shorelines we gen- 

 erally find that the strata diminish in thickness, at first quite rapidly, 

 but afterwards more slowly. The materials deposited near the shores 

 are, in many cases, of coarser texture than those deposited at a distance 

 from them. This is not always true of every distinct bed, but if we con- 

 sider any group of strata with many members we shall usually find it 

 true of the group as a whole. In the case of the Mesozoic strata of the 

 terraces, they are remnants of beds deposited in a sea or bay, the shore- 

 line of which lay to the westward and northwestward. The position of 

 this shoreline, no doubt, varied during the Mesozoic periods, now ad- 

 vancing and now receding ; but in general terms its mean position ap- 

 pears to have been nearly along what is now the boundary of the Basin 



