LOCALITIES WHEEE THE GROUPS CAN BE STUDIED. 55 



forming the summit of the walls of these gorges. They are also well exposed 

 along Marble Canon ; and Cataract Canon at the junction of the Grand and 

 Green furnishes another good section. Good sections are obtained at Horse- 

 shoe Canon, the Canon of Lodore, Whirlpool Canon, and Split Mountain 

 Canon in the Uinta Mountains. Its junction with the Shinarump Group 

 above, in all these places can be plainly seen, and in like manner its junction 

 with the Lower Aubrey Group is apparent. To the southward in the Grand 

 Canon country these beds are a series of cherty limestones. At the junction 

 of the Grand and Green they are a series of sandstones with intercolated 

 cherty limestones, with a homogeneous sandstone at the summit 150 feet 

 in thickness. In the Uinta Mountains we have a homogeneous gray Sand- 

 stone which we call the Yampa Sandstone, from 1,000 to 1,200 feet in thick- 

 ness, capped by a bed which is believed to be the equivalent of the one 

 mentioned as found at the summit of the series at the junction of the Grand 

 and Green, and varies from 150 to 200 feet in thickness. On the south side 

 of the Uinta Mountains it is an indurated, calciferous sandstone, but on the 

 north side of the mountains it is a cherty limestone, and on both flanks of 

 these mountains it is characterized by a species of bellerophon. Here we 

 have called it the Bellerophon Limestone. 



LOWER AUBREY GROUP. 



The Lower Aubrey Group is seen underlying the Upper Aubrey at 

 all the localities mentioned for that group. In the Grand Cation it is a 

 conspicuous group, its relations to the Upper Aubrey and the Red Wall 

 Groups being well marked. At the junction of the Grand and Green the 

 lines of demarkation cannot be so closely drawn but they appear again 

 very clearly in the Uinta Mountains. 



RED WALL GROUP. 



The Red Wall Group is the most conspicuous feature of the Grand 

 Canon of the Colorado and its tributary gorges. It often stands in a vertical 

 wall 2,000 feet high or more, and is everywhere carved into a series of 

 grand amphitheaters, which I have elsewhere tried to describe. There are 

 two well defined members in the Grand Canon country ; the upper one 



