AUTnOE'S PEEFACB. XXI 



PAvant the Carboniferous is represented as occupying exclusively the west- 

 ern side of the range. It is believed, however, that a few remnants of 

 Triassic beds are to be found in that locality, but I am not able to desig- 

 nate accurately their positions. On the northwestern side of the Tushar 

 also I am informed that there are some Archiean rocks, of which the exact 

 location cannot be specified. A portion of the northwestern flank of the 

 Tushar and the western side of the Pavant I have not visited, and the geo- 

 logical coloring is adopted in those portions as representing merely the 

 dominant rocks. A considerable portion of the country lying south of the 

 Wasatch Plateau is colored from data derived in part from my own observa- 

 tions and in part from those of Mr. Edwin E. Howell. There was some 

 difficulty here in fixing in the field the demai'kation between the Tertiary 

 and Cretaceous, since the two series are not always well distinguished either 

 by lithological characters or by fossils. But if the horizon chosen was 

 properly selected the delineation is believed to be accurate. South and 

 southwest of the Markagunt Plateau a similar difficulty occurred in sepa- 

 rating the Jura from the Trias, and the uncertainty here is somewhat 

 greater. Tlie boundary between those two formations, as delineated upon 

 the map, may, upon more thorough investigation, receive some notable 

 modifications, though I believe it represents very approximately the truth. 

 In the valley of the Paria some slight modifications also may be necessary in 

 locating with precision the same boundary line; and again upon the south- 

 eastern slopes of the Aquarius Plateau, around the net-work of canons 

 tributary to the Escalante, the Trias and the Jura were utterly inaccessible, 

 and the location of the separating horizon was inferred from the colors of 

 the beds and the arrangement of the rocky ledges viewed from a distance. 

 The colors and sculptural forms are most exceptionally characteristic in 

 these two formations, and in this locality there is no possibility of mistak- 

 ing them whenever they can be distinctly seen, whether from great or small 

 distances 



The large area of the map devoted to the trachytes should be under- 

 stood as meaning that in that area the trachytes are the dominant rocks. 

 Commingled with them are the principal bodies of conglomerate and very 

 extensive masses of andesite and dolerite. To define these intercalary 



