BOliDER LINES OF PROVINCES. 7 



the fact that before these " mountains were brought forth " the phitform 

 of the country from which they arose had been plicated, and the phcations 

 planed down again by erosion. The Basin area is the oldest of the West,* 

 its final emergence being of older date than the Jurassic, and most probably 

 as ancient as the close of the Carboniferous. 



Between the Plateau and Park Provinces there is no definite boundary. 

 Gradually as we proceed westward from the easternmost ranges of the Rocky 

 system the valleys widen out, and the country gradually expands into a 

 medley of terraces bounded by lofty cliff's, which stretch their tortuous 

 courses across the land in every direction, yet not without system.. The 

 boundary separating the Plateau Province from the Basin is, on the contrary, 

 tolerably definite, and in some portions of its extent remarkably so. It 

 lies along the eastern flank of the Wasatch, south of the Uintas, as far as 

 Nebo ; thence along the Juab Valley, in the Pavant Range, as far as the 

 Tushar Mountains. Here for a time it is concealed by immense floods of 

 old lavas, and is not seen for a distance of 50 miles. It reappears near the 

 southern end of that range, continuing south-southwest along the western 

 base of the Markagunt Plateau, near a string of Mormon settlements scat- 

 tered along the route from Beaver to Saint George, and follows the great 

 fault which makes the Hurricane Ledge to the Arizona boundary. Here 

 an offset carries it to the westward to another fault which walls the Grand 

 Wash, and it then extends southward to the mouth of the Grand Canon 

 of the Colorado and crosses the river. Here is the maximum westinof of 

 the Plateau Province. A few miles south of the crossing it swings back 

 to the southeastward, and continues beyond the explorations of this sur- 

 vey. This boundary is frequently very sharp and distinct, and throughout 

 the greater portion of its extent the breadth of the doubtful or transitional 

 zone lies wholly within the limits of a narrow valley or a narrow moun- 

 tain range. The Pavant is a range of which the eastern side presents 

 conspicuously the features of the Plateau type, while the western side pre- 

 sents those of the Basin type The Tushar Range shows a distinct plateau 

 form in its southern half, while the northern half is masked by floods of vol- 

 canic rock. From Toquerville to Parowan the Markagunt Plateau faces 



* I refer only to largo areas. There may be, ami jirobably are, small areas of equal or greater 

 antiquity. 



