DAVIS: THE GRAND CANYON OF THE COLORADO. 113 



recession of cliffs is greater in the plateau blocks of greater uplift than 

 in those of less uplift (a, p. 191). Volcanic action began with the 

 period of displacement and continued till after the Grand canyon had 

 been eroded {a, pp. 201, 94, 131) ; a close relation is inferred between the 

 lines of fracture and the location of the volcanic vents (a, pp. 6, 94, 196). 

 An arid climate is thought to have long prevailed, for otherwise the 

 cliffs and canyons could not have maintained their sharp forms (a, pp. 

 204, 209, 211). Some additional details concerning the sti'atigraphy of 

 the plateau series are presented in Powell's " Geology of the Uinta 

 Mountains " (b, pp. 43, 54, 62, 70). 



Button, first presented his conclusions in a report on the "Geology of 

 the High Plateaus of Utah " (1880) ; they were afterwards elaborated in 

 his " Tertiary History of the Grand Canyon District " (1882). Reference 

 is here made chiefly to those points which carry the study Of the region 

 beyond the stage reached by Dutton's predecessors. Several periods of 

 uplift and displacement are given geological dates, but in the absence of 

 the later Tertiary deposits, such terms as Miocene and Pliocene are 

 used only in a general way (c, p. 192). One of the oldest displacements 

 is the "VVaterpocket flexure, involving all strata up to and including the 

 Cretaceous ; it is crossed by the Colorado in Glen canyon, and is 

 unconformably buried in the northwest by the horizontal Eocene strata 

 of Thousand lake mountain, one of the high plateaus (a, pp. 44, 288, c, 

 215). The whole region began to rise in the early Tertiary (a, pp. 14, 

 c, 219), and several broad gentle swells were at the same time locally 

 elevated above their surroundings ; one of these was the San Rafael 

 swell (Miocene) north of the Henry mountains, and another of much 

 greater size was the Grand canyon district (late Eocene, a, p. 19). 

 Faults and flexures in the Grand canyon district were of later date. 

 The whole uplifted region was greatly eroded in Miocene time, and more 

 slowly in Pliocene time (a, pp. 18-21). The first chapter of erosion, 

 frequently named " the great denudation " and placed in Miocene time, 

 witnessed the removal of strata having an average thickness of six 

 thousand feet from a broad area in the Grand canyon district, reducing 

 the surface to moderate relief, "a very flat expanse " (c, p. 77, also pp. 

 119, 224). On such a surface, the volcanic eruptions which had begun 

 at an earlier period poured forth great flows of lava; eruptions con- 

 tinued still later, the vents being generally independent of fault lines, 

 even though new vents were made after the time of faulting (c, pp. 105, 

 107). The high plateaus of Utah are residual table-lands capped by 

 Tertiary strata and lavas, remnants of the great denudation that 



