DAVIS: THE GRAND CANYON OF THE COLORADO. 1t3 
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 stratigraphy of 
the plateau series are presented in Powell’s “Geology of the Uinta 
Mountains ” (6, pp. 43, 54, 62, 70). 
Dutton 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 Waterpocket 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, ¢, 
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 (¢, pp. 105, 
107). The high plateaus of Utah are residual table-lands capped by 
‘Tertiary strata and lavas, remnants of the great denudation that 
