258 Davis — Notes on the Colorado Canyon District. 



certainly offer abundant opportunity for the development of 

 new stream courses, even if the drainage of the region had 

 been chiefly antecedent at the beginning. Moreover, some of 

 the valleys actually follow the strike of weak monoclinal struc- 

 tures, and are therefore to be explained as of subsequent rather 

 than as of antecedent or consequent origin ; other valleys slope 

 against the dip of the strata, and may come to be classed as 

 obsequent instead of antecedent. Altogether, this problem is 

 so complicated that its solution seems out of reach at present, 

 The main trunk of the Colorado may be antecedent, but such 

 an Origin seems improbable for the side streams. 



Climatic changes. — The change from a moist Miocene cli- 

 mate to an arid Pliocene climate, already referred to in connec- 

 tion with the Toroweap valley, does not seem to be essential to 

 the development of existing forms. House Rock valley, for 

 example, is explained by Dutton as having been eroded by a 

 member of the original antecedent drainage system, whose 

 waters dried up at the time of the change from the moist Mio- 

 cene to the dry Pliocene period ; but may this valley not be 

 equally well explained by headward erosion under a persist- 

 ently dry climate along the weak Permian monoclinal strata 

 that lie between the resistant Carboniferous strata of the Kaibab 

 arch on the west and the heavy Triassic sandstones of the 

 Paria plateau on the east? The valley seems to belong to the 

 class of subsequent valleys above referred to ; the depth of its 

 floor having been determined by the sill of resistant Car- 

 boniferous strata in the platform of the Marble canyon on the 

 southeast. The valley is not yet significantly deepeued in 

 response to the uplift which has permitted the erosion of the 

 main canyon by the full-bodied Colorado. 



The ravines which dissect the flanks of the Kaibab and the 

 uplands of the Paria plateaus are ascribed by Dutton to the 

 increased rainfall of the Glacial period, by which the aridity 

 of Pliocene time was interrupted (1. c, 196, 202, 228) ; but in 

 view of the active although intermittent erosion and transpor- 

 tation now going on in at least some of the flanking ravines of 

 the Kaibab, it seems inadvisable to limit the origin of ravines 

 of this class to any special period of time. Effective work is 

 done on them to-day when thunderstorm torrents flush their 

 channels, and similar work has probably been done ever since 

 the uplands were stripped of the weak overlying Permian 

 strata and exposed to erosion. 



It may be added that the longitudinal summit valley of the 

 Kaibab, classed by Dutton with House Rock and Toroweap 

 valleys as the work of Miocene streams that became extinct 

 with the coming of a dry Pliocene climate (1. c, 193-195), 



