CORRELATIONS 283 



continuing to the present time, forming broad flood-plains. The river is 

 continually filling its channel and changing its course, either gradually, 

 by lateral cutting, or suddenly, by establishing some new course during a 

 flood. The rapidity of the filling is indicated in many places where the 

 river has cut laterally, exposing the roots of trees and shrubs now buried 

 to a greater or less extent. Many places were noted where living arrow 

 weeds are standing in five feet or more of silt. In other words, the sur- 

 face on which the arrow weed had begun to grow had been built up 5 feet 

 or more during the life of that shrub. The flood-plains contain numer- 

 ous sloughs, lagoons, oxbow lakes, and other evidences of change in the 

 rivers course. 



Correlations 



The various epochs described can not at present be assigned definite 

 places in the geologic time scale, but with a knowledge of their order 

 which is apparently clear, and of their relative duration, which detailed 

 investigation might furnish, the establishment of any one in the time 

 scale would give relative place to all. Unfortunately this is not possible 

 at present, although a probable correlation is found with the Gila con- 

 glomerate, which has been referred to early Quaternary.* This conglom- 

 erate is presumably equivalent to the great detrital accumulation of the 

 Lower Colorado river region (epoch 6 of the following table), the corre- 

 lation being based on the similarity of the beds in composition, physio- 

 graphic position, and general field relations, as well as their mutual asso- 

 ciation with flows of basaltic lava. 



If this detritus and the Gila conglomerate are correctly referred to 

 early Quaternary, the order of events described places the rise of the 

 plateau (4) and the origin of Grand canyon at or near the close of the 

 Tertiary period, and the filling of Grand Wash trough (3), which ante- 

 dates the erosion of the Grand canyon, in the Pliocene, as suggested by 

 Mr Spurr.f It also makes the great masses of andesite and rhyolite (2) 

 equivalent in age to the extensive lava flows of Oregon and "Washington, 

 which Professor Le ConteJ has shown were outpoured at the close of the 

 Miocene. 



It is altogether probable that the history as here outlined is imperfect 

 and will be modified by further investigation. It is possible that phe- 

 nomena which are conspicuous in this region may not be as far-reaching 

 as they seem to be, and that events of great importance in the recent 



* G. K. Gilbert : Wheeler Survey, vol. iii. 1875 ; Geology, p. 540. 



P. L. Ransome : U. S. Geological Survey, Professional Paper no. 12, 1903, pp. 47-57. 

 r J. E. Spurr: U. S. Geological Survey Bull. no. 208. p. 132. 

 X Joseph Le Conte : Am. Jour. 8ci., 3d ser.. vol. 7, 1874, pp. 176-178. 

 XXIV — Bull. Geol. Soc. Am., Vol. 17. 1905 



