FKKCH.i SECTION IN CONGRESS CANYON. 481 



were subsequently removed by erosion; (3) or, no sedimentation what- 

 ever took place in the Silurian sea. In either ease the changes observed 

 were effected without any dynamic movement in the earth's crust. On 

 the other hand, the unconformity between the Devonian and the Car- 

 boniferous is evidently due to some change in the sea level. 



7. With the earlier Carboniferous begins a period of regular marine 

 sedimentation, which went on uninterruptedly until the close of Per- 

 mian time. Between Permian and Trias (in the Triassie, Shinarnmp, 

 conglomerate), and again in Trias and Jura "we lind instances of these 

 peculiar unconformities by erosion without any unconformity of dip 

 in the beds." 94 



In early Tertiary time a period of disturbance (folding and faulting) 

 again set in in the Grand Canyon region, where no such changes had 

 taken place since the Algonkian epoch. A typical instance of a 

 flexure in the massive Red Wall limestone was observed in descending 

 through the upper part of Congress canyon. The same phenomenon has 

 been observed by Walcott, who pointed out the interesting fact that 

 the pre-Cambrian and Tertiary movements took place on the same line 

 of displacements. The upturning of the strata on the western side of 

 the fault was effected during pre-Cambrian time; that on the eastern 

 side during Tertiary time. 97 



It was at this time, probably, that the period of volcanic activity 

 commenced, during which the San Francisco mountains were formed. 

 After the formation of these andesitic mountains, basaltic eruptions 

 took place, in which Dutton distinguishes an earlier and a later period. 

 During the earlier, the plateau surface, now formed by the Aubrey 

 limestone, was covered with Permian shales and clays. The basaltic 

 flows of this time protected these Permian beds from snbaerial erosion. 

 Bed Butte, which was passed during the second day's journey to the 

 canyon, and consists of Permian capped by basalt, is a characteristic 

 instance of such protection. 



The cinder cones, lava flows, and ash beds near Flagstaff, passed 

 during the first day's journey, belong to the very latest eruption and 

 may be of very recent origin. 

 451 ge 31 



