﻿J. LeConte — Elevation of the Sierra Nevada. 171 



10,000-12,000. Besides this, the Grand Canon is itself 6000 

 feet deep, making an extreme erosion of 18,000 feet ; and 

 taking the bottom of the Grand Canon at 2000 feet above sea- 

 level, it makes the whole rise during the Tertiary (for this 

 whole region was sea-bottom during the Cretaceous) at least 



Ideal section across the Grand Canon, showing the successive erosion- cliffs to 

 the northward. The dotted lines show the amount of erosion. 



20,000 feet. The outer canon walls b b may be regarded as 

 commencing cliffs, or the successive cliffs as far-receded and 

 therefore very old canon walls : although the forms are cer- 

 tainly unique in this case. The exceptional verticality of the 

 widening canon-walls is the result partly of the very gentle 

 outward dipping of the strata composed of alternate hard and 

 soft material, and partly of the air and climate, and the absence 

 of vegetation to hold the soil and humus to decompose the 

 rocks. 



2. Rivers of the eastern drift region. 



Over the whole area of the drift region in the eastern part of 

 the continent we have, as is well known, the reverse condition 

 of things. The rivers there have sought their base-level by 

 sedimentation instead of erosion, because the land has subsided 

 from a previous more elevated condition. Beneath the present 

 river-beds in all this region we find much deeper and wider 

 beds, now filled with drift or with old river deposits, as 

 shown in the ideal section, fig. 3. In this figure the wide 



Ideal section across a river bed in Drift region, r, Present river bed; r' r', 

 old river bed ; a, country rock : b b, old river sediment. 



river bed r' r' r' is the tertiary and r the present river bed. 

 This relation between the old and present river beds shows 

 that in later Tertiary and early Quaternary times the land was 

 higher than now, and the rivers cut to their base-level ; and 

 that this state continued a long time until the channels had 



