16 



AQUEOUS AGENCIES. 



larged and deepened by water and by ice. But there are some, at least, 

 which may be proved to have been produced wholly by erosion, and 

 that during the present or at least during very recent geological times. 

 We refer especially to those which have been cut through lava-streams. 

 In Middle and Northern California are found lava-streams which 

 have flowed from the crest of the Sierra. By means of the strata on 

 which they lie, these streams are known to have flowed after the end of 

 the Tertiary period. Yet the present rivers have since that time cut 



Fig. 8. — Lava-Stream cut through by Rivers: a a, Basalt; b b. Volcanic Ashes; c c, Tertiary; d d, 

 Cretaceous Rocks. (From Whitney.) 



great canons through the lava and into the underlying rock, in some 

 cases at least 2,000 feet deep. Such facts impress us with the immen- 

 sity of geological times. This important point is discussed more fully 

 in a subsequent part of this work. 



Fig. 9.— Buttes of the Cross (Powell). 



But nowhere in this country, or in the world, are the phenomena of 

 canons exhibited on so grand a scale, and nowhere are they so obviously 

 the result of pure erosion, as in the region of the Grand Plateau of 

 Utah, Arizona, New Mexico, and Colorado. This plateau is elevated 

 7,000 to 8,000 feet above the sea, and composed entirely of nearly hori- 

 zontal strata, comprising nearly the whole geological series from the 

 Tertiary downward. Through this series all the streams have cut their 



