PHYSICAL GEOGEAPHY. xlv 



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" causes which shall he explained afterwards, cut their way 



^^rough them in channels, which deepen continually as they 



Tance, and also present fewer and fewer open valleys as 



^ ey progress, to break the narrow and sunless perpcndicu- 



* "^"rity of tlieir gigantic walls^ until, in the case of the Colorado, 



^is penctratiye tendency culminates in a gorge or caiion, 



P^mi 3,000 to 6,000 feet in depth, \7hich, for at least 500 



tf)iai 



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lies, is totally inaccessible from above. 





■'*• The plateau of the Colorado — that is, the district through 

 hich this river runs for 5"^ of latitude — has been raised to 

 1 average elevation of 7,000 feet. It extends in a IS'.N.W. 

 rection, from a point south-east of the San Francisco 



w. 



their fountains, across the Colorado into Utah, and includes a 

 ^'^re portion of the country traversed by Grand and Green rivers, 

 tlio ii'fore they unite to form the Eio Colorado, as well as a more 

 ) ifloro 'Qsidorablc ^^^.^ ^f ^i^^^ crossed by the Colorado Chiquito 

 ,jii3 W'-d the Eio San Juan in the latter part of their coui'se. 

 jjjisniff'i Q^,^j. ^i^-g plateau," says Dr. IN'ewberry, "the Eio Colo- 

 -' striiVlo formerly iiowed for at least 500 miles of its course ; but 

 F rise. ' the lapse of ages its rapid current has cut its bed down 

 ' 1 32'' rough all the sedimentary strata, and several hundred feet 

 to the granite base on which they rest. 



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For 300 miles the cut edges of the table-lands rise 

 forni^jruptly, often perpendicularly, from the water's edge, 

 l^li'Hrming walls from 3,000 feet to over a mile in height. This 

 M^ the Great Canon of the Colorado — the most magnificent 

 j^i^.orgo, as well as the grandest geological section, of which we 

 ^(oJ-'ave any knowledge." 



"That portion of the table-lands lying between the mouth 



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f the Vii'gen and the Little Colorado is composed of over 

 j^u'-,000 feet of sedimentary rocks, representing the Silurian, 

 "Devonian, and Carboniferous epochs." 



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