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PHTSICxiL GEOGRAPHY. xlix 



I '' What the character of this upper mesa is I had no means 

 determining at that time, and eyen now there may be some 

 estion about it ; but I have scarcely a doubt that it is 

 mposed of the Upper Cretaceous strata^ the equivalents of 

 e true chalk of Europe." This has not yet been recognised 

 y any geologist on the American Continent.* 

 From what has been said, we can now answer the question 

 How are canons formed^ and what are the physical condi- 

 ons necessary for tlieii* formation ? 



Canons are usually formed by the action of water only, 

 maided by yolcanic force, which is often erroneously said to 

 DC the primary cause of the so-called fissures and cracks 

 through which the waters floA7. 



The physical conditions are, — 1st, A dry climate, in which 

 even periodical rains do not fall to any considerable amount ; 

 2nd. The passage of never-failing streams from tLeir distant 

 and exterior sources tlirough tliis dry country; 3rd. It is 

 requisite that the surface strata should be of such a nature as 

 easily to yield to the action of the current ; but when once a 

 gi-oove has been furrowed, and the water channel definitely 

 fixed, it does not appear to matter of what the underlying 

 rocks are composed, since the unceasing attrition of ages has, 

 in some instances, succeeded in cutting tlu-ough 1,000 feet of 

 the hardest granite ; 4th. It is requisite that the fall of the 

 land should be sufficiently great to insure a rapid cui-rent. 



It is impossible for a country in which canons abound to 

 te anything but sterile and utterly worthless, for the deep 

 cuttings everywhere drain it to the utmost ; and the waters, 

 buried deep in the bowels of the earth, lie far beyond the 

 reach of animal or vegetable life on the surface. 



In answer to the question, To what causes are due the mesa, 



* Vessels are ballasted with true chalk from London to New York. 



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