PHYSICAL GEOGExVPHY. xliu 



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How are Canons formoclj and wiiat physical conditions are 



' : ccssary to tlieir formation ? 



'^In answer to the first of these q^nestions. Dr. Kewberry 



xites as follows : 



^' " The qnestion of the origin of the sediments composing 



^ e stratified rocks of the table-lands T>f the Colorado can 



, arcelj^ he intelligently dis.cussed till we know more than we 



^w do of the geology of a large area lying north of the 



^' )lorado, and of the broad and compound belt of monntains, 



• " oich we have covered by a single name (Eocky Mountains), 



it which^ when carefully studied^ will probably not be found 



, form a geological unity. 



■^ -'u rp]-^jg much, howeyer, we can fahly infer from the observa- 



:ns already made on the geological structure of the Far 



est, yiz., that the outlines of the Korth American continent 



re approximately marked out from the earliest Palaeozoic 



les, not simply by areas of shallower water in an almost 



-- indless oceaiij but by groups of islandsj and broad con- 



ental surfaces of dry land. Since the erosion of rocks is 



/ays sub-aerialj or at least never takes place more than 



feet below the ocean surface, it follows, that to form the 



ui * 



Fatified rocks of only that portion of the great central 



.teau which borders the Colorado^ an island 300 miles in 

 mieter, and at least 6,000 feet high — or, what is more 



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li^'obable, a continent of six times that area, and 1,000 feet 

 ^^h — was worn down by the action of waves and rains, and, 

 the form of sediments, sand, gravel, clay, or lime, deposited 

 P^ the sea bottom. 



^'Whcn we reflect that, with the exception of narrow 

 edges of eruptive material in the mountains, an area, having 

 ■1 the 36th parallel, the breadth of the entire distance betAveen 

 ■le great bend of the Colorado and the Mississippi (1,200 



