2IO NATURAL SCIENCE. ^l^; 



chain of canons of the Green and Colorado Rivers, he emerged, 

 after fourteen weeks of adventurous travel, at the mouth of the Rio 

 Virgen, below the Grand Canon. When this voyage through an 

 unknown country was projected, the engineers opposed it as an 

 impossibility. On a river falling more than 5,000 feet in a thousand 

 miles it was held that great water-falls must be met with. Powell 

 was convinced to the contrary : there could be no falls on so muddy 

 a river. In other words, he saw that a stream so intensely active 

 in erosion, as this detritus-laden torrent must be, would always keep 

 its channel cut down to a fairly uniform grade. This foresight is 

 worth quoting as an example of the value of geological reasoning in 

 an untravelled region. 



Although there are no falls, there are terrible rapids. The 

 most formidable of these occur where the bed of the rapid river is 

 choked with the huge boulders brought down by its far more rapid 

 tributaries. 



The Grand Canon is the last of the long chain of deep gorges 

 through which these waters flow. Below it, the Colorado emerges 

 into an open country at a lower level, and so pursues its way with 

 more of the usual characteristics of a large river. 



It is difficult in any description to avoid emphasising whatever is 

 superficially striking, but the impression of strangeness produced by 

 great magnitudes or unusual simplicity of structure is one to be 

 combated by those who cherish the principles on which modern 

 geology is founded. If the science is built on the laws of Nature, 

 these laws must be the same in the Old World as in the New. Deserts, 

 high plateaux, monoclines, fault-scarps, canons — these things are not 

 peculiar, but only peculiarly striking manifestations of principles with 

 which geologists have to reckon ; and British geologists especially 

 will not fail to find in such features valuable clues to the problems 

 offered by the history of their own island. 



Alfred Harker. 



