Notices respecting New Books. 387 



strange gorges. Thus the Colorado itself emerges from the plateau 

 region in a canon 4000 feet deep. 



The chief economic product of the area is coal ; and though the 

 amount of glacial drift is inconsiderable, at least a tenth part of the 

 sedimentary rocks of the region is buried beneath vast fields of lava. 



As before remarked, the disturbance in the Colorado district can 

 scarcely be considered violent. The faults and folds are numerous, 

 but simple in character; and the " monoclinal fold," or one single 

 bend in the strata, sometimes accompanied by faulting at one 

 extremity of the long ridge or slope formed by the disturbance, is 

 not merely "a feature of occasional occurrence," but is " a charac- 

 teristic type of displacement, and is rivalled in frequency only by 

 the fault." These monoclinal folds divide the area into a series of 

 " blocks," and sometimes culminate in lines of displacement. A 

 very clear sketch showing the occurrence of these, as well as of 

 the canons which seam the plains between them, from Major 

 Powell's report on the Exploration of the Colorado, is given to 

 illustrate the region. 



But it is in the concluding section of the memoir that its value 

 chiefly lies : in fact the introductory matter is chiefly designed 

 to lead up to the theory of how the canons of the region were 

 formed ; and this is argued out with great completeness. 



Their cause is entirely erosion, the natural processes of which 

 may be called disintegration and transportation — the former 

 effected by " weathering" and by the action of running water (for 

 which the term " corrasion " is used), the latter by the action of 

 the streams which carry away the disintegrated material to deposit 

 it at a lower level. 



The chief agents of weathering are solution by water, which 

 dissolves the cementing material, splits the rock-mass when frozen 

 in fissures, occasioned by changes of temperature, or, as rain, beats 

 against and destroys the feeble coherence of earths. Plants, too, 

 lend their aid by increasing the power of the percolating water. 

 In corrasion, solution and mechanical wear are the active agents ; 

 " the effective force is that of the current ; the tools are mud, 

 sand, and boulders ;" in the latter case, clear pure water "corrades 

 their beds " by its solvent powers only. 



Both these causes have tended to the formation of the canons ; 

 but corrasion has outstripped weathering in its marked effects. 



But the rapidity of erosion depends again on declivity, character 

 of rock, and climate, though the former does not affect the dis- 

 solving power of water as it does its corrading power; so that 

 it may be fairly assumed that " a deep declivity favours trans- 

 portation, and thereby favours corrasion ;" and " the rapid, but 

 partial, transportation of weathered rock accelerates weathering," 

 by admitting air, frost, and damp, though still the complete 

 removal of the products retards weathering. 



It stands to reason, too, that " erosion is most rapid when the 

 eroded rock offers least resistance ;" and here the nature of the 

 material is important. The solubility of the rock is increased 



2C2 



