388 Notices respecting New Books. 



when the cement is soluble and the texture of the other ingredients 

 open ; " disintegration by frost is most rapid in rocks which 

 absorb a large percentage of water," detrition by mechanical wear 

 in soft rocks ; and transportation is easiest when the results of cor- 

 rasion are finely comminuted. 



As a rule, climatal effects depend rather on rainfall or dampness, 

 than on changes of temperature, as the latter must be extreme to 

 produce much useful Assuring, though still cracks in a country 

 where frost has an opportunity of acting would facilitate the 

 destruction of the mass. One of the most valuable results of 

 rainfall is the increase the vegetable products afford to the solvent 

 power of water ; but nevertheless, on the other hand, vegetation 

 acts as a protective agent to the surface, both in checking the force 

 of the rain-drops and in retarding the removal of the soil by its 

 rootlets and fallen leaves. Thus in arid regions of sufficient steep- 

 ness " absence of vegetation is accompanied by absence of soil," 

 the rain runs off the rock-surface, and both solution and weathering 

 are therefore retarded, though the disintegration by corrasion 

 would in many cases be increased. 



From these data the author arrives at the following conclusions, 

 in a very instructive summary. 



That " a stream of water flowing down its bed expends an 

 amount of energy that is measured by the quantity of water and 

 the vertical distance through which it flows ;" and its capacity for 

 transportation is enhanced by comminution, according to the fine- 

 ness of the detritus on which some portion of the stream's energy 

 must be expended. Thus a stream with a maximum load of fine 

 material will be slower than when carrying a maximum load of 

 coarse debris ; and the capacity of a stream for transportation is 

 greater for fine debris than coarse. 



Still a river cannot do its full work with too full a load, as it is 

 then on the verge of deposition ; nor can it do as much work with 

 small or soft debris as with stouter materials, as the blows dealt 

 with the fragments on the river-bottom are the more effective as 

 the current is strong, the bottom soft, and the fragments hard. 



Hence it follows that in the Colorado region the erosion that 

 commenced with the first lifting above the ocean has progressed 

 continuously. In the uplands which border the canons the rain- 

 fall is small and the declivity great, so that weathering is less 

 favoured than transportation ; but in the gorges themselves corra- 

 sion becomes more important, owing to the " quantity of water, 

 which belongs to the mountain sources of the stream and not to 

 the plateau which they divide;" while the great declivity of the bed, 

 coupled with a moderate supply of debris " sufficient for the work 

 of excavation, but not so great as to consume the energy of the 

 current," are important factors in calculating the amount of work 

 done. If the amount of loose material were such as to cause the 

 energy of the stream to be used up in transportation, the result of 

 a river's action would simply be to generally degrade the plain ; but 

 where weathering is not rapid and the stream is well supplied with 



