230 IRRIGATION. 



most subject to aridity, opportunities occur for construct 

 ing such reservoirs on the grandest scale, at the most 

 moderate cost. Deep, narrow canons, which open out 

 into extensive, and sometimes vast valleys, now of little 

 use, for want of soil and on account of their rocky sur 

 face, might be easily and cheaply closed, and thus a res 

 ervoir of great magnitude might be made. The normal 



Fig. 111. VALLEY IN INCLINED STRATA. 



flow of the issuing stream might thus be regularly main 

 tained, and destructive torrents from &quot; cloud bursts,&quot; 

 and rapidly melting snow banks, might be prevented. 

 But before any expenditure is made in such operations, 

 the geological character of the valley should be examined, 

 lest from unfavorable conditions failure might ensue. 

 This will clearly appear by a glance at the three annexed 

 illustrations. At fig. Ill is shown a section of what is 

 known as a valley of erosion, situated in an inclined 

 formation. It is apparent, that if such a valley be 

 dammed, the water might escape through any one of the 

 strata on the lower side, that might happen to be porous. 

 In this case failure might be expected. 



At figure 112 is shown a section of a valley occupying 

 an anticlinal axis. It is equally apparent here, that the 

 escape of the impounded water might be looked for, and 

 that upon both sides if any of these strata be porous. 

 Failure would be certain in this case also. 



