THE IRRIGATION AGE. 51 



summer by an inflow of living streams and in other cases it is not. 

 This will make the one more durable than the other, but even it has 

 its limits. Where they are we know not, but it is advisable not to 

 try to find them. For the chances are very strong that if such a 

 reservoir is once emptied it will take much longer to fill again than 

 if it were a reservoir above ground. If we have in the meantime 

 expanded settlement too much on that basis it will not only suffer 

 but the great resource upon which we have fallen back in the last 

 two years will also be gone and legitimate settlement will suffer 

 also. 



To understand how easy this may be you have only to wander 

 back in fancy 10 the time when our valleys were hundreds of feet 

 lower and the mountains thousands of feet higher. You have only to 

 apply the conditions we have seen in our own time of the disposition 

 of the wash from the mountains. You know how the streams change 

 their channels, swinging in the course of ages Irom one side of the 

 valley to another Here they leave a bed of gravel this time, and 

 when in time the bed returns again it may cover it with fine sand or 

 finer rnud that makes a perfect cut-off from the gravel bed below. 

 "When in time another wash of heavy gravel comes upon that, followed 

 in time with another layer of finer stuff, the two beds of gravel become 

 independent reservoirs, perhaps with different pressures. They may 

 or may not be connected at some point above by a common thread of 

 supply. They might last for a year or two or three and then require 

 three years to fill again. In this way the valleys are filled from side 

 to side with old stream beds of various depths and widths, lying one 

 upon another lor hundreds and in places near the coast for thousands 

 of feet perhaps. For it is quite certain that the whole country has 

 once been much higher above the sea than it now is as well as much 

 lower at some time. 



This cutting off of the different channels from each other is in- 

 creased by the decay into clay of gravel and sand in streaks and strips 

 as they lie in place below ground. With the pressure these layers 

 flatten into perfect dams. This increases the number of independent 

 channels and pockets of water-bearing gravel and sand. It is easy to 

 understand how the friction on the lower part of these keeps the 

 water from passing away to the sea; so that as long as it is not drawn 

 on too heavity in any other way they will retain the water accumu- 

 lated from years of good rainfall. There is therefore no need of re- 

 sorting to the theory of any far distant or inexhaustible supply, but 

 on the contrary it is against probabilities and, if so, it is dangerous to 

 expand on. 



There is another kind of reservoir, even more valuable than any 

 of the others and which is in most cases wholly within the control of 

 the irrigator. This is the subsoil the subsoil for many feet or even 

 yards in depth. In most years the rainfall attends to it sufficiently 



