AND THEIR RELATION TO IRRIGATION. 
5 
friction in the intervening distance may be more than 
sufficient to make up for the difference in level. The 
pervious stratum may consist of any material which will 
allow T water to pass through it, but most commonly it 
consists of sand or sandstone. The more open and 
porous this stratum is, the more abundant will be the 
flow with any given pressure. No rocks are perfectly 
impervious, but thickness will compensate to a great ex¬ 
tent for a slight porosity. The confining stratum gener¬ 
ally consists of clay or shale. 
The region where artesian wells are found is gener¬ 
ally spoken of as an artesian basin, largely because the 
typical form of such a region is a genuine basin, with the 
rim higher than the center. A section of the Denver 
basin is of this form. The figure may represent an 
exaggerated section of such a basin, with the porous 
strata outcropping at B, D, C and A. Anywhere 
lower than the line AK flowing wells might be 
expected if the strata are continuous, but as we reach K, 
or some point nearer B, it will be found that water comes 
only to the surface, and still higher it may fail to reach 
the surface. It is also evident that while at P flowing- 
water will not be obtained from the upper stratum, by 
going deeper it may be secured, because the outcrop of 
the stratum which furnishes it is higher. 
The figure also shows why the pressure is generally 
greater as the depth is greater. This fact has given 
rise to a popular belief that if one only goes deep enough 
