14 GEOLOGICAL SURVEY OF ALABAMA. 



not :a percentage, it necessarily forms a much larger propor- 

 tion of a heavy annual rainfall than it does of a small annual 

 rainfall. For instance, in the Crystalline region of Georgia 

 or Alabama where the annual precipitation is 45 to 55 inches, 

 the run-off from the water-sheds is equal to fully one-half 

 of the rainfall, while in regions having a precipitation of 

 only 10 to 20 inches annually, the run-off is frequently less 

 than one-fifth of the rainfall. Again, the geological charac- 

 ter of the water-shed makes a vast difference in the run-off, 

 even where the annual rainfall is the same, and where practi- 

 cally the same conditions of climate, topography, forest area 

 and cultivation exist. There will be a smaller run-off from 

 the water-shed having permeable geological strata under- 

 neath it, into which a part of the rain water can percolate, 

 and furnish the supply to artesian wells in the lower country 

 under which the same strata run, without regard to surface 

 topography. In a comparison of two such water-sheds, one 

 in the crystalline region, and the other in a regularly strati- 

 fied formation, the difference of run-off should form a basis 

 for estimating the artesian supply obtainable from the latter 

 as a fountain head. 



