116 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
amount of water available is limited on the one hand by the amount of rainfall upon the catchment area and the facility with which the rainfall can obtain entrance to the porous stratum, and on the other hand by the capacity of the water-bearing rock to transmit the water over long distances and diminution through leakage and seepage. These two limiting conditions are usually of sufficient magnitude to render the overdrawing of the supply a practical and present danger which should be constantly kept in mind."
With regard to the artesian basin at Denver, Colorado, the failure of which was unusually rapid, Slichter says: "This basin was discovered in 1884, and in a few years about 400 wells had been drilled within an area extending a distance of 40 miles along South Platte River in a strip about 5 miles wide on both sides of the stream. Most of the wells were within the limits of the city itself. Many .of the wells had a good pressure and strong flow when first constructed. In 1886 it was not thought that any general decrease in the flow of the wells could be detected. Between 1888 and 189o, however, a continuous decrease in the flow of the city wells took place, and by the end of the latter year all but six of the city wells had to be pumped, while numerous wells in the basin were permanently abandoned."
CAUSE OF THE LOSS OF FLOW.
The loss of flow may be due to several causes. It is frequently the case that the life of an artesian well is limited. The escape of water through the well relieves the pressure which results in a reduced flow. In some instances pressure has so far been relieved that wells have ceased to flow entirely. This may be regarded as a natural and unavoidable loss of flow.
The second cause of reduced flow which may have affected the Jacksonville and other wells is interference of wells. Numerous instances are on record where one artesian well has affected surrounding wells.
A third possible cause is clogging of the wells through accumulation of sand or other material in the pipes or in the formations through which the water comes. In addition to the accumulation of sand it is not impossible that the porosity of the formation immediately around the well may have been more or less affected by chemical deposition since the well was drilled. It seems probable, however, that the clogging of the pores of the rock is more likely to be caused by material mechanically transported than by chemical deposition.
Improper casing is likewise a frequent cause of failure. It is frequently the case that an insufficient length of casing is used in