Page:Popular Science Monthly Volume 67.djvu/683

Rh of underground currents, but these are all clearly of volcanic origin, and here and there may be seen the remnants of the secondary craters whence came the ejections.

Oahu resembles Kauai in the dispersal of friable material in layers alternating with hard basalts, and adds marine accumulations to the igneous. The meteorological conditions explain the source and spread of the waters. Bain is profusely abundant on the highlands. The trade-winds, laden with moisture, drop their burdens on coming in contact with the land surfaces. The maximum rainfall is at the altitude of about 1,200 feet. The preponderance of the discharge, being upon the windward side, determines the place of the most copious streams and the more effective erosion. Hence the domes have been worn away unequally. One side may be entirely removed, and the other be scarcely affected at the surface. If the ridge is narrow at the altitude of greatest precipitation both sides will be extensively worn down. This is well shown on the Koolau upland, where the southeast end has been greatly denuded upon both sides from Mokapu point to the pali, while to the north, at a greater height, the canons are less conspicuous on the west side.

The laying bare of the interior of the dome allows the water to sink into the pervious layers, and to flow beneath the surface towards Kaala and the southwest. Only the needful alternation of pervious and impervious strata is necessary to give rise to the subterraneous streams which will send water to the surface when pierced by the artesian wells.

The borings upon Oahu prove the alternation of basalt, clay, earth and limestone to the depth of several hundred feet. The principal water-bearing stratum is a very porous basalt, from 300 to 400 feet below the sea level by the shore. It has a hard, impervious cover, sufficiently tight to prevent the passage of water through it. The following general statements concerning the artesian conditions seem to be well established:

1. The presence of a porous water-bearing stratum beneath an impervious cover.

2. Water is reached usually at the depth of from 300 to 500 feet.

3. The water flows freely without pumping only in a narrow belt of territory adjacent to the coast line, where the surface is but slightly elevated; which is 42 feet at Honolulu, 32 feet at Ewa and 26 feet at Kahuku, at the northeast angle of the island. Wells sunk in higher ground show the water rising to the level indicated. Thus at the height of 100 feet the water will rise to the level of 42 feet at Honolulu, above which it will discharge only by the application of a pump.

4. For convenience in obtaining a proper supply several wells are sunk adjacent to each other. Naturally, as development takes place, the number of the wells increases. Thus the Ewa plantation had at first six ten-inch wells about thirty feet apart connected to a single pump,