FRESH-WATER SPRINGS IN THE OCEAN. 677 



velopment 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 im- 

 pervious 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, 



