GEOLOGY AND TOPOGRAPHY OF HAWAII 127 



island in Pearl Harbor and fresh water was secured at a depth of 2-10 feet. 

 The natural principle involved in the fresh water spring and especially the 

 spring in the ocean, was turned to practical account. To secure water, wells 

 were driven deep enough into the earth to puncture the more or less impervious 

 strata overlying the water-bearing strata beneath, with the result that owing to 

 the pressure or head on the empounded water, it rose in the well, and in the 

 lower zone about the island often overflowed to form an artificial spring or 

 flowing artesian well. The principle involved in wells which do not overflow 

 is the same as that in those that do; for which reason all deep wells are now 

 called artesian. Wells in which the water is raised to the surface by pumps are 

 liable to become brackish, through excessive pumping, while those which flow 

 naturally seldom show a marked change in the amount of salt carried in their 

 waters. 



The water-bearing stratum on Oaliu at the sea-shore, is usually found to be 

 between three and four hundred feet below tide level, and is us\uilly a very 

 porous basalt, capped with an overlaying impervious stratum \isually of basalt. 

 Wells drilled in the vicinity of Honolulu at an elevation above forty-two feet 

 above the sea have to be pumped. The flowing wells are, as a rule, found at 

 the lower levels. It is of interest to note in this connection that as a rule the 

 shallowest wells are those bored aliout the ends of radiating lava ridges and 

 that usually their depth increases the nearer they are to the sea-coast. Wells 

 drilled in the middle of valleys are usually deeper than those at either side. 

 All of these facts taken together iiidicntc that the island has been sul)merged 

 to considerable depth before the subsequent elevation of the rai.sed coral reef 

 on the costal plain about the island, and that the reefs were laid down in suli- 

 merged valleys that were already deejjly eroded before the reefs were formed 

 in them. 



In several places, notably at Waianae and Oahu plantations, as well as else- 

 where in the group, underground streams have been encountered through hori- 

 zontal tunnels driven into the mountains, and the underground v.'ater supply has 

 been tapped near its head. The tunnel is then extended to the right and left, form- 

 ing a Y-shaped drain, which brings the water to the surface, far above possible 

 contamination with sea water. Such tunnels are usually driven at altitudes suffi- 

 cient to admit of distributing the water by gravity over extensive fields well 

 upon the slopes of the mountain. On Maui a dail,>' flow of six million gallons 

 lias l)ecn secured in this way at an elevation of 2,600 feet. The wduderful W.iia- 

 hole tunnel on Oahu, built on a modification of this principle, delivers twenty 

 million gallons of water each twenty-four hours. 



Economic Products. 



Of the economic products, clays are the most important and are found on 

 Oahu. .AFaui and Hawaii, in many places, in varying amounts. A numlier of 

 years ago a brick kiln was opened in Nuuanu Valley and brick of fair quality 

 was manufactured. Unfortunately, the attempt was abandoned in a short time. 



