126 XATLKAL ilLSTOKY OF HAWAII. 



bi-oii-lit the ancient reefs above tlie sea. Tlu'se ele'vated reefs contain extinct 

 fossils, probably those of Eocene time. The dawn of the Eocene is generally 

 placed by ^aH)logists at four million years ago. How much older then must be 

 tlir moimtain mass in which the valleys of the AYaianae region were so deeply 

 carved before the reefs were laid down across the emba^-ments at the mouths of 

 their valley streams? 



Artesi.vn Wells. 



Hcfci-ence has been made above to the artesian water supply of the island, 

 and the important geologic facts that the sinking of five hundred or more artesian 

 wells on Oahu has brought to light. The wealth of water, amounting to millions 

 of gallons per hour, now poui-ed out on what was formally in many places semi- 

 arid, and tlici-efore. un|)i'o(hictiv(^ land, has been the prime factor in the modern 

 development of the agricnltural resources, not only on the island under con- 

 sideration, bnl all the islands of the grou]), where conditions favorable to the 

 development of artesian wells are found. 



The erosion of the sloping volcanic lava flows in the mountains offers condi- 

 tions favoi-a])le foi- storing in the ground much of the excess of the copious 

 precipitation occurring in the higher altitudes. As we have seen, the strata of 

 igneous cock exposed in the mountains are often buried several hundred feet 

 beneath the surface when they reach the costal plain. The Avater which enters 

 the exjHjsetl portion of the more porous strata, especially when the water-bearing 

 strata lie between more impervious strata, tends by gravity to flow as under- 

 ground M'ater down to the lower levels. Eventually, this underground stream 

 descends to the sea, often several miles distant from the point in the highlands 

 where it was taken into the porous rock or soil. 



ir the lower ends of the water-bearing strata open into the sea beneath its 

 surface, the fresh water gradually forces its way out at the lower end of the 

 natural conduit, to mingle quietly with the water of the ocean, or, as often occurs 

 about the shore line of the group, to l:)u1)ble to the surface forming fresh water 

 springs in the ocean. 



Owing to the pressure exerted by the sea, the subterranean water moves out 

 nuich more slowly than the surface water which rushes from the mountains to the 

 sea in the forin of rivers. If the pressure of the water in the imderground stream 

 is greater tlian tlie pressure exerted by the water of the sea, the stream con- 

 tinues to flow into the latter as fresh water. If the pressure of the ocean exceeds 

 that exerted by the underground waters, the two waters commingle, and brackish 

 water occui's in the underground basin. So long as the fresh water level in the 

 underground stream or basin is maintained at a level above sea-level, the water 

 in the undergr-ound sti-eam or l)asin seems to remain free from salt. 



An appreciation of the geologic conditions existing in the strata of rock 

 underlying the island, and the need of a more abundant water supply, led to 

 the practical utilization of this great natural resource through the development 

 of artesian wells. The first well was sunk in 1879 by James Campbell on an 



