266 



Waianae, being much the older, has sufifered much the greatest 

 change. 



Besides that form of change, the whole island, probably from 

 a very early period, has experienced great subsidence of level 

 beneath the ocean. This subsidence has been doubtless more than 

 1500 feet. The evidence of this is the depth at which coral has 

 been found in artesian borings, coral not being possible of 

 growth at more than perhaps 20 fathoms of depth. Also the 

 vesicular structure of the lava found at all depths reached by the 

 borings proves the original position of those lavas to have been 

 above sea level, since the pressure below that level would have 

 prevented the occluded gases of the magma from expanding. 



At an extremely recent period the whole island of Oahu under- 

 went an apparently simultaneous elevation of level, amounting to 

 from 30 to 50 feet, those being the altitudes at which formerly 

 submarine formations are now found in position above sea-level. 

 Among such elevated calcareous formations of greatest altitude 

 are: coral reef on bank of Pauoa stream, a little below School 

 Street bridge in the city ; reefs in Waianae near the railway, 

 adjacent to high lava promontories, and at the north end of the 

 island, a number of masses of ancient sand dunes which had been 

 submerged and cemented into hard sandstone by marine action, 

 but were later elevated, and now indicate that marine action to a 

 present height of 50 feet. 



As to the depth of the coral in your locality, I would suggest 

 an approximate estimate as follows : I find by the latest map of 

 Oahu that Barber's Point lighthouse, near which I believe is the 

 observatory, is about three and a half miles from the nearest 

 point of the base of the mountain where it meets the plain. Also 

 from that base to the nearest peak is but a slightly greater dis- 

 tance. I think it safe to assume that the grade of descent was 

 the same from the same present base of the mountain to the 

 bottom of the coral reef at the Point. The present altitude of 

 that peak, "Manawahua," is given on the map as 2450. It has 

 doubtless suffered much degradation by weathering. It seems, 

 therefore, quite safe to estimate the depth of the coral at Bar- 

 ber's Point at 2500 feet. 



As to the method of formation of coral reefs, that is very 

 fully and well treated in many books on the subject. A coral 

 reef seems to be an accumulation of the debris of various corals 

 and shells beaten by the heavy surf to powder, and cemented by 

 the sea-water. A gradual accretion grows up as the land sub- 

 sides beneath it, forming first a "fringing" reef, and later, if fresh 

 water from the land interferes with marine growth at the shore. 

 f<jrming a "barrier" reef. There being little fresh water from 

 the adjacent mountain, only a fringing reef was formed al I'.ar- 

 bcr's Point, many miles in width and of great solidity. 



The formation of such reef on the western shore along Waia- 

 nae seems to have been ])revcntcd l)y the violence of the westerly 



