J. D. Dana — Geological History of Oahu. 



101 



the modern reef, and also has often the corals in the positions 

 of growth. But the wind-drift beds show the quaquaversal or 

 variously-striking dip common in wind-made drifts, as rep- 

 resented in the two sections below. 



Kahuku bluffs of coral rock and drift-sands, with two sections of the 

 drift-sand rock. 



The change of level along northern Oahu, according to the 

 facts from Kahuku, appears to have been at least sixty feet, or 

 twenty feet greater than on its southern side. Even with an 

 accurate measurement of the height of the reef -rock about Oahu 

 the amount of elevation would remain doubtful because the 

 coral reefs off the island are at present nowhere up to low- tide 

 level ; and this may or may not have been the fact before the 

 change of level took place. 



The surface of the elevated reef of Oahu is exceedingly un- 

 even from unequal construction and erosion, and its interior has 

 in some places large and winding caverns, so that an overlying 

 formation, were there one, would afford an example of uncon- 

 formability by denudation. It is obvious that with greater 

 elevation, the unevenness would be as much greater, large 

 enough to get the credit, perhaps, of representing an interval 

 of many thousands of years, although results of the " modern " 

 period in geology. Denudation works rapidly among lime- 

 stones and especially so when the limestones have just left the 

 water, with the usual irregularities of upper surface and texture. 



2. Subsidence. — A gradual subsidence of the island is appar- 

 ently indicated by the coral reefs, through the depth to which 

 they have been found to extend in Artesian borings. In these 

 borings, described on page 96, a depth of TOO to 800 feet was 



