Branner — Geology of the Ilavmiian Islands. 305 



from the sniTOunding land and deposited in the harboi''s quiet 

 waters. In the first edition of his work npon volcanoes Dana 

 sajs (p. 282) that it would be necessary to cut a channel 

 through the outer reef in order to make a harbor of this place. 

 Later it has been found that there is a natm^al opening quite 

 through the reef, and dredging is now going on to remove the 

 bar of sand just ontside of the narrow entrance. The shallow- 

 ness of the water outside and its greater depth in the neck of 

 the harbor is caused by tidal scour. 



The depth of the silts in the harbor has probably been deter- 

 mined, but I have not been able thus far to get any informa- 

 tion on this subject. The topography and geology offer no 

 suggestion further than that the depth is likely to be approxi- 

 mately the same as that of the harbor at Honolulu, that is, 

 about thirty feet. The depth of the silts can be readily deter- 

 mined by driving steel rods down into them. 



We shall readily understand the process by which this 

 topography was made if we imagine the whole island elevated 

 fifty or seventy-five feet and the original rock-beds restored 

 right across the present channels. AVe should then have the 

 streams that now enter the upper parts of Pearl Harbor flow- 

 ing across a table land of horizontal rocks, uniting at or near 

 the point marked A on the map and entering the sea below 

 the boat landing through a single channel. In the course of 

 time these streams would all cut steep-sided gorges, and where 

 the gorges, by bends of the streams, approached each other the 

 watershed between the two streams would be lowered below 

 the general land surface. Such a place would eventually be 

 an isolated bit of high land and after depression would form 

 an island, such as we have in Mokuumeume. A depression of 

 the island would back the sea into the valleys ; the cutting of 

 the streams would then cease, and the land silts would settle 

 in the quiet water of the submerged channels, forming shallows, 

 later mud-flats, and then the swampy lands. 



The map of the island (see Plate XY) shows that the 

 entrance to Pearl Harbor through the coral reef is very like 

 that opening into the harbor of Honolulu and like another 

 similar one known as the Kalihi entrance about two miles east 

 of the Honolulu harbor. It seems probable, therefore, that all 

 three of these harbors and their passages through the coral 

 reef have been formed in the same way. 



Aside from the form of Pearl Harbor there is evidence of 

 depression of Oahu and the other islands of this group. The 

 evidence of depression on Oahu consists partly of the great 

 depth at which coral rock has been found in deep wells ; some 

 of the evidence on the island of Hawaii is mentioned in the 

 preceding note upon the Waipio and other canyons. 



