Branner — Geology of the Havxtiian Islands. 303 



The ^reat height of the sea bluffs is due to the long encroach- 

 ment of the sea upon this old land margin. The flat bottoms 

 of the largest of these canyons are due to the fact that the 

 canyons were formed as Y-shaped gorges on the land and have 

 sunk until their lower ends were filled by the sea, forming deep 

 fjords which were soon filled by the material cut by the waves 

 from the headlands and thrown back into them, and by the 

 debris brought down from the land by the streams. An 

 approximate idea of the depression might be obtained by bor- 

 ings in the lower ends of these valleys. I am unable to learn 

 of any deep wells having been put down in them. The accom- 

 panying photograph (fig. 2) gives some idea of the striking 

 topography of the Waipio Yalley, the largest of the group. 



Professor W. T. Brigham, the able director of the Bishop 

 Museum at Honolulu, tells me that there are precisely similar 

 valleys on the north side of the island of Kauai. An assistant 

 in the ofiice of the government surveyor at Honolulu, who has 

 lately visited the region, also states that the bluifs on that part 

 of Kauai are from 1,000 to 1,500 feet high, and that some of 

 these valleys are flat-bottomed while others are Y-shaped and 

 truncated above tide level. 



The origin of Pearl Harhor (Plate XY).^ — The topography 

 of Pearl Harbor is so different from that of most harbors that 

 it is sometimes spoken of as being unusual and diflicult of 

 explanation. 



Briefly, this harbor has been formed by the depression 

 beneath the sea of a s?nall group of dendritic valleys previously 

 carved by subaerial erosion in horizontal beds of rocks. If this 

 explanation is not at once suggested by a glance at the map, it is 

 made quite plain by a brief study of the rocks exposed about the 

 harbor. The rocks are horizontal and consist of alternate beds 

 of volcanic tuff and coral rocks. The so-called coral rocks, 

 however, are not necessarily all of coral but are often mixtures 

 of shells and other fragmental calcareous materials commonly 

 found in and about coral reefs. 



The low bluffs that surround the harbor are nearly all hori- 

 zontal beds of tuff. Coral rocks are shown on the government 

 map of the harbor at a few places. Those particular points I 

 did not examine, but there is no reason for doubting this 

 identification of the rock, for a few^ miles east of the harbor 

 the coral rocks are exposed ten or more feet above tide level. 

 In the wells put down in the vicinity of the harbor the coral 

 rock is also found a very short distance below tide level. 



The harbor is now quite shallow over most of its area, but 

 this shallowness is due to the silts having been washed down 



* The explanation here given was presented before the Social Science 

 Association of Honolulu in March, 1903. 



