TRACHYTE ON THE ISLAND OF HAWAII 519 



The magnitude of the erosion by which the mountains and valleys 

 of Kauai and Oahu have been carved out of former basaltic piles can- 

 not now be measured. All writers upon these islands have recognized 

 that erosion has been great. The imposing cliffs or pali which face 

 the sea upon both islands, the canyons with walls 2,000-3,000 feet in 

 height, the general ruggedness of mountain contours, are comparable 

 in scale with the same features of the Rocky Mountain country. The 

 erosion periods of the two districts must also be comparable. The 

 former volcanic centers are rudely indicated by the attitude of lava 

 flows, but enormous sections of the old volcanoes have been engulfed 

 by faulting or wholly destroyed by erosion. 



The discovery by Dall of marine fossils in a raised reef or beach 

 rock about Diamond Head, the well-known tuff crater near Honolulu, 

 fixes a datum point in the history of Oahu which is manifestly of 

 great importance. Close determination of the age of the fossiliferous 

 deposit is at present impossible because, to quote Dr. Dall, 



we have no standard of comparison in the whole Polynesian region by which the 

 species could be compared with those of Tertiary beds of known age; but the 

 fossils have every characteristic of those generally assigned to the Pliocene or 



upper Miocene in their general aspect, and state of fossilization To sum 



up, it is concluded that the reef rock of Pearl Harbor and Diamond Head lime- 

 stones are of late Tertiary age, which may correspond to the Pliocene of west 

 American shores, or even be somewhat earlier.^ 



The view of Dr. Dall that "the whole mass of Diamond Head 

 had been slowly deposited in comparatively shallow water and 

 gradually elevated without being subjected to notable flexure" 

 seems to the writer incorrect for various reasons, some of which 

 have been pointed out by Dr. J. C. Branner^ and Dr. S. E. Bishop. ^ 

 This point is not here at issue, for whatever the relations of the 

 marine deposits to the Diamond Head tuffs, they exist and are younger 

 than the great erosion of Oahu. Even if of early Pleistocene date, 

 they indicate that the enormous denudation of the Oahu volcanoes 

 must be referred to the late Tertiary and the lava eruptions themselves 

 to a still earlier period. 



From the considerations above presented it appears that the 



^ Loc. cit., pp. 58, 60. 



'American ournal of Science, Vol. XVI (1903), pp. 306, 307. 



3 American Geologist, ]a.nua.ry, 1901. 



