GEOLOGY AND TOPOGRAPHY OF HAAVAII. 117 



nearly with the low land tract utilized for eaue and sisal from Barber's Point 

 to Koko Head; perhaps to the altitude of 300 feet entirely around the island." 

 Small patches of the rock appear at Waianae, Waialua, Kahuku Plantation. 

 Laie and other places on the northeast coast, the highest reef being on the south- 

 west end of Mailiilii at 120 feet above the sea. The rock is also extensively dis- 

 tributed beneath the surface, as is developed in boring artesian wells. 



Age of Oahu. 



Dr. W. II. D(dl. who also studied the deposits in the vicinity of Pearl 

 Harbor and Diamond Head, found species of sea shells" seemingly extinct, 

 which are referable to the Pliocene. In conclusion he says, "that the reef rock 

 of Pearl Harbor and Diamond Head limestones, are of the late Tertiary age 

 which may accord with the Pliocene of West American shores or even be some- 

 what earlier, and in the region studied there was no evidence of any Pleisto- 

 cene* elevated reefs whatsoever. It is probable that Oahu was land inhabited 

 by animals as early as the Eocene," which period preceded the Miocene, and 

 marked the opening period of the Cenozoie era, or the era of modern life. 



Bl.vck Volc.\nic Sand. 



Over much of the region about Honolulu, but especially on the slopes of the 

 Punchbowl and Tantalus group of cones, are to be found extensive deposits of 

 black ash, a volcanic product usually formed from basalt when erupted in associa- 

 tion with much steam. The maximum thickness of the deposits is exposed at the 

 base of the Tantalus cone, in Makiki Valley, where a bed twenty-five feet thick 

 occurs. This coarse-grained sand has found many uses in the city; such as in 

 making sidewalks and grading roads, and to some extent as sewers in the early 

 days, while recently it has been found to be of some value as a fertilizer owing to 

 the presence of potassium. The sources of the deposits referred to seems to have 

 been Tantalus and Punchbowl ; but practically all of the smaller cones have 

 given more or less volcanic ash, which varies in fineness and color, as well as in 

 amount, in each eruption and at different times during the same eruption. On 

 Punchbowl especially this ash overlays the tuff, and, owing to the pronounced 

 weathering of the latter, it seems to indicate two quite distinct periods of activity 

 from the same source, with a long period of time between them. In the first 

 eruption the material came up through the sea as the character of the tuff 

 deposits indicate, while the later eruption or eruptions, including the ash, the 

 basalt-like dikes which radiate from the rim, as well as the cinder-like beds on 

 the upper part of the rim, found its way up a pipe within the cone from a 

 deeper source of basalt, apparently without coming in contact with the water of 

 the sea or its limestone deposits. 



Limestone is also abundant about tlie crater at Diamond Head, at Koko 

 Head, and at the Salt Lake crater, where porfitms of the old reef are saitl to be 

 present on the inside of the crater. 



