VOLCANIC ASHKS 53 



In Olaa aiul elsewhere near the sea shallow canyons are beginning to 

 show themselves. 



Volcanic Ashes 



At the New York meeting of the American Association for the Ad- 

 vancement of Science, in 1887,'^ I re})resente(l that what had been called 

 oceanic sediments f in various benches and biittes were reall}' volcanic 

 ashes. I have examined these deposits more carefull}^ during the past 

 year, and substantiate the earlier })osition. 



The annexed outline map J is constructed so as to show the areas oc- 

 cupied by these ashes so far as known. It is needful first to describe 

 the location of the forest. In Hamakua, Hilo, and Puna districts it 

 flanks Mauna Kea and Mauna Loa from about 300 to 7,000 feet altitude. 

 Interrupted for 10 miles by Kilauea, it commences again near Aina})0, 

 runs down to Kahuka, and curves to the north, extending through Kona. 

 Its presence is due ))rimaril3'' to the rainfall, which is excessive (175 to 

 200 inches) in the more eastern section. Sugar plantations are located 

 below the forest, on the windward sloi)es, and the planters have discov- 

 ered that when the ground has been cleared of trees the land is well 

 fitted for the growth of the cane and coffee. In Olaa there seems to be 

 a connection between the fertility and the prevalence of the ashes. For- 

 merly the Volcano road from Hilo avoided the forest, except for about 

 three miles, near the lower end, because of the mud. Since the recent 

 opening of the region to settlement a new macadamized road has been 

 constructed through a dozen miles of the forest, and this portion of the 

 woodland is underlaid by the ashes. Away from the trees the ashes 

 have been covered up, and the hard basalt has not disintegrated suffi- 

 ciently to form a soil. On much of this fertile land a slender twig can 

 be easily thrust down for six feet, so loose is the material. 



The area below the forest land in Puna is very extensive, and this for 

 much of the district is due to the absence of friable material, not to 

 aridity. It is here that the lava flowed to the sea at Nanawale from 

 Kilauea in 1840. Taking a road recently constructed from the ninth mile- 

 post out of Hilo on the Volcano road to Pohoiki (Rycrofts), I discovered 

 the reason for the partly fertile and partly sterile tracts. Near the coast 

 there is a dense growth of the Pandanus or screw ])ine. Higher up there 

 is much guava and various shrubs. The flow of 1840 is still conspicu- 

 ous by the sparse vegetation upon it, and sufficient time has not ela])sed 



*Proc. Amer. Assoc. Adv. Sei., vol. xxxvi. 



t Fourth Annual Report of the U. S. Geol. Survey. 



J Figure 1 is copied from Alexander's published map, drawn by W. A. Wall, 188C. 



IX— BuLi,. Geol. Soc, Am., Voi,. 12, 1900 



