OF THE HAWAIIAN ISLANDS. 465 



in the valley of the Wailuku, the columns are very perfect. So on Kauai near Huleia 

 and in the Hanapepe valley. The dykes so common all over the group are usually 

 of basalt, and the structure is generally prismatic, the prisms often separated by 

 a thin calcareous film. 



Lavas which have been ejected through Water. 



On all the islands there are found shore craters, and those on Oahu have attained con- 

 siderable size. The tufa of which they are principally composed is simply comminuted lava, 

 cemented together by ferruginous or calcareous cement. The stone thus formed 

 is tolerably firm and compact. It is in layers varying in thickness from half an 

 inch to a foot. Where the eruption has taken place through the coral reef, the limestone 

 thus mingled with the ash or mud is gradually decomposed, and fills the cracks with 

 calcareous deposits. Sometimes this is so abundant as to whiten the whole surface of the 

 rock. In a climate where frosts prevail, the loose porous tufa could not long hold together ; 

 and even in the tropics the rains fast reduce the hardest to soil. Sometimes the grain is 

 coarse and much mottled with lime, while at the same place other layers may be fine and 

 homogeneous throughout. In the tufa of Koko small crystals of augite are seen, and also 

 what closely resembles these, particles of obsidian. At Aliapaakai, these and garnets, mica 

 and chrysolite, have been found, the latter abundantly in the tufa. 



Soils. 



All the soil about Honolulu and Waikiki is simply tufa debris, and it does not differ 

 essentially from that resulting from the decomposition of other lava. It is often bright red, 

 like the original tufa. We have spoken of the clay soils, but they are not common ; the 

 usual earth is light, recpiiring constant irrigation to render it fertile. In the valleys and on 

 the windward slopes of the mountains the rains are abundant; but where these fail all is dry 

 and sterile ; yet the soil is ecpually good, covering itself with verdure during the rainy 

 season, although dry and brown all the rest of the year, and yielding good crops of cane, 

 bananas, and cotton whenever artificially irrigated. The vegetable acids produce a rich black 

 soil in the forest. 



It will be seen that the lavas contain but little potassa, no titanium, so far as analyzed, 

 and a large amount of iron oxides. The different varieties run into each other, and a 

 complete chain may readily be formed from the clinkstone of Mokuaweoweo to the basaltic 

 dykes of Hanapepe. 



MEMOIRS HOST. SOC. NAT. HIST. Vol. I. Pt. 3. 118 



