464 W. T. LRIGHAM ON THE VOLCANIC PHENOMENA 



Kilauea result from the former agency, and are usually bright red, tenacious, and gritty. The 

 undecomposed lava contains more silica than is required to form clay with the 

 alumina, and consequently clays are not always produced. At the sulphur banks 

 a curious alternation of earths containing more or less silica has been observed, the more 

 clayey containing twigs or other vegetable matter from the cliffs above, in a more or less 

 fossil state, while the siliceous layers are thin and comparatively free from foreign substance. 



Decomposition by Meteoric influences. 



Under this head we may range all the ancient lavas, and thus almost the entire mass of 

 the Hawaiian Islands. I have already pointed out that the lava rock in the centre of the 

 volcanic mountains is by no means the oldest ; like granitic veins penetrating sedimentary 

 strata its actual formation may have been long subsequent to the surface overflows which 

 have formed the innumerable coatings of the dome. Wherever this central mass is visible, 

 as in the deep valleys, it exhibits an amorphous, compact structure, gray or dark blue in 

 color, and of considerable specific gravity. The composition is slightly different on the 

 different islands, Haleakala containing a rock of lighter color and less specific gravity than 

 Konahuanui. I have never seen in either any cells which would permit the formation of 

 zeolites. 



The clinkstone beds on the summit of Maunas Lua and Kea are of two kinds, 



Clinkstone. 



differing chiefly in color, one being gray, the other pale red or light brown. \. 

 partial analysis by Mr. J. C. Jackson gave : — 



Si 3 Fe + Fe 2 3 Al 2 3 Ca + Mg + Na 

 52.0 25.2 20.6 2.2 = 100.0 Sp. gr. 2.91. 



This is a very tough compact rock, chips with a conchoidal fracture, and is used for adzes. 

 In ancient times, the only implements for cutting wood and shaping canoes were made 

 from the rock of Hawaii, which was considered better for the purpose than any other. 

 The composition of most of the summit lavas is trachytic. They take a good polish, and 

 have the characteristic ring of phonolites. Olivine is quite rare, and I have never seen a 

 single specimen from these compact rocks. The phonolites, like the other lavas, are in beds 

 and much fissured. Both Kea and Loa are capped with this rock, mingled with the ordinary 

 trachydolerite of Abich, which is the only stone on the other mountains. 



The ordinary rock of the walls of the craters, both on Hawaii and on the other islands, 



is a compact heavy graystone, not cellular to any extent, and containing much 



olivine, usually of a pale yellow color. A mass in Kilauea, evidently from the 



outer walls, has a specific gravity of 2.2. The summit of Haleakala, and consequently the 



walls of the crater, are of a vesicular trachydolerite, light brown on the surface where 



acted upon by the weather, but brownish black within. Fracture conchoidal and splintery. 



Specific gravity, 2.7. Much of this rock has in spots a structure similar to that of highly 



fermented dough — coalescing bubbles with intervening bundles of fibres. A specimen of 



stony lava procured by the Exploring Expedition gave — 



Si0 3 FeO MgO Na O 



59.80 31.33 1.71 4.83 = 97.67 B. Silliman, Jr. 



Specific gravity, 2.93. Soluble in Hydrochloric acid, 24.55 ; insoluble, 75.45. 

 Although the trachytes are often columnar, it is in the basaltic varieties that this structure 

 is most beautifully shown. In many of the deep flows of lava from Ke"a and Loa, especially 



