OF THE HAWAIIAN ISLANDS. 457 



glossy colorless crystals. It is doubtless a component of many of the basalts. An analysis 



of a specimen from Maui by Schlieper gives — 



Si 3 Al a 3 Fe 2 3 Ca Na K Mg 



53.98 27.56 1.14 8.65 6.06 0.47 1.35 = 99.21. 



Orthoclase — Feldspar. 



Feldspar has not been found in separate crystals, but enters into the composition of the 

 phonolites of the higher mountain regions, and the solid mountain core. The decomposition 

 of feldspathic rocks by sulphurous vapors sets free the silica, and results in the recombination 

 of the elements into various species. Clays are found on all the islands, especially on Oahu 

 and Maui, and in the district of H6na. On the latter island, much coarse pottery was manu- 

 factured some years ago. Bricks also have lately been made. 



Prehnite. 



It is found in the cavities of ancient basaltic rock on Oahu, in pale green compact gran- 

 ular masses. Seldom found near the surface of the rock, so only seen when the rock is 

 quarried for building purposes. Not common. 



Thompsonite. 



This mineral is found in lavas containing much olivine on Oaliu and Hawaii ; it usually 

 completely fills the cavities, forming an amygdaloid. 



Natrolite. 



Natrolite is found in the same rock as the last, and is usually of a gray color, vitreous 

 lustre and radiating structure. I have never seen it in situ, but in fragmentary rocks the 

 centre of radiation seems to be always on the same side of each cavity in the same specimen. 



Scolecite. 



A radiated crystalline mass, much resembling natrolite, gave the well-known blowpipe 

 indication of scolecite, but was not further analyzed. Other zeolitic minerals will doubtless 

 be found in the older lavas whenever the rocks are excavated to any extent. In many 

 places I have been struck with the resemblance between some Hawaiian lava beds, and 

 those in the Ghats of Western India, where such rich deposits of apophyllite and other 

 zeolites have been found, especially in the deep excavations at the Bore ghat. The so- 

 called amygdaloid in this place is no older, apparently, than some of the cellular lavas on 

 Oahu, and is of the same nature. 



Gypsum. 



This is found beautifully crystallized in Kilauea, both beneath the crust around the fuma- 

 roles and on the tube stalactites in the caves. Sometimes the long slender tubes pendant from 

 the roof are completely covered with opaque white rhombic crystals thickly agglomerated, 

 and at other times with long transparent acicular crystals often stellate. Both varieties are 

 found in the same cave and even on the same tube, but always where both forms are so 

 closely associated, the transparent crystals are at the end of the tube, and as this grows by 

 successive deposits of silica and other soluble parts of the superincumbent rock, the opaque 



MEMOIRS BOST. SOC. NAT. HIST. Vol. I. Pt. 3. 116 



