OF THE HAWAIIAN ISLANDS. 



463 



as will be seen from the figure, which represents three actual forms, 

 not occurring, however, in the same cave. Specimens have been found 

 which exceed eight inches in diameter, and these are usually low and 

 flat-topped. The more slender ones sometimes rise to a height of two 

 feet ; and so rapidly is the silica deposited, that they seldom increase in 

 diameter, but are true acrogens, none of the suspended silica running 

 down the sides. In one cave the growth of the stalactites was at about 

 the rate of an inch a week, but owing to the varying amount of water 

 or steam the production is quite irregular. They are often coated with 

 beautiful white crystals of gypsum, sometimes tipped with needle-like 

 transparent crystals of the same mineral, where the cave is high. The 

 natives collect them with the upper open joint of a long bambu. 



The process of formation is this : the water from the frequent rains, and 

 the condensing steam, act upon the soluble portion of the superincum- 

 bent rock, carrying along the silica and lime to be deposited in the form 

 of tubes and their encrusting gypsum, and the resulting stone is quite 

 anhydrous, as will be seen from the following analysis of specimens not 

 coated with gypsum. 1 



Mn Ca Mg Na K 

 0.8 9.6 4.8 3.0 1.1 = 100.1. 



The temperature of the caves is usually from 



Fig. 49. 



Si 3 Al 2 3 Fe 2 3 

 51.9 13.4 15.5 



Specific gravity, 2.9. 

 S0°-95 c Fahr. 



Other specimens, examined by Prof. Dana, had a hardness of 5-5.25, and a specific gravity 

 of 1.656. 2 



The imitative forms arising from the evaporation of the siliceous solutions in the caves, 

 are often quite curious, some resembling bunches of dried raisins, from a partial collapse of 

 the encrusting bubble. The structure of all is stony and very cellular. 



Acid vapors exercise a more extensive influence, but only in combination with steam. 

 The surface of the black lava is first colored white or yellow, and the decomposition extends 

 throughout the mass, rendering it after a while friable, and if the aqueous vapor is in 

 excess, reducing it to an earth which is often again consolidated by the dissolved silica. A 

 portion of spattered lava exposed to this action may be colored quite differently from the 

 rest of the mass. Thus yellow or orange drops appear on a red ground, brown on a. purple, 

 and so on in great variety. 



In the sulphur banks changes are going on quite rapidly, and the soil which results from 

 the decomposition of the ancient lava of the outer walls of Kilauea does not differ in 

 appearance from that formed in the crater from the fresh black vitreous lava. The stony 

 lava is always first decomposed, and the vitreous crust resists longest. In Puna I have 

 raised large slabs of the crust which were nearly entire, wdiile the stony stream underneath 

 was reduced to a red earth for more than eighteen inches deep. Wherever cracks permit 

 the passage of gases through the crust the interior is attacked. Most of the salts are 

 deposited in these cracks and beneath the crust. 



Clays are formed both by acid vapors and by simple meteoric influence. Those near 



1 Analyzed by Mr. John C. Jackson in the laboratory of Dr. C. T. Jackson. 



2 Geology of Ike United Stales Exploring Expedition, p. 201. 



