32 



KihiHca and Mauna Loa. 



irregular. They are sometimes coated with beautiful white crystals of gypsum, some- 

 times tipped with ueedle-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 drip 

 on the floor of the cave forms much thicker and more irregular stalagmites, as may be 

 seen in Plates XLVIII and XLIX. 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 

 acrogois^ none of the 

 suspended silica run- 

 ning down the sides. 

 Fig. 28. The process of 

 formation is this: the 

 water from the fre- 

 quent rains, and the 

 condensing steam, act 

 upon the soluble por- 

 tions of the superin- 

 cumbent rock, carrying 

 along the silica and lime 

 to be deposited in the 



form of tubes and their incrusting gypsum, and the resulting stone is quite anhydrous, 

 as will be seen from the following analysis of specimens not coated with gypsum:'^ 



SiO, ALO3 Fe.O,, MnO Ca O MgO Na.O K.O 



51.9 13.4 J5.5 0.8 9.6 4.8 3.0 i.i = 100. 1 



Fig. 29. BLOW-HOLE SI'KCIMKN. 



Specific gravity, 2.9. The temperature of the caves is usually from So- 95 Fahr. 

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

 gravity of 1.656.'^ The imitative forms arising from the evaporation of the siliceous 

 solutions in the caves are often quite curious, some resembling bunches of raisins, from a 

 partial collapse of the incrusting bubble. The struAure of all is stony and very cellular. 

 The microscopic structure is clearly described b_v Dana in the volume referred to. 



'■•Aualy/.L-d by Mr. John C. Jackson in the laboratory of Dr. C. T. Jackson, Boston. 

 "Geology of the United States Exploring E.xpedition, p. 2ot. 



[410] 



