462 W. T. BRIGHAM ON THE VOLCANIC PHENOMENA 



When the melted lava is thrown into water, the black gravel called lapillo or 



rapilli results, and the iron which it contains as peroxide becomes magnetic. 



From the loose form of this gravel it is readily transported by water, and is generally 



spread out in horizontal layers near the shore, often covered, as near Honolulu, with other 



alluvial deposits. 



In one blow-hole, Mr. Rexford Hitchcock found some very curious forms of lava which, 

 „. . for want of a better name, may be called as in the margin, for I conceive them 



to have been formed by the action of electric currents on a plastic mass 

 containing much iron. The masses are of considerable specific gravity (2.857), and the 

 surface is arranged precisely as the iron filings place themselves around the poles of a magnet. 

 Similar specimens were found in Vesuvius, and are in the Museum of the Society; but in 

 Kilauea only this one locality has been discovered. 



Only in the eruption of 1789 has volcanic sand or ash been ejected, and this is simply 



irregular, angular, and rounded grains of the ordinary lava of the pools, comminuted 



perhaps by the explosive violence of the eruption. It is not so fine as the ash of 



Vesuvius, and although it is apparently but little acted upon by meteoric influences, rounded 



particles of calcareous matter are found in it in considerable quantities. 



Heated Lara acted upon by Gases. 



The immediate result of gases, usually sulphurous vapors, is to change the external color 

 of the lava, so that as it cools it assumes various shades of brown, red, blue, yellow, and 

 often a metallic hue, so as to resemble brass. The color imparted in this way is quite 

 superficial, and the action of the gas is but momentary, but the results are often very 

 beautiful. The heat of gas-jets seems to remelt the angles of the consolidated lava. 

 Much more important, however, is the 



Decomposition by Aqueous or Acid Vapors. 



A formation which always excites the curiosity of visitors to Kilauea, is found in many 



of the caves in the floor of the crater which have been undisturbed for several years. At 



„, , the first glance the tubes which hang from the roof, and the curiously formed 



Tubes. ° ° '. . J 



droppings beneath these, seem to be of igneous origin, or droppings of melted 



lava from the roof. An examination in situ shows that this was not the case. The roof of 

 these caves is about two feet thick and generally unbroken ; the stalactites do not occur 

 under cracks, and indeed there is often no fresh lava over the surface. The formative 

 process may be clearly seen, as the tubes grow from day to day ; and I have caught the 

 steel gray deposit in the drops on the end of the tubes upon my finger and watched its 

 solidification. Usually the tubes are straight cylinders, from one to three eighths of an inch 

 in diameter, and sometimes more than two feet long. The bore is almost never continu- 

 ous, and while externally they are smooth, within, a mass of stony cells of considerable size 

 is presented. As long as these tubes grow downward in the quiet upper region of the 

 cave they hang perpendicularly, but when they reach farther down the currents of air 

 and steam blow the deposits to one side and the tube becomes distorted ; it may even return 

 on itself. The drip on the bottom forms much thicker and more irregular stalagmites 1 



l A very beautiful stalagmite of shining black obsidian from Iceland, formed with surprising regularity, is in the cabinet of 

 Uarvard College. 



