J. D. Dana— Volcanic Action. 105 
brittle material may be shattered in the projection, under fric- 
tion and sudden cooling, into the finest dust; those of the 
streams flowing from the vents or-pools are cellular and in 
part scoriaceous; over the bottom of Kilauea in 1841, they had 
a scoria-crust of 2 to 4 inches, easily separable from the stony 
part below. They were /roth-covered streams. On the con- 
trary, the melted rock escaping from fisstres in non-voleanic 
regions, and those of the remoter fissures about a volcano, are 
generally stony, withcut a scoriaceous aspect in any part. 
Even over the bottom of Kilauea, lava that had come up 
through fissures distant from the lava-vents was of the more 
solid, stony kind “either solid stone throughout, or so except- 
ing a compact, glassy exterior half an inch thick.”* 
The making of scoria is dependent on the kind of action 
characterizing the open lava vent—that is the aerially exposed 
extremity, or extremities, of a lava-conduit. In a fissure-ejec- 
tion, some one or more places in the fissure may be broad 
enough to remain open as_lava-vents with the usual phenom- 
ena. 
7. Abundant discharge of vapors of water—The exposed ex- 
tremity of a lava conduit discharges vapors in great volume, the 
amount of which increases with the increase of activity. When 
at Kilauea in 1841, an immense column of heated vapors stood 
over the great lava lake Halema’uma’u (1,500 1,000 feet by 
Wilkes’s measurement) and the southern half of the crater— 
its base some hundreds of feet above the lake, owing to the inten- 
sity of the radiated heat. It had the appearance of a threatening 
storm-cloud as Wilkes observes (iv, 225).t Yet this was six 
months after a great eruption. At times of extreme activity 
the pit is sometimes so densely filled with the vapors as to 
‘obscure all the glowing fires.” 
Many facts show that the vapors are chiefly those of water, 
as is generally believed. ‘The features of the clouds were those 
of ordinary clouds, and the shrubbery and ferns about the 
margin of the crater derived luxuriant growth from the con- 
densed moisture. 
No direct experiments were made; but we have from Mr. 
Hmerson on page 90 of this volume, the statement that he 
was half an hour at his station near Halema’uma’u W 
exposed to the vapors that came with the wind from over 
the pit, and suffered nothing from the sulphurous fumes pres- 
ent; a fact showing that they constitute a very small part of 
the vapors. 
While vapors are so freely discharged from the craters, the 
amount from fissure eruptions, even those in a volcanic region, 
* My Expl. Exped. Report, p. 180. + Narrative Expl. Exped. 
