228 J. D. Dana — History of the Changes in Kilauea. 



and levels of the lavas of conduits. If only to a depth of 

 200 feet, an average of 20 per cent, of vesicles would add 

 only 40 feet to the height or level of the surface. 



But if the vapor particles at all deeper depths are, through 

 their expansive force, undergoing gradual expansion as they 

 work their way or are carried upward, we are still further in 

 the dark as to the amount of effect of vapors on the bulk of 

 the lavas in a conduit. After my observations of 1840, 1 was 

 led to question, as I state in my Expedition Report, whether 

 the effects from this means might not be sufficient to account 

 for much of the excess of elongation of the Mt. Loa column 

 over that of Kilauea. This is obviously not so. But how 

 much the elongation, is an important question, and it has still 

 to remain unanswered. 



4. WORK OF VAPORS GENERATED OUTSIDE OF THE CONDUIT: FRACTURES, 

 DISPLACEMENTS AND OTHER RESULTS. 



The conduit has hot rocks around it ; and beneath the floor 

 of the crater there are hot rocks abont and over its upper ex- 

 tremity. The descending waters are driven back as vapor, and 

 usually in a harmless manner. But a sudden incursion of sub- 

 terranean waters happening under any circumstances, might 

 produce confined vapors of great force. The natural effects 

 of the pressure of such confined vapors are fractures, elevations 

 and subsidences, and, where pressure is brought to bear in a 

 confined place on a source of liquid lavas, their injection into 

 any open fissure at hand. 



These effects belong mostly to times of eruption ; but in a 

 lighter form, they may be part of the ordinary work of the 

 crater. The lava-lakes of the bottom, even in quiet times, 

 often have large over-flows, and also out-flows through fissures, 

 that is both sujperfiuent and effluent discharges ; and it is probable 

 that the cause here considered may be the occasion of part of 

 them. 



Confined vapors are often generated also by the action of 

 the heat of a lava-flow on moisture underneath. As rains fall 

 almost every day at Kilauea, there must be more or less mois- 

 ture underneath many parts of the cold floor ; and if a few 

 hours flow from the great lake should flood it with liquid rock, 

 its 2000° F. which the bottom of the stream carries along and 

 does not at once lose, would make vapor out of the moisture, 

 having great expansive force. The large dome-shaped bulg- 

 ings of the lava-streams and other undulations of the surface are 

 thus accounted for on a former page (xxxiv, 356) ; and many 

 of the steaming fractures of the floor as well as those of the 

 domes may have the same origin. 



The next topic under the head of . the Ordinary work of 

 Kilauea is " the Ascensive force of the conduit-lavas." 



[To be continued.] 



