DALY. — THE NATURE OF VOLCANHC ACTION. 97 



mical stresses do not seriously deform an abyssal injection durin",' its 

 lifetime as the feeder of a central vent. During such a period, which 

 may be thousands of years in length but seldom or never millions of 

 years in length, crustal readjustments must be minute, for even the 

 greatest lava flows that could have been thus squeezed out at central 

 vents are always very small in relative measure. The last-mentioned 

 fact and the persistent recurrence of eruption at the main vent appear 

 to forbid the h}'pothesis that renewal of activity at central vents is due 

 to renewal of injection along new abyssal fissures. It would be highly 

 improbable that the vent of a second injection would coincide with that 

 of the first injection ; and on the other hand, the great crustal disturb- 

 ance accompanying the second injection should normally cause first- 

 class lava floods at the initial vent, instead of the comparatively 

 insignificant flows actually observed at central vents. Difficult as 

 the problem is, the change from donnancy to activity does not, in 

 general, seem to call for anything so drastic as a strong deformation 

 of the earth's crust in its entire thickness. 



In conclusion, the gas-fluxing hypothesis appears to be worthy of a 

 leading place among those which can be constructed to account for the 

 stubborn persistence in the revival of activity at a vent like Mokua- 

 weoweo, Vesuvius, or Etna. 



Small Size of Central Vents. 



The gas-fluxing hj^pothesis accounts for other general features of 

 central eruptions. The small cross-sections of the vents at Kilauea, 

 Hualalai, Mauna Loa, and even at Mokuaweoweo, as everywhere else 

 in the world, are all of the order of size expected if the fluidity of each 

 lava column is due to the slow passage of relatively minute masses of 

 gas through those vents. 



The writer is not able to agree with J. D. Dana, that the conduits 

 beneath Kilauea and Mauna Loa are nearly equivalent in horizontal 

 section to the great sinks (" calderas ") in which the lava lakes are sit- 

 uated. Each of those sinks measures roughly five kilometers by three 

 kilometers. The periodic rise and fall of the floor of the Kilauean sink 

 (the only one carefully studied) can be explained on the assumption 

 that its conduit has a much smaller cross-section. The " New Lake " 

 after five years of activity, was emptied in 1 SSfi, and was proved to have 

 had a depth of only a few meters. It was a saucer-shaped sheet of lava 

 resting on solid rock. (Compare Plate III.) When the present Ilale- 

 maumau is emptied, the lava runs out through a very narrow hole appar- 

 ently less than thirty meters wide, and leaves a broad, funnel-shaped 

 cavity. The action is like that of water running out of a domestic sink 



VOL. XLVII. — 7 



