DALY. — THE NATURE OF VOLC.YNIC ACTION. Ill 



strongly convex to the eastward. They there portray a long arch- 

 shaped spur running out from Mauna Loa. From sea-level at the cape, 

 the crest-line of the spur rises at the average rate of about twenty 

 meters per kilometer, or 1 in 50. At Kilauea the spur is plateau -like, 

 and immediately to the westward of the Kilauean "sink" there is a 

 faint downward slope to the west. This arrangement of slopes does 

 not mean that Kilauea is a crater surrounding a cone. It is rather a 

 large pit or "sink" in the side of ]\Iauna Loa (Plate II, A). Along 

 the Kau coast, southwest of Kilauea, the Mauna Loa slope varies from 

 1 in 10 to 1 in 15, averaging about 1 in 12. 



The broad spur is capable of two interpretations. It has either been 

 built up by the specially prolonged extrusion of lava in this region, or 

 it is due to local deformation of the general Mauna Loa slope, as if by 

 deep-seated intrusion of the laccolithic type. 



There is no sign that Kilauea has ever overflowed the outer walls of 

 its great sink. The writer tried, in the field, to answer that question 

 definitely by plotting the attitudes of the festoons of " ropes " on the 

 pahoehoe flows forming the surrounding surface, but was defeated, 

 through the failure to find a sufficient amount of that surface not 

 covered by ash deposits. In any case, however, the general topography 

 shows that no significant part of the spur could have been built up by 

 overflows from this vent. Lava seems never to have issued from most 

 of the many pit-craters situated on the back of the spur, and the total 

 effect of activity of that kind at the other craters in the whole Puna 

 district may have been a vanishing quantity so far as the development 

 of the arch is concerned. 



On the other hand, it is evident that lava flows from the upper 

 slopes of Mauna Loa could not have constructed the spur in its present 

 form. Such flows as do run down the southeast flank of the main vol- 

 cano are deflected by the arch and run either northeastwardly toward 

 Hilo, or southwardly into southern Kau. Finally, the spur does not 

 apjjear to owe its principal volume to a succession of fissure eruptions 

 along its a.xis. The flow of 1840 may be an exception tending to prove 

 the rule. It represented a subterranean discliarge of the magma 

 chamber of which Kilauea is still the active vent. The formation of 

 that chamber is really the point at issue. The injection of its magna 

 might have followed or directly caused the formation of the spur; in 

 either case extrusion of lava from the chamber is a wholly subsidiary 

 and unessential fact. Furtlier field work is necessary to determine how 

 far the surface of the spur has been rai.sed by local eruptions like that 

 of 1H40. 



The alternative explanation of the spur-arch regards it as a mass of 



