DALY. — TUE NATURE OF VOLCANIC ACTION. 113 



has been decreasing in its activity with fair regularity. The very rapid 

 rate of decline suggests a relatively small feeding chamber. 



3. The iuiperfect observations so far made seem to show that the lava 

 fountains of Mokuaweoweo more nearly a])proach brilliant-white heat 

 than do those of Kilauea in the hottest state of its present eruption. 

 The difference of temperature may be as much as 200°C.'*2 Assuming 

 that the magma of the Kilauean laccolith originally had the higher of 

 these temperatures, the cooling would be of the order expected if the 

 laccolith were of good size and not more than a few thousand years old. 



If the average slope of Mauna Loa, before the laccolith was injected, 

 was one in twelve, as now in southern Kau (obviously an uncertain 

 assumption), a minimum estimate of the volume of the laccolith may 

 be made. The top of the broad arch just east of Kilauea must have 

 been raised at least 1000 meters, which is, therefore, the minimum 

 thickness of the laccolith in that large area. The average thickness, 

 as shown by the surface deformation, would be at least 500 meters and 

 the area at least 500 square kilometers. The corresponding volume 

 of the laccolith is 250 cubic kilometers. A body of magma of that 

 size and endowed originally with the high temperature demonstrated 

 at Mokuaweoweo, might, in part, remain fluid for 2000 years. There 

 is nothing to indicate that Kilauea is so old. 



Clearly little stress is due any such computation of the size. It can 

 give only an order of magnitude. The magmatic body may be much 

 larger, and it may have a form not ideally laccolithic. 



4. Kilauea is the largest, the last, and most persistent of all the pit- 

 craters for some good reason. The shape of the arch on which it stands 

 suggests that it overlies the highest point in the roof of the laccolith, 

 where the fluxing gases would naturally accumulate in largest quantity 

 and where they would finally tend to emanate from the magma cham- 

 ber. The smaller, extinct craters may easily represent so many points 

 where irregularities in the roof, lower down, temporarily caused local 

 accumulations of the gases. 



5. Like the pit-craters, important systems of earthquake cracks like 

 those of 1868 southwest of Kilauea, are concentrated in the Kau-Puna 



'' Quite recently Dr. Tempest Anderson has recorded that, even in day- 

 light, the lava of the active Matavanu lava lake in Samoa was, at the time of 

 his vi.sit in 1909, of brilliant-white incandescence (Quart. Jour. Geol. Soc. 

 London, 66, 1)27 and (J.J2, 1910). This indicates a temperature little, if any- 

 thiuR, short of 1400° C. The color of the hottest lava at the "Old Faithful" 

 fountains of Kilauea of the same year was, in the daytime, a brijiht orange, 

 corresjjonding to a temperature not far from 1200° C. The intensity of tiie 

 heat at Matavanu has been, for years, of the order represented in the great 

 eruptions of Mauna Loa. 

 VOL. XLVII. — 8 



