86 J. D. Dana — Summit Grater of Mt. L 



oa. 



lengths from the end of one year to the beginning of another, 

 reckoned in months, being 5, 3, 10, 6, 12. After February of 

 1877, there was the longer interval of 3-J years. Such short- 

 period alternations seem to imply the recurrence after each of 

 a subterranean discharge somewhere, if not a subaerial. The 

 display of 1877 quite certainly ended in a submarine eruption, 

 and probably that of 1872, (pp. 29, 26). 



4. The changes in depth of the summit-crater. — The changes 

 since the year 1834, when the crater was visited by Douglas, 

 have diminished its depth by at least 400 feet, if we may 

 trust — as we probably ought to do — his measurement " with a 

 line and plummet," making it 1,270 feet. In 1840, Lieut. 

 Eld, U. S. N., of the Wilkes Exploring Expedition, made the 

 depth on the west .side 784 feet (p. 16), and in 1885, J. M. 

 Alexander, 800 feet, (plate II). 



We know nothing as to variations in the level of the floor 

 after and before an eruption, and nothing as to the down- 

 plunges which have followed discharges. The terrace-levels 

 situated at the north and south ends of the crater may mark 

 high lava-levels just previous to some ancient eruption ; but 

 they antedate history ; for Wilkes's map (p. 17) shows that they 

 existed in 1840 very much as now. The map, Plate 2, by J. 

 M. Alexander, which contains his '' estimates" of the depths of 

 the terraces or "plateaus" below the highest point or summit, 

 makes the terrace at the south end on a level with the uj^per 

 of the two at the north end, suggesting thus that the two may 

 mark one of the high-lava levels of the crater. In addition, 

 it places the bottom of the South Crater (D), and that of the 

 pit in the upper north terrace or plateau ( A'), at or below the 

 level of the bottom of the central crater, favoring the view that 

 all three parts of Mokuaweoweo are still in active connection ; 

 which view is sustained by the facts (1) that the fountain of 

 May, 1880, was a South Crater fountain, and (2) that the pit 

 A' was formed since 1874, as it is not in Lydgate's map of the 

 crater of that year. 



2. The Ordinary activity of the Mt. Loa crater. 



1. General course of action. — Although but few ascents to 

 the summit-crater have been made since the first by Douglas 

 in 1834, and only four of these found the crater in action, 

 there are still facts enough for important conclusions. The 

 cycle of changes has been, beyond doubt, the same essentially 

 as in Kilauea ; that is, when a discharge takes place : (1) the 

 lava of the lava-column within the central conduit of the moun- 

 tain falls to a level some distance below the crater (say one 

 or more hundred feet), as a consequence of the loss by the out- 



