50 Dana — Dodge's Observations on Halemct umci? u. 



It will be remembered that in April, 1886, a month after the 

 eruption, Mr. J. S. Emerson found the basin 570 feet in depth 

 at middle and 175 to 200 feet deep over a broad border re- 

 gion. The condition is represented approximately (from Mr. 

 Emerson's measurements) in the profile section, A. Three 

 months later, July 20th, Prof. Van Slyke reported that " a cone 

 of loose blocks" had been formed within the basin "perhaps 

 150 feet high." This is the first notice of the cone.* 



In the first week of the following October (1886), less than 

 six months after Mr. Emerson's survey, Mr. Dodge made his 

 first survey. He found the cone standing at the center of the 

 basin, with the "broad border region" around it little changed 

 from the condition observed by Mr. Emerson ; but owing to its 

 depth, the top of the cone at s was only 2 to 5 feet above the 

 level of the western rim and at t, 28 '7 feet below the same 

 level. The width of the cone at top on an east-and-west line 

 was found to be 1100 feet. The general profile, deduced from 

 the survey, is shown in section B. Ten months later, in August, 



1887, the writer found the position of the cone, judging from 

 his estimates, as represented in fig. C. After another eleven 

 months, in July, 1888, Mr. Dodge made his recent survey. 

 The basin was very nearly obliterated, and some parts, as 

 already mentioned, were higher than the level of the rim ; as a 

 consequence the debris-cone stood with its whole height emerged. 

 This is illustrated in the fourth of the above sections, D. 



From the levels obtained by Mr. Dodge at his two surveys 

 in October, 1886, and July, 1888, and by Mr. Emerson in April, 



1888, we have data for determining the rate of change of 

 level. (1) The change in the western rim of Halema'uma'u 

 was nothing; (2) in the summit s, 167*2 feet; in the sum- 

 mit t, 171 "4 feet. The time during which this rise of ap- 

 proximately 170 feet took place was about 650 days, giving for 

 the mean daily rate of rise 3 - 15 inches. 



The small ejections, going on over the basin outside of the 

 cone during the two years past, raised to some extent the level 

 of the floor. But whatever the amount it does not affect the 

 calculation, this being based on changes in .the level of the 

 summit, which received no additions from ejections or any 

 other source. 



The conclusion of . Mr. Dodge that the cone within Hale- 

 ma'uma'u, and the floor of the basin about it had been "floated 

 upward " on the rising lavas appears, therefore, to be the only 

 satisfactory explanation of the change of level. 



* Emerson, this J., Ill, xxxiii, 87; Van Slyke, ibid., 95, 1886. 



