16 J. D. Dana — History of the Changes in Kilauea. 



and 1868 — distinguished by a down-plunge in \;he floor of the 

 crater making in each case a lower pit several hundred feet 

 deep. Others, as those of 1849, 1855, 1879, 1886, were minor 

 discharges, discharges simply of the active lakes, without any 

 appreciable or noticed sinking of the floor of the crater. The 

 eruption of 1849 might be questioned ; but it was preceded 

 by far more activity in the crater than that of 1886. Other 

 subterranean discharges may have occurred since 1840 of which 

 no record exists. Even small breaks below might empty 

 Halema'uma'u. 



The mean length of interval between the first three erup- 

 tions was 8 to 9 years (xxxiv, 81). The great eruption of 1789, 

 the only one on record before that of 1823, occurred 34 years 

 back of 1823, or 4 X 8-J years ; and the 1868 eruption was 

 3 X H years after that of 1840. 



The above approximate coincidences in interval and multiples 

 of that interval seem to favor some law of progress. But it is 

 not yet proved that they have any significance. The minor 

 eruptions which have been referred to above have intervals 

 varying from 6 to 13 years. Moreover, looking to the summit 

 crater of Mt. Loa for its testimony, we find still greater irregu- 

 larity, the successive intervals between its six great outflows 

 from 1843 to 1887 being 9, 4, 3J, 9, 12^, 6J years. 



A partial dependence of the activity of the fires on seasons 

 of rains was suggested by Mr. Coan ; and there is some 

 foundation for the opinion in the times of occurrence of the 

 Kilauea discharges mostly within the four months, March to 

 June, as shown in the following table : 



1823 March? 1855 October. 



1832 June (Jan. ?) (xxxiii, 445) 1868 April 2. 



1840 May. 1879 April 21. 



1849 May. 1886 March 6. 



In addition, there was a brightening of the fires around the 

 crater in October of 1863 ; and again in May and June of 1866 ; 

 whether followed by a discharge of the Great Lake is not 

 known. The future study of the crater should have special 

 reference to this point. 



2. Mean rate of elevation of the floor of the crater after the great 



eruptions. 



After the eruption of 1823, between the spring of that 

 year and October of 1829, an interval of 6^ years, the 

 bottom, if the depth was 800 feet as inferred after the 

 measurement of the upper wall by Lieut. Maiden, rose at a 

 mean annual rate of 138 feet, or, taking the depth at 600 feet, 

 of 93*3 feet. Lieut. Maiden's 900 feet for the upper wall, 



