28 J. I). Dana — History of the Changes in Kilauea. 



cone of 8° or 10° with one of 30°, the latter the average angle 

 of the greater volcanic mountains of western America ; and this 

 suggests important differences in the results of volcanic action 

 independent of those consequent on the possible prevalence of 

 cinder-ejections in the latter. But somehow or other Mauna 

 Loa breaks easily — very easily, its quiet methods say — and it 

 seems to be because such rocks, however thick, can offer but 

 feeble resistance to rupturing volcanic agencies. 



In the discussion beyond of the operations going on and of 

 their causes, I speak, I, of Kilauea as a Basalt- volcano, the basis 

 of its peculiarities ; II, of the size of the Kilauea conduit ; III, 

 of the ordinary work of the volcano ; IY, of its eruptions ; 

 and Y, of the contrast in volcanic action between Kilauea and 

 volcanos of the Yesuvian type. 



I. KILAUEA A BASALT-VOLCANO. 



1. The mobility of the lavas. — The phenomena of Kilauea 

 are largely due to the fact that it is a basalt-volcano in its nor- 

 mal state. By this I mean, first, that the rock-material is doleryte 

 or basalt, and secondly, that the heat is sufficient for the perfect 

 mobility of the lavas, and therefore for the fullest and freest 

 action of such a volcano. It is essentially perfect mobility 

 although there is not the fusion of all of its minor ingredients, 

 that is of its chrysolite and magnetite. This is manifested by 

 the lavas, whether they are in ebullition over the Great Lake, 

 throwing up jets 20 to 30 feet high, throughout an area of a 

 million square feet or more, or when only splashing about the 

 liquid rock and dashing up spray of little lava drops from areas 

 of a few square yards. There is in both conditions the same 

 free movement, almost like that of water, and suggesting to the 

 observer no thought of viscidity. Of the two conditions just 

 mentioned, the former was that of November, 1840, the latter 

 that of August, 1887 ; and that of August seemed to be the more 

 wonderful, because we naturally look for some of the stiffening 

 of incipient solidification where only a few square yards of lava 

 are in sight. 



2. This mobility is dependent largely on the fusibility of 

 the chief constituent minerals of the lava. — Along with 

 augite, a relatively fusible species, the rock contains, as its 

 other chief constituent, labradorite, almost as fusible as augite, 

 and the most fusible of the feldspars. Andesine and oligoclase 

 are less fusible feldspars, and orthoclase is of difficult 

 fusibility. Thus in this prominent physical character the 

 feldspars widely differ, and accordingly there should be, 

 and are, volcanoes of different characteristics, for example, 

 Andesyte volcanoes, in which oligoclase or andesine is the pre- 



