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



immense basaltic outflows of other regions exemplify. In 

 Hawaii the heat required for the existing mobility is no greater 

 than the deep-seated conditions below the mountain can keep 

 supplied, in spite of cooling agencies from the cold rocks, the 

 subterranean waters and the air ; it is no greater than it can 

 continue to supply for half a century and more, as the records 

 have shown ; and supply freely to the top of a conduit 3000 

 to 3500 feet above the sea-level, and even to the top of 

 another conduit but twenty miles off, rising to a height of 

 13,000 feet above the sea-level. The temperature needed for 

 this mobility judging from published facts, is between 2000° 

 F. and 2500° F. The fusing temperature of augite and lab- 

 raclorite has not yet been determined. We are certain that 

 a white heat exists in the lava within a few inches of the 

 surface ; for the play of jets in a lava-lake makes a dazzling net- 

 work of white lightning-like lines over the surface ; and white 

 heat is equivalent to about 2400° F. Considering the height 

 of Mt. Loa and the greatness of its eruptions, and the vastness 

 of basaltic outflows over the globe, we may reasonably assume 

 that the temperature needed for the normal basalt-volcano has 

 long been, and is now, easy of supply by the earth for almost 

 any volcanic region ; and that the difficulty the earth has in 

 supplying the higher heat for equal mobility in a trachyte or 

 rhyolyte volcano is the occasion of the common semi-lapidified 

 pasty condition of their outflowing lavas. 



Even if the higher temperature required for orthoclase- 

 lavas, were always present quite to the surface in the volcano, 

 the ordinary cooling influences of cold rocks and subterranean 

 waters and air would be sure to bring out, in some degree, 

 on a globe with existing climatal conditions, the characteristics 

 of the several kinds of volcanoes designated. 



I do not say that this higher heat required for the complete 

 fusion of trachyte or rhyolyte is wanting at convenient depths 

 below ; for it has been manifested in the outpouring of vast 

 floods of these rocks through opened Assures, many examples 

 of which over the Great Basin are mentioned in King's 

 " Systematic Geology" of the 40th Parallel. But in the volcano, 

 whose work, after an initial outflow, is carried forward by 

 periodical ejections and requires for long periods a continued 

 supply of great heat, the more or less granulated or pasty 

 condition of the outflowing orthoclase-bearing lava streams is 

 the usual one. Consequently, when a volcano changes its 

 lavas from the less fusible to the more fusible, as sometimes has 

 happened, some change in the features of the volcano should 

 be looked for, except perhaps when the change occurs directly 

 after the initial discharge. 



