J. D. Dana — History of the Changes in Kilauea. 31 



Here the question suggests itself whether the temperature 

 existing at depths below may not be one of the conditions that 

 determine whether the discharged lavas shall be of the less 

 fusible or the more fusible kind. 



But a basalt- volcano also may fail to have heat enough for 

 perfect fusion, and hence have partially lapidified or pasty 

 lavas, and thus be made to exhibit some of the characteristics 

 of the other kinds of volcanoes. This condition may result 

 from three causes : (1) A decline in the supply of heat of the 

 conduit, as when the partial or complete extinction of the vol- 

 cano is approaching ; (2) When the lava is discharged by lat- 

 eral openings or fissures, in which case the lateral duct of lava 

 may not be large enough to resist completely the cooling agen- 

 cies about it; (8) The sudden entrance of a large body of 

 water into the conduit. 



The effects from the first of these conditions— declining heat 

 connected with approaching extinction — are strikingly exem- 

 plified in two great volcanic mountains of the Hawaiian 

 Islands, Mt. Kea on Hawaii, and Haleakala on Maui. Those 

 of the second, in which the ejections are from lateral openings, 

 are abundantly illustrated in the cinder and tufa cones of the 

 islands, and also in widespread cinder or ash deposits through 

 the drifting of the ejected material by the winds. The third, 

 a sudden incursion of waters through an opened fissure, if a 

 possibility, should both lower the temperature and produce vio- 

 lent projectile results, and even Kilauea bears evidence of at 

 least one eruption of great magnitude which was thus catastro- 

 phically produced ; for the region bordering the crater on all its 

 sides, and to a distance of ten or fifteen miles to the southwest, 

 is covered with the ejected stones or bowlders, scoria and ashes 

 of such an eruption. 



4. Eruptive characteristics of a Basalt-volcano. — The ob- 

 vious results of superior mobility and density in lavas, are, as 

 in other liquids : 



(1) First : greater velocity on like slopes, and thus an easier 

 flow, with less liability to be impeded by obstructions ; a lower 

 minimum angle of flow, and consequently a less angle of slope 

 for the lava cones. 



(2) Secondly : The vapors ascending through the liquid lava 

 encounter comparatively feeble resistance, and hence the ex- 

 pansive force required for escape of bubbles through the lava to 

 the surface is feeble; and so also are the projectile effects due to 

 the explosion of the bubbles. Hence the projected masses com- 

 monly go to a small height — it may be but a few yards — and 

 fall back before cooling, instead of reaching to a height that 

 involves their cooling and solidification in the fall and the 

 making thus of cooled fragments of lava or scoria, called cinders 

 and volcanic ashes. 



