J. D. Dana — Relations of Vesuvius to Jit. Zoa. 173 



There is no reason to regard the forces as different in kind 

 or mode of action. If the outside waters gain slow access, at 

 depths below, to the lavas for the ordinary action of a volcano 

 in Hawaii, they can at Vesuvius ; and the force from the 

 escaping vapors that, in this ordinary action, will make jets of 

 lava of 30 feet to 600 feet, will make jets of cinders of far 

 greater height. Moreover, as the erupting force at Mt. Loa in 

 non-explopive eruptions, is not due to vapors inside the lava-col- 

 umn, since it does its chief fracturing part way down and some- 

 times far down the mountain instead of about the summit, and 

 causes a quiet condition in the crater instead of violent action, 

 so it is essentially at Vesuvius. In explosive eruptions at Ve- 

 suvius, on the contrary, the explosive force is due to vapor-gen- 

 eration inside of the lava- cauldron, the projectile action being 

 vastly increased,. as at Tarawera in 1886 and Krakatoa in 1883 

 (page 104:). 



As the observations at Vesuvius of Scacchi and others have 

 shown (and my own two visits to Vesuvius, one just before an 

 eruption, enable me to appreciate) high-lava mark in the vol- 

 cano, or that of readiness for a discharge, is attained in the 

 same way essentially as in Kilauea, After a down-plunge fol- 

 lowing an eruption (as a result of the undermining), leaving 

 the crater hundreds of feet in depth and the upper extremity 

 of the lava-column at a still lower level, work again soon com- 

 mences, provided the lava- column was not so profoundly cooled 

 off by the aggressive waters and vapor-generating as to be left 

 too deeply buried. For a while the fractures in the bottom of 

 the crater emit only vapors. Later, projectile action begins at 

 one or more points, making conical cinder-deposits by the peri- 

 centric action, with now and then an addition to the inside accu- 

 mulations from small outflows of lava about the bases of the 

 cones or from their vents. The throws of cinders and flows of 

 lava are kept up at irregular intervals, and the level of the floor 

 rises. After the height within has become much increased, 

 small fissures occasionally open through the outside 'slopes and 

 let out some lava ; but the ejections are mostly retained inside 

 except in the later period of progress when some of the high- 

 thrown cinders may fall over the outside of the mountain or 

 drift away with the wind. Years pass ; and finally the crater's 

 bottom, bearing a large cinder cone, or more than one, reaches 

 that high level in which it becomes actually the summit-plain 

 of Vesuvius, and the fires are visible in the cracks of the plain 

 because the liquid lavas are not far below it.* High-lava mark 

 is thus attained and an eruption may be at hand. Severe earth- 

 quakes are not needed in the work any more than at Kilauea. 



* I may refer here to a cut representing Vesuvius in this condition in my Text- 

 book of Geology, made from my sketch in 1834, and to a paper in this Journal 

 for 1835. 



