﻿J. D. Dana — Dissected volcanic Mountain. 253 



the lower or outer part of the mountain. Toward the coast 

 stratified lavas are seen along the valleys, and the layers are 

 usually but twenty feet thick or less. The vesicular lavas, the 

 larger part a chrysolitic basalt, alternate with layers of tufas 

 and conglomerate. But five or six miles up one of the valleys 

 (the Papenoo), the layers of lava were much thicker, and, as 

 my Keport states, some bluffs a thousand feet in height consti- 

 tute apparently a single bed, and are more or less columnar 

 throughout. This greater thickness of the beds on approach- 

 ing the center appears to show that the eruptions were far 

 more copious early in the history of the volcano than they were 

 later. But this is not an explanation of the massive structure 

 of Orohena. The uniform massiveness through so great height 

 at the volcano's center has plainly come, as I long since stated, 

 from the cooling of continuously liquid lava at the center, or 

 in the region of the great central conduit of the cone. 



We appear to have an explanation of the facts, or a sug- 

 gestion toward an explanation, in the condition of Mount Loa, 

 on Hawaii. At the beginning of one of its great eruptions the 

 central mass of liquid lavas stands at a height of more than 

 12,000 feet above the sea-level; and the breadth of the liquid 

 mass at top, if of the size of the crater (as it sometimes has 

 been), is over 8,000 feet. To appreciate the conditions it is to 

 be considered that the summit-crater of this great volcano has 

 been in eruption on a grand scale eight times since 1843, — the 

 last time in 1880-81; or, on an average, once in five years; 

 and that the fires at some of these times of activity have con- 

 tinued active, at heights of 10,000 to 12,000 feet, for 10 to 18 

 months. A mass of liquid rock at the mountain's center 

 10,000 to 12,000 feet high above the sea level and at least 

 5,000 feet in breadth near the top is not an exceptional condi- 

 tion in the volcano ; it may be often true.* If 5,000 feet 

 broad at top, the breadth would be greater below as it de- 

 scended toward the deep region of continuous or wide-spread 

 liquidity. Suppose such a mountain to have its last eruption 

 and then to cool off, and become extinct. The central liquid 

 mass, in such a case might perhaps retreat downward as a conse- 

 quence of escape, either outside or subterranean, and a great 

 cavity be left. But if pressure from below and resistance to 

 fracture should hold the lavas to their place, as it did in the 

 case of the ejections making the laccoliths of the Gilbert 

 Mountains described by Professor Gilbert, they would cool 

 there, and the liquid central mass would become the solid 

 central mass. 



* On the size of the crater (called Mokuaweoweo) and the height of the moun- 

 tain, see page 235 of this volume ; and on the area of sinking, and therefore 

 of liquid lavas, in Kilauea at one of its eruptions, see the last volume, p. 398. 



