68 PROCEEDINGS OF THE AMERICAN ACADEMY. 



were built along the fissure, which emitted floods of basalt on the 

 greatest scale recorded by man. Escape of lava from the abyssal in- 

 jection was evidently much easier at some points along the visible fis- 

 sure than at others. The case is analogous to the formation of the 

 " Dewey craters " (cinder-cones) on the Mauna Loa lateral fissure 

 opened in 1899, and of scores of similar accumulations on the flanks of 

 Mauna Loa, Etna, etc. Dutton^^ gives this explanation for some of 

 the necks occurring in the well known Mount Taylor district of New 

 Mexico. In all such instances certain poitits in the fissure-lines are 

 favored in the eruptivity, while the remainder of each fissure was 

 either never opened clear to the surface, or else was rapidly sealed up 

 by congealing lava. 



The continuance of eruption at any point depends on victory in the 

 strwjgle with cold. That victory in its turn depends in part on a suffi- 

 cient width of vent to permit of a column of lava which is not chilled too 

 greatly by conduction into the wall-rock. Since erupting fissures are 

 never more than a few meters in width at the surface, it seems neces- 

 sary to postulate a widening of each fissure where it carries cone and 

 crater of prolonged activity. The widening may be conceived to de- 

 pend on four different factors : solution and mechanical removal of 

 wall-rock by emanating lavas ; melting and explosive abrasion of the 

 wall-rock by magmatic gas emitted through the lava column. It is not 

 important here to decide on the relative efficiency of these processes in 

 merel}' enlarging the original fissure to full vent size. Their relative 

 efficiency becomes of fundamental significance in the problem of the 

 persistence of eruptivity at a central vent. In the following discussion 

 of this topic it is concluded that the vent is kept hot, and therefore 

 active, because of the emanation of free juvenile gas rising from great 

 depth — a process which may be styled "gas-fluxing." Since a great 

 enlargement of an original fissure, below the bottom of any possible ex- 

 plosion funnel, demands much time, it would follow that most of the 

 enlargement is due to gas-fluxing. Gaseous explosion and erosion of 

 the walls by emanating lava might be more eff'ective in the widening 

 of smaller and more short-lived vents. 



It is an easy step from the observed case where central eruptions 

 are developed on fissures of lava-flooding, to the case of the formation 

 of central vents on surface fissures from which no true fissure-eruption 

 has ever taken place. Such a crack may be too narrow to permit the 

 extrusion of gas-free lava, which, through quick chilling, seals the fis- 

 sure, and yet the crack may be wide enough to allow passage of the 



*• Sixth Annual Report of the United States Geological Survey, 1885, 

 p. 172. 



