272 H. S. WASHINGTON PERSISTENCE OF VENTS AT STROMBOLI 



larger phenomenon we are not here concerned; nor have we to do with 

 the shifting of activity along a radial fissure, snch as that of Manna Loa 

 or those of Etna. 



Confining onr attention chiefl}^ to Stromboli, as this represents the 

 more nsnal type of volcanic activity, and as I am personally unacquainted 

 with Kilauea, we may examine into the cause of this persistence of a vent 

 or vents for a considerable time at one of the halting places of the general 

 shift and discuss briefly its bearing and that of the other features on some 

 questions of the mechanism of volcanic action. 



There is considerable vagueness in our ideas as to the conditions which 

 obtain beneath a volcanic crater. The presence of some sort of magma 

 reservoir seems to be a necessary assumption; but in the standard works 

 on volcanoes and volcanism opinions either differ or are not expressed as 

 to the depth of the top of this below the crater floor, and the origin, char- 

 acter, shape, and size of the conduit which connects the magma chamber 

 Avith the surface. It may be added that practically all the ideal sections 

 which have been published, not being drawn to scale (for obvious rea- 

 sons), tend to give erroneous and misleading ideas of the true relations. 

 In general, the magina reservoir is supposed to lie at a very considera- 

 l)le depth (of the order of more than a kilometer) and to connect with the 

 surface through one conduit, which is usually regarded as a local widen- 

 ing of the fracture line on which the volcano is supposed to stand. Very 

 little mention is made of the size of this conduit, but it would appear that 

 Dana's view,^^ that the area of the conduits at Kilauea and Loa is as 

 large as or larger than that of the crater, is generally accepted as appli- 

 cable to most volcanoes. Daly, on the other hand, thinks** that the diam- 

 eter of the conduit at Kilauea and elsewhere is very small relatively to 

 that of the crater and is of the order of magnitude of some tens of meters. 

 It may be said at the outset that the persistence of the Stromboli vents 

 Avould seem to be best in accordance with Daly's view. Were the lava con- 

 duit of a size commensurate with the area of the crater terrace, the top 

 of the lava column being close beneath the floor of this, one would expect 

 vents to break out through the thin cover without regularity or continuity, 

 at different chance points of weakness over the terrace floor at each suc- 

 cessive eruption or accession of activity and not persist in their locations, 

 as they have done. 



On the contrary, from the persistence in location, the size, and mode 

 of activity of the vents, we may safely draw the following conclusions : 

 1. The vents are the mouths or surface openings of conduits which 



« J, D. Dana: Characteristics of volcanoes, New York, 1890, p. 151. 

 *^ R. A. Daly : Igneous rocks, New York, 1914, p. 280. 



