52 PROCEEDINGS OF THE AMERICAN ACADEMY. 



over most of the area covered by that ocean. For example, geodetic 

 results seem to suggest that the Pacific sediments rest, more or less 

 directly, on a terrane whose average density is nearer the density of 

 crystallized basalt than that of granite or common gneiss. 



In the case of the greater lava-fields, the abyssal fissures have been 

 prolonged, as true fissures, to the surface ; there a leading condition of 

 extrusion is probably the simple hydrostatic adjustment of magma to 

 a breaking earth-crust. Most central eruptions have occurred in locali- 

 ties where emitting fissures are not visible. In emphasizing this fact 

 some geologists have been disposed to deny that fissuring of the crust 

 is a prerequisite of vulcanism. Their reasoning is strong so far as the 

 actual opening of many vents is concerned, and one result has been to 

 sharpen the eyes of vulcanologists as to field relations. But we may 

 agree with Supan that " one should not throw out the baby with his 

 bath-water." Though a vent, for the last few hundreds or thousands 

 of meters, may have been opened by explosion or by other means than 

 simple fissuring, a fair study of geological maps shows the existence 

 of strong abyssal fissures beneath most of the volcanic piles which 

 are not visibly located on surface fissures. Obviously, an immediate 

 tendency of volcanic action is to cover such feeding fissures. Yet their 

 existence is to be inferred from the group alignment of the greater 

 cones. Such is, of course, the traditional explanation of groups like 

 those of the American Cordilleras, of the Hawaiian series, the Samoan 

 series, or the Antillean series. Some clusters of vents show no definite 

 alignment ; instances like the " embryo " volcanoes of Suabia will be 

 discussed on later pages. 



We may, therefore, assume abyssal fissuring and magmatic injection 

 to be the essential prelimi?iary to vulcanism, as it is, indeed, to all 

 igneous action. Probably most vulcanologists have made the same 

 assumption, but the literature of vulcanism nowhere contains a full 

 statement of some of the chief theoretical consequences of the assump- 

 tion. Some authors, either expressly or tacitly, refuse to believe in 

 the principle, but their arguments against it have never been sufficiently 

 supported by the facts of structural and areal geology. The matter is 

 so fundamental that it has here been duly stated ; it may savor of 

 a truism to many working geologists. 



Conditions of Abyssal Injection. 



The principle may be accepted, though the circumstances permitting 

 of injection at depths of ten, twenty, or more kilometers are not yet 

 completely understood. This much more difficult subject has been 



