Russell — Massive-Solid Volcanic Eruptions. 265 



should be best defined in the central part of the mass, and 

 radiate outward from a central vertical axis toward the sides. 

 The most that could seemingly be expected, however, would 

 be irregular and confused jointing without the formation of a 

 columnar structure. Once more checking deductions by obser- 

 vation, we find no evidence of columnar structure in the mas- 

 sive-solid material extruded from either the Panum crater or 

 the crater near Pauline Lake. As to the other examples cited, 

 information in this connection is lacking. 



There is one other feature of volcanic rocks which might rea- 

 sonably be expected to occur in those forced to the surface in 

 a massive-solid condition, namely, a brecciated structure ; that is, 

 the presence of angular fragments of the parent lava, adhering 

 one to another or united by portions of the same magma which 

 consolidated about the fragments produced by the fracturing 

 of the portion which cooled earlier. Such volcanic breccias 

 are common in the lava flow about the Mono craters, and in 

 the similar lava stream near Pauline Lake, but have not been 

 observed in the massive-solid extrusion of those localities. 

 From the manner in which massive-solid eruptions take place, 

 however, it seems as if the conditions would favor the pro- 

 duction of brecciated rock of the type just mentioned. 



The crucial test of the above deductions w T ill come when the 

 activity of Pele has decreased sufficiently to permit of a criti- 

 cal study of its obelisk. The prediction may be ventured, 

 however, that it will be found to be composed of massive but 

 somewhat porphyritic rock since the products of the fragmen- 

 tal-solid eruption are of this nature, and without definite col- 

 umnar structure, although irregular or confused jointing will 

 no doubt be present. 



In reference to the topographic forms analogous to those pro- 

 duced by massive-solid eruptions and which might be mistaken 

 for them, there are seemingly but three directions in which uncer- 

 tainty is likely to arise. These are : (1) the similarity in shape 

 and in location of the crags, spines, obelisks, etc. produced by 

 massive-solid eruptions, and the ordinary cones of eruption 

 such as are typically illustrated by the one which forms from 

 time to time within the crater of Vesuvius ; (2) the similarity 

 of obelisks, etc. to the tower-like forms produced by the ero- 

 sion of craters within the conduits of which lava has cooled 

 and hardened, so as to form what are termed volcanic necks / 

 and (3) the tower-like forms resulting from the weathering of 

 lava sheets which rest on less resistant and as is most commonly 

 the case, incoherent sedimentary strata, or beds of tuff, and 

 frequently termed erosion columns. 



1. The necessity of distinguishing between massive-solid erup- 

 tions and cones of eruption arises principally from the fact that 

 in fresh and well-characterized examples of each class, an eleva- 

 tion is present within an encircling crater. Cones of erup- 



