266 Russell — Massive-Solid Volcanic Emcptions. 



tion, however, in all known instances are composed of highly 

 scoriaceous material, consisting of lava blocks, lapilli, bombs, 

 cakes formed of splashes of liquid lava, dust, etc., and have a 

 tube or conduit within, leading upward to a crater at the top. 

 Such structures are in fact miniature volcanic cones of the 

 same general character as the greater cones in the craters of 

 which they are formed. In all of the characteristics mentioned 

 the differences between cones of eruption and massive-solid 

 extrusions are obvious and need no further discussion. 



2. The similarity between volcanic necks exposed by the 

 removal of their enclosing cones, and the tower-like forms pro- 

 duced by massive-solid extrusion, as in the case of Pele, 

 is most striking. This similarity approximates to identity, 

 inasmuch as a volcanic neck and a volcanic obelisk may be por- 

 tions of the same lava column ; the former being the material 

 within a conduit which cooled in place, or if forced upward 

 did not emerge from its enclosing tube, while the latter repre- 

 sents the summit portion of a congealed lava column that has 

 been forced out of its parent conduit. 



Fresh and uneroded obelisks are not to be mistaken for vol- 

 canic necks exposed by erosion, because of their freshness and 

 the presence about them of crater walls,, or evidence of the 

 destruction of such encircling rims by explosions, or their 

 burial beneath the debris falling from the obelisks themselves, 

 etc. It is in drawing distinctions between much weathered 

 obelisks afnd volcanic necks when exposed by erosion, that 

 difficulty is likely to arise. 



In the case of well-characterized volcanic necks and good 

 although much weathered examples of volcanic obelisks, it 

 seems possible to draw a distinction, although their shapes, 

 positions, etc., are essentially the same. The material forming 

 a volcanic neck cools slowly on account of the insulation 

 afforded by its enclosing cone, and would be expected to form 

 a well crystallized and, in the case of most lavas, a porphyritic 

 rock. Owing to slow cooling, also, the rocks of volcanic necks 

 should exhibit a w r ell defined columnar structure. Such we 

 know to be the case in certain typical examples situated in the 

 northwestern part of New Mexico and having a height of from 

 800 to 1000 feet above the adjacent plain, concerning which 

 Major C. E. Dutton* writes as follows : "In all of these necks 

 the basalt is columnar. The columns stand or lie in all sorts of 

 attitudes, and in most cases are curved. Frequently they are 

 grouped in radiating fascice, and at times are flexed and re- 

 flexed." The columns are described as varying in size from 

 Ave or six inches to more than twenty feet across ; the larger 

 ones being generally vertical. 



* Mount Taylor and the Zuni Plateau, in Sixth Annual Report of the U. S. 

 Geological Survey, Washington, 1886, p. 172. 



