Russell — Massive- Solid Volcanic Eruptions. 267 



Then, too, volcanic necks may contain rocks of any chemical 

 composition ranging from nitra-basic to ultra-acid, that are 

 extruded by volcanoes ; while, as already stated, there seem to 

 be good reasons for concluding that massive-solid eruptions 

 occur only in the case of volcanoes which are supplied with 

 highly refractory lavas. 



With these considerations in mind, any monumental rock 

 that simulates an obelisk in form, which is composed of basic 

 or, more strictly, easily fusible material, might with greater prob- 

 ability be classed as a volcanic neck, then as a volcanic obel- 

 isk. And again, if the material composing such a monument 

 is coarsely crystalline, and traversed by definite systems of 

 joints, producing a well-defined columnar structure, the evi- 

 dence is seemingly conclusive that it is not of the nature of a 

 volcanic obelisk. In this connection the conditions revealed by 

 the uncovering and erosion of subterranean intrusions, such as 

 dikes, plutonic plugs, laccoliths, etc., need to be borne in mind, 

 but space does not permit of considering them at this time. 



Reverting to the case presented by Fernando de Noronha ; 

 the topographic form and prominent position of the Peak are 

 such as to simulate in a remarkable way the conditions that 

 may reasonably be supposed to pertain to a weathered obelisk ; 

 the rock of which it is composed, as reported by Darwin and 

 Branner, is phonolite, and although^ so far as I am aware, no 

 analysis is available of this particular example, phonolites in 

 general have approximately the composition indicated in the 

 table presented above, and are to be included among the igneous 

 rocks of medium fusibility. The rocks associated with the 

 phonolite which forms the Peak of Fernando de Noronha and 

 other neighboring elevations, as stated by Branner,* are of a 

 basaltic type and compose the greater part of the island. From 

 this same authority also, we learn that the phonolite of the 

 Peak is conspicuously columnar ; " The direction of the columns 

 varies in some cases as much as fifty degrees. The lowest rocks 

 of the Peak exposed in place are the irregular columns upon 

 the eastern side. The columns are here very nearly vertical ; 

 but higher up, even upon this side, they twist and bend to the 

 northeast and thus, form the overhanging projection which is 



so remarkable a feature of this great rock On its 



western side the columns stand at various angles with the 

 meridian, and usually at a high angle with the horizon." A 

 comparison of this description with that of the columnar struc- 

 ture of the volcanic necks of New Mexico, quoted above, is 

 highly suggestive. 



It thus appears that the facts recorded concerning the Peak 

 of Fernando de Noronha do not furnish positive evidence as 

 to its mode of origin. Its shape and prominent topographic 

 position are similar to those of the obelisk of Pele, but on the 



*This Journal, III, vol. xxxvii, 1889, p. 150. 



