extend uninterruptedly for a great length in a vertical direction, and
apparently parallel to the walls of the dike-like mass. Von Buch
("Description des Iles Canaries" page 184.) has described at Teneriffe, a
stream of lava containing innumerable thin, plate-like crystals of
feldspar, which are arranged like white threads, one behind the other, and
which mostly follow the same direction. Dolomieu ("Voyage aux Isles de
Lipari" pages 35 and 85.) also states, that the grey lavas of the modern
cone of Vulcano, which have a vitreous texture, are streaked with parallel
white lines: he further describes a solid pumice-stone which possesses a
fissile structure, like that of certain micaceous schists. Phonolite, which
I may observe is often, if not always, an injected rock, also, often has a
fissile structure; this is generally due to the parallel position of the
embedded crystals of feldspar, but sometimes, as at Fernando Noronha, seems
to be nearly independent of their presence. (In this case, and in that of
the fissile pumice-stone, the structure is very different from that in the
foregoing cases, where the laminae consist of alternate layers of different
composition or texture. In some sedimentary formations, however, which
apparently are homogeneous and fissile, as in glossy clay-slate, there is
reason to believe, according to D'Aubuisson, that the laminae are really
due to excessively thin, alternating, layers of mica.) From these facts we
see, that various rocks of the feldspathic series have either a laminated
or fissile structure, and that it occurs both in masses which have injected
into overlying strata, and in others which have flowed as streams of lava.

The laminae of the beds, alternating with the obsidian at Ascension, dip at
a high angle under the mountain, at the base of which they are situated;
and they do not appear as if they had been inclined by violence. A high
inclination is common to these beds in Mexico, Peru, and in some of the
Italian islands (See Phillips "Mineralogy" for the Italian Islands page
136. For Mexico and Peru see Humboldt "Essai Geognostique." Mr. Edwards