2 GEOLOGICAL MEMOIRS. 
published nearly forty years ago*, we have a very remarkable instance 
of a trachytic rock having a parallelism of structure; and in his in- 
structive memoir on Trap-porphyry or Trachyte+, he pointed out 
that which has since been so frequently observed, viz. that beds of 
trachyte not unfrequently occur in which the crystals of felspar assume 
a parallel arrangement. He mentions still more remarkable exam- 
ples of this structure in his work on the Canary Islands. Thus, at 
p- 215, he describes a dyke of trachyte near Angostura in Teneriffe, 
the rock of which is composed of thin parallel layers of erystalline 
plates of felspar, so that it has quite a slaty texture, and has been 
taken for white silvery mica-slate; and at p. 244 he refers to a simi- 
lar slaty trachyte in the vicinity of Perexil on the Cumbre; and at 
p- 274 he speaks of a trachyte from the Caldera of Tiraxana, which 
is so slaty that at every step one fancies it a mica-slate. 
This peculiar structure, which Von Buch was the first to describe, 
in the case of trachyte, was afterwards pointed out by Beudant, in his 
excellent work on Hungary, as occurring in Perlitet. He describes 
this parallelism of structure as a very remarkable appearance, shown 
by an alternation of stony and glassy layers; and it is perceptible in 
hand. specimens as well as in the mass of the rock. The stratification 
of the latter, which is parallel to the slaty structure, is often horizon- 
tal, often contorted in various ways, and often combined with a dis-— 
position to separate into slabs, or at least to split into such forms. 
Afterwards Poulett Scrope, when examining M. Beudant’s specimens 
of Hungarian perlite at Paris, came to the very just conclusion, that 
their parallelism of structure must be ascribed “to the substance of 
the rock haying been drawn out in the direction of the zones,” as also 
“to the flowing of the matter in obedience to the impulse of its 
own gravity§,” just as happens with the obsidian-lava of Lipari, 
Teneriffe, and Iceland, which respectively exhibit a similar bedded 
structure. 
Scrope, in the above-mentioned memoir, also describes a rock in 
the island of Ponza, which he calls a prismatic trachyte, and speaks 
of its parallel structure. This trachyte, when viewed in mass, ex- 
hibits a striped appearance, derived from a kind of stratiform alter- 
nation of texture and colour, combined with a corresponding extension 
of all the pores of the stone; the brighter layers bemg porous and 
softer, the duller bemg compact and harder, more siliceous, and some- 
times almost like hornstone. This parallelism of structure passes 
right across the prisms of the rock; and as the axes of these prisms 
are always at right angles to the plane of the bed or of the dyke, 
so it may be seen that the direction of the structure-planes depends 
upon that of the resisting surfaces. } 
In the island of Palmarola the stratiform structure of the trachyte 
is still more remarkable; the layers in it are more continuous, and 
* Geognost. Beobacht. auf Reisen durch Deutschland und Italien, 1809, ii. 
s. 209. 
t+ Abhandlungen d. Berlin. Akademie d. Wissenschaften, 1816, 127. 
~ Voyages en Hongrie, tome iii. p. 403, 1822. 
§ Transactions of the Geol. Soc., 2nd Series, vol. ii. p. 225. 
