that lava in these instances had been erupted with its ingredients mixed in
the same proportion, as in the concretionary obsidian.

LAMINATION OF VOLCANIC ROCKS OF THE TRACHYTIC SERIES.

We have seen that, in several and widely distant countries, the strata
alternating with beds of obsidian, are highly laminated. The nodules, also,
both large and small, of the obsidian, are zoned with different shades of
colour; and I have seen a specimen from Mexico in Mr. Stokes' collection,
with its external surface weathered (MacCulloch states "Classification of
Rocks" page 531 that the exposed surfaces of the pitchstone dikes in Arran
are furrowed "with undulating lines, resembling certain varieties of
marbled paper, and which evidently result from some corresponding
difference of laminar structure.") into ridges and furrows, corresponding
with the zones of different degrees of glassiness: Humboldt ("Personal
Narrative" volume 1 page 222.), moreover, found on the Peak of Teneriffe, a
stream of obsidian divided by very thin, alternating, layers of pumice.
Many other lavas of the feldspathic series are laminated; thus, masses of
common trachyte at Ascension are divided by fine earthy lines, along which
the rock splits, separating thin layers of slightly different shades of
colour; the greater number, also, of the embedded crystals of glassy
feldspar are placed lengthways in the same direction. Mr. P. Scrope
("Geological Transactions" volume 2 second series page 195.) has described
a remarkable columnar trachyte in the Panza Islands, which seems to have
been injected into an overlying mass of trachytic conglomerate: it is
striped with zones, often of extreme tenuity, of different textures and
colours; the harder and darker zones appearing to contain a larger
proportion of silica. In another part of the island, there are layers of
pearlstone and pitchstone, which in many respects resemble those of
Ascension. The zones in the columnar trachyte are generally contorted; they