appear to me eminently curious, and shall be first described, and
afterwards their passage into the obsidian. They have an extremely
diversified appearance; five principal varieties may be noticed, but these
insensibly blend into each other by endless gradations.

FIRST.

A pale grey, irregularly and coarsely laminated (This term is open to some
misinterpretation, as it may be applied both to rocks divided into laminae
of exactly the same composition, and to layers firmly attached to each
other, with no fissile tendency, but composed of different minerals, or of
different shades of colour. The term "laminated," in this chapter, is
applied in these latter senses; where a homogeneous rock splits, as in the
former sense, in a given direction, like clay-slate, I have used the term
"fissile."), harsh-feeling rock, resembling clay-slate which has been in
contact with a trap-dike, and with a fracture of about the same degree of
crystalline structure. This rock, as well as the following varieties,
easily fuses into a pale glass. The greater part is honeycombed with
irregular, angular, cavities, so that the whole has a curious appearance,
and some fragments resemble in a remarkable manner silicified logs of
decayed wood. This variety, especially where more compact, is often marked
with thin whitish streaks, which are either straight or wrap round, one
behind the other, the elongated carious hollows.

SECONDLY.

A bluish grey or pale brown, compact, heavy, homogeneous stone, with an
angular, uneven, earthy fracture; viewed, however, under a lens of high
power, the fracture is seen to be distinctly crystalline, and even separate
minerals can be distinguished.