These five varieties, with many intermediate ones, pass and repass into
each other. As the compact varieties are quite subordinate to the others,
the whole may be considered as laminated or striped. The laminae, to sum up
their characteristics, are either quite straight, or slightly tortuous, or
convoluted; they are all parallel to each other, and to the intercalating
strata of obsidian; they are generally of extreme thinness; they consist
either of an apparently homogeneous, compact rock, striped with different
shades of grey and brown colours, or of crystalline feldspathic layers in a
more or less perfect state of purity, and of different thicknesses, with
distinct crystals of glassy feldspar placed lengthways, or of very thin
layers chiefly composed of minute crystals of quartz and augite, or
composed of black and red specks of an augitic mineral and of an oxide of
iron, either not crystallised or imperfectly so. After having fully
described the obsidian, I shall return to the subject of the lamination of
rocks of the trachytic series.

The passage of the foregoing beds into the strata of glassy obsidian is
effected in several ways: first, angulo-modular masses of obsidian, both
large and small, abruptly appear disseminated in a slaty, or in an
amorphous, pale-coloured, feldspathic rock, with a somewhat pearly
fracture. Secondly, small irregular nodules of the obsidian, either
standing separately, or united into thin layers, seldom more than the tenth
of an inch in thickness, alternate repeatedly with very thin layers of a
feldspathic rock, which is striped with the finest parallel zones of
colour, like an agate, and which sometimes passes into the nature of
pitchstone; the interstices between the nodules of obsidian are generally
filled by soft white matter, resembling pumiceous ashes. Thirdly, the whole
substance of the bounding rock suddenly passes into an angulo-concretionary
mass of obsidian. Such masses (as well as the small nodules) of obsidian
are of a pale green colour, and are generally streaked with different