shades of colour, parallel to the laminae of the surrounding rock; they
likewise generally contain minute white sphaerulites, of which half is
sometimes embedded in a zone of one shade of colour, and half in a zone of
another shade. The obsidian assumes its jet black colour and perfectly
conchoidal fracture, only when in large masses; but even in these, on
careful examination and on holding the specimens in different lights, I
could generally distinguish parallel streaks of different shades of
darkness.

(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The upper
ones are externally marked with parallel ridges. The internal radiating
structure of the lower ones, is much too plainly represented.

FIGURE 7. A LAYER FORMED BY THE UNION OF MINUTE BROWN SPHAERULITES,
INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly the
natural size.)

One of the commonest transitional rocks deserves in several respects a
further description. It is of a very complicated nature, and consists of
numerous thin, slightly tortuous layers of a pale-coloured feldspathic
stone, often passing into an imperfect pitchstone, alternating with layers
formed of numberless little globules of two varieties of obsidian, and of
two kinds of sphaerulites, embedded in a soft or in a hard pearly base. The
sphaerulites are either white and translucent, or dark brown and opaque;
the former are quite spherical, of small size, and distinctly radiated from
their centre. The dark brown sphaerulites are less perfectly round, and
vary in diameter from the twentieth to the thirtieth of an inch; when
broken they exhibit towards their centres, which are whitish, an obscure
radiating structure; two of them when united sometimes have only one
central point of radiation; there is occasionally a trace of or a hollow