and scoriae, of different colours, and slightly consolidated. Each
successive saucer-shaped layer crops out all round the margin, forming so
many rings of various colours, and giving to the hill a fantastic
appearance. The outer ring is broad, and of a white colour; hence it
resembles a course round which horses have been exercised, and has received
the name of the Devil's Riding School, by which it is most generally known.
These successive layers of ashes must have fallen over the whole
surrounding country, but they have all been blown away except in this one
hollow, in which probably moisture accumulated, either during an
extraordinary year when rain fell, or during the storms often accompanying
volcanic eruptions. One of the layers of a pinkish colour, and chiefly
derived from small, decomposed fragments of pumice, is remarkable, from
containing numerous concretions. These are generally spherical, from half
an inch to three inches in diameter; but they are occasionally cylindrical,
like those of iron-pyrites in the chalk of Europe. They consist of a very
tough, compact, pale-brown stone, with a smooth and even fracture. They are
divided into concentric layers by thin white partitions, resembling the
external superficies; six or eight of such layers are distinctly defined
near the outside; but those towards the inside generally become indistinct,
and blend into a homogeneous mass. I presume that these concentric layers
were formed by the shrinking of the concretion, as it became compact. The
interior part is generally fissured by minute cracks or septaria, which are
lined, both by black, metallic, and by other white and crystalline specks,
the nature of which I was unable to ascertain. Some of the larger
concretions consist of a mere spherical shell, filled with slightly
consolidated ashes. The concretions contain a small proportion of carbonate
of lime: a fragment placed under the blowpipe decrepitates, then whitens
and fuses into a blebby enamel, but does not become caustic. The
surrounding ashes do not contain any carbonate of lime; hence the
concretions have probably been formed, as is so often the case, by the