sight to be the shade of the rock on which it grows."

Mr. Glas adds, that the best sort is of the darkest colour, and nearly
round; and that the more white spots or scabs it exhibits the better.
It is found in considerable quantities in the Canary Islands, the Cape
de Verds, the Azores, and the Madeiras, and such are the nice varieties
and properties incidental to the different soils, (if they may be so
called,) or climates, that although the above clusters of islands are
at no great distance from each other, the difference in the produce
makes a very considerable difference in the value of the article. It is
also found on the coast of Barbary, and the Levant, and on that part of
the coast of Africa, which lies adjacent to the Canary-Islands; but,
owing to the want of seasonable rains, the produce of the latter is not
rapid or abundant, although the quality is excellent. It has been
suggested, that the orchilla was probably the Gertulian purple of the
ancients; a conjecture which is strengthened by the fact, that the
coast of Africa, where the orchilla abounds, was formerly called
Gertulia. That the vivid dye which resides in this weed was known to
the ancients, does not admit of any doubt.

The plant belongs to the class Cryptogamia, and order Algae, of the
Linnean system, and to the class Algae, and order Lichenes, of the
natural system. Professor Burnett, in his Outline of Botany, informs
us, that "Roccella, a corruption of the Portuguese Rocha, is a name
given to several species of lichen, in allusion to the situation in
which they are found; delighting to grow on otherwise barren seaward
rocks, that thus produce a profitable harvest. Tournefort considers
that one species at least (R. tinctoria) was known to the ancients,
and that it was the especial lichen (Greek: leichaen) of Dioscorides,
which was collected on the rocky islands of the Archipelago, from one