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 of which it received the name of the ‘purple of Amorgus.’”

Of all the known varieties of orchilla, that which is grown in the Canary Islands stands the highest in estimation, and brings the greatest price.  In the collection of the weed, which is always performed by the natives, the risk is imminent:  they are obliged to be suspended by ropes over the cliffs, many of which are of stupendous height, and loss of life frequently occurs in these perilous efforts to contribute to the luxury of man.  Such is the esteem in which the orchilla of the Canaries is held, that it has recently reached the enormous value of 400l. per ton.  That from the Cape de Verds is next in quality, but of much greater importance, in reference to the quantity produced.  Madeira and the Azores produce the next qualities.  The same plant, though of a very inferior character, is found in great abundance in Sardinia, in some parts of Italy, and also on the south coast of England, Portland Island, Guernsey, &c. but of so poor a kind that it would not reward the expense of collection.