It is somewhat remarkable that Alpine lichens generally are more or less of a brown or black color. This peculiarity seems to be owing to the presence of usnine or usnic acid, which in a pure state is of a green color, as in the lichens which grow in shady forests, but which becomes oxidized, and changes to every shade of brown and black, when exposed to the powerful agencies of light and heat on the bleak barren rocks on the mountain side and summit. These gloomy lichens, associated as they always are with the dusky tufts of that singular genus of mosses, the Andræas, give a very marked and peculiar character to many of the Highland mountains, especially to the summit of Ben Nevis, where they creep, in the utmost profusion, over the fragments of abraded rocks which strew the ground on every side, otherwise bare and leafless, as was the world on the first morning of creation, and reminding one of the ruin of some stupendous castle, or the battlefield of the Titans. Some of the Alpine lichens, however, are remarkable for the vividness and brilliancy of their colors. The mountain cup-moss, with its light green stalk clothed and filigreed with scales and emerald cup studded round with rich scarlet knobs, presents no unapt resemblance to a double red daisy. It grows in large clusters on the bare storm-scalped ridges, and forms a kind of miniature flower-garden in the Alpine wilderness. The loveliest, however, of all the mountain lichens is the Solorina crocea, which spreads over the loose mould in the clefts of rocks, and on the fragments of comminuted schist on the summits of the highest Highland mountains, forming patches of the most beautiful and vivid green, varied, when the under side of the lobes is curled up, by reticulations of a very rich orange-saffron color. This species is not found at a lower elevation than 4,000 feet; hence it is unknown in England, Ireland, and Wales, whose highest mountains fall considerably short of this altitude. I have gathered it on Cairngorm, Ben Macdhui, and Ben Lawers. In this last locality, which is well known to botanists as exhibiting a perfect garden of rare and beautiful Alpine plants, it grows in greater abundance, I believe, than in any other spot in the Highlands.