It is the turn now of other lichens to colonize it. These may be the little trumpet or horn and cup lichens, _Cladonias_, or perhaps the larger grey kinds, _Parmelias_ and _Physcias_, which have leaf-like fronds and form circles of perhaps eight to ten inches in diameter. The crust-lichen is overgrown, broken up, disorganized, and devoured by the _Parmelias_ and _Cladonias_, who are helped by bacteria, insects, and animalcula which shelter below them. These leafy lichens grow much more rapidly. They may increase two-thirds of an inch in one year. But very soon after this, one notices a few inconspicuous green mosses; at first in crevices between the stones or in hollows, and not remarkable, they soon increase and form trailing sprays or branches which grow very quickly. Branches of moss four or five inches long extend over the leafy lichens in a season. The _Parmelias_ and _Cladonias_ struggle on, but they cannot keep pace with the rapid life of the moss, and soon our wall is covered by beautiful moss turfs. Underneath such a turf there may be an inch or so of good soil (dead moss and dust with lichen and insect bodies). Worms, insects, etc., shelter and flourish and multiply in this soil. But the turn of the moss is coming. Here a few grass-blades, there a tiny plant of Sandwort, possibly a Rock Bedstraw, begin to root themselves in the moss. If people would only let the wall alone, it would soon be festooned with hanging plants, and producing quantities of grass, but somebody is sure to find that it looks very untidy, and everything is torn off the wall, which again looks new and raw and clean. Then of course the pioneer lichens begin again! Some very interesting and remarkable facts have been discovered about the way in which lavas and basalts have been occupied by the plant world. In the great volcanic eruption of 1883, the whole island of Krakatoa was covered by hot lava and glowing ashes. In 1884 and 1885 the sunsets were remarkable for a curious fiery red or orange glow, which was popularly supposed to be due to the volcanic dust of that explosion. It is said that the dust travelled three times round the earth, though I do not know on what authority. However, on Krakatoa island there was left a clean "slate." There were neither bacteria, nor leaf-mould, nor living plants of any kind; no spores or seeds could have endured the fiery furnace of the eruption. Three years afterwards the botanist Treub visited the island. He found that the rocks had been first covered by thin layers of minute freshwater Algæ, but that ferns were then occupying and inhabiting the lavas. Eleven kinds of ferns, and but very few other plants, were discovered.