150 THE HABITABLE EARTH ORIGINALLY WOODED. 
moisture, from perpetual frost, and from the depredations 
of man and browsing quadrupeds. Where these requisites are 
secured, the hardest rock is as certain to be overgrown with 
wood as the most fertile plain, though, for obvious reasons, the 
process is slower in the former than in the latter case. Lichens 
and mosses first prepare the way for a more highly organized 
vegetation. They retain the moisture of rains and dews, and 
bring it to act, in combination with the gases evolved by their 
organic processes, in decomposing the surface of the rocks they 
cover; they arrest and confine the dust which the wind scatters 
over them, and their final decay adds new material to the soil 
already half formed beneath and upon them. A very thin 
stratum of mould is sufficient forthe germination of seeds of 
the hardy evergreens and birches, the roots of which are often 
found in immediate contact with the rock, supplying their trees 
with nourishment from a soil deepened and enriched by the 
decomposition of their own foliage, or sending out long root- 
lets into the surrounding earth in search of juices to feed them. 
The eruptive matter of volcanoes, forbidding as is its aspect, 
does not refuse nutriment to the woods. The refractory lava 
of Etna, it is true, remains long barren, and that of the great 
eruption of 1669 is still almost wholly devoid of vegetation.* 
But the cactus is making inroads even here, while the volcanic 
sand and molten rock thrown out by Vesuvius soon become 
productive. Before the great eruption of 1631 even the in- 
* Biven the volcanic dust of Etna remains very long unproductive. Near 
Nicolosi is a great extent of coarse black sand, thrown out in 1669, which, 
for almost two centuries, lay entirely bare, and can be made to grow plants 
only by artificial mixtures and much labor. 
The increase in the price of wines, in consequence of the diminution of the 
product from the grape disease, however, has brought even these ashes under 
cultivation. ‘‘ I found,” says Waltershausen, referring to the years 1861-62, 
*‘ plains of voleanic sand and half-subdued lava streams, which twenty years 
ago lay utterly waste, now covered with fine vineyards. The ashfield of ten 
square miles above Nicolosi, created by the eruption of 1669, which was en- 
tirely barren in 1835. is now planted with vines almost to the summits of 
Monte Rosso, at a height of three thousand feet.’’—Ueber den Sivilianischen 
Ackerbau, p. 19. 
