DISTRIBUTION OF THE RAINFALL. 285 



inroads even here, while the volcanic sand and molten rock thrown 

 out by Vesuvius soon become productive. Before tlie great eruption 

 of 1631 even the interior of the ci-ater was covered with vegetation. 

 George Sandys, who visited Vesuvius in 1611, after it had reposed 

 for several centuries, found the throat of the volcano at the bottom of 

 the crater ' almost cholied witli broken rocks and trees that had falne 

 therein.' ' Next to this,' he continues, ' the matter thrown up is 

 ruddy, light, and soft : more removed, blacke and ponderous : the 

 uttermost brow, that declineth like the seates in a theater, flourish- 

 ing with trees and excellent pastui'age. The midst of the hill is 

 shaded with chesnut trees, and others bearing sundry fruits.' " 



A.nd Mr Marsh adds in a foot-note : — " Even 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 labour. 



" The increase in the price of wines, in consequence of the diminu- 

 tion 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 volcanic 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 entirely barren 

 in 1835, is now planted with vines almost to the summits of Monte 

 Rosso, at a height of three thousand feet.' — Ueber den Sicilianischen 

 Ackerhati, p. 19." 



I accept as probable the statements which have been cited in re- 

 gard to the tendency of trees and arborescent shrubs, in common with 

 other vegetables, to diffuse themselves extensively, and in the struggle 

 for life to retain possession of the soil ; but there are conditions 

 essential to their success in doing so, and one of these is an adequate 

 supply of water, not in excess, and varying with varying require- 

 ments. An excess of water may prevent vegetation, — it may pre- 

 vent germination, prevent vigorous growth, prevent the formation 

 of flowers and the maturation of fruit ; and a deficiency of water at 

 any corresponding critical period may have the same effects. There 

 are general laws in accordance with which is governed the distribution 

 of all vegetables, and the consideration of some of these laws may 

 make this manifest. 



We may study the phenomena in the growth of mould as well as 

 in the growth of a forest, and find some advantage in doing so. 



