On the Food of Plants. _ 323 
their vegetation, such as the numerous lichens and traga- 
canths, of which genera the former were discovered by Saus- 
sure on the highest of the Alpine granite rocks. In lower 
situations these form a soil for the genista, for the cistuses, 
and more especially for rosemary and lavender, which abound 
on the miost elevated mountains of the Pyrennees. These 
again, by their decay, form vegetable earth, in which the 
luxuriant pine-trees and the ilex grow. 
This vegetable matter, being washed down into the val- 
leys, helps to form and to increase their soil to a considera- 
ble depth, and to give them that fertility which is not rea- 
dily exhausted. 
When we analyse a soil, we never fail to find it composed 
of substances derived from a superior level. If the hills are 
quartzose, calcareous, argillaceous, or magnesian, so is the 
soil in all the valleys which communicate with them. But 
with these earths in a rich soil we find a great proportion of 
vegetable matter, or of animal exuvize; and as these are de- 
ficient or abound, vegetation languishes, or is exceedingly 
Juxuriant. : 
Good mould abounding with vegetable matters is com- 
monly of a dark colour, pulverises easily, and has therefore 
what is called a mellow look; but when exhausted or im- 
poverished by frequent crops, the richest soil, such as I have 
here described, becomes arid, of a lighter colour, compact, 
and comparatively barren. In a maiden soil, or where every 
shower of rain brings down from more elevated regions a 
quantity of vegetable matter, a succession of luxuriant crops 
may be taken incessantly without any diminution of ferti- 
lity.. Thus it is in the country newly occupied by the Ame- 
ricans, in Kentucky, on the Ohio, and in the whole extent 
of territory watered: by the Mississippi or by its tributary 
streams. Thus also in some parts of Spain, where an ex- 
tensive plain happens to receive the spoils of rich cireumja- 
cent hills, as in the well-watered vale of Orihuela, near Mur- 
cia, of which they say, ‘ Let it rain or not rain, corn never 
fails in Orihuela.” Indeéd, so productive is wheat in that 
highly-favoured district, that the farmers commonly receive 
100 for 1 upon their seed. 
X 2 Tn 
