8 TROPICAL AGRICULTURE chap. 



in a town in England, a new stone pavement was laid down, 

 and, to every one's surprise, after a time the heavy stones were 

 mysteriously lifted out of their places. An examination into 

 the matter showed that the earth under the stones had been 

 mixed with the spores of mushrooms, the spores had grown 

 and the mushrooms by their increase in size had lifted 

 up the pavement. In the West Indies, where vegetation is 

 vigorous in growth, the splitting up of rocks by the entry of 

 roots into cracks and crevices can easily be investigated 

 by any one. 



The formation of soil by the combined action of the atmo- 

 sphere and vegetation is a very important factor in the pro- 

 duction of what is known as vegetable mould. A chemical 

 change which may be described as a slow burning (or decay) 

 is brought about by the action of the oxygen of the atmo- 

 sphere on moist organic bodies, by means of which they are 

 gradually consumed and resolved into their various con- 

 stituents. In the case of trees and plants in the tropics this 

 decay is very rapid, and the result is the large quantities of 

 vegetable mould met within tropical forests. In a very small 

 An illustra- way the Same thing may be observed on rocky precipices, 

 formation of or even on walls. Lichens, which are flat, leathery kinds of 

 ^°' ^' plants, first attach themselves to the rocks and eat into the 



surface. The lichens die and decay, and add vegetable 

 matter to the scanty soil formed by them with the atmo- 

 sphere and moisture. Mosses then grow up, and they in 

 turn die and decay, adding to the soil by their action during 

 life and by their remains after death. Then small hardy 

 plants appear on the scene, to give place in time to larger 

 plants, until at last a shrub or a tree grows up and sends its 

 roots into the cracks and crevices of the rock, with the final 

 result that the rock is split up and crumbled away to form 

 the soil for more vigorous vegetation. All this, of course, 

 takes a very long time, but then the process has been going 

 on for ages, so that it must have exerted a very considerable 

 influence on the formation of soils. 



