28 Pioneer Labourers 



forth streams of lava, often buries the surrounding 

 country many feet deep in the finest dust and ashes, 

 or in mud, if the eruption be accompanied, as it often 

 is, by rain. 



Such, then, are the principal pioneer -labourers 

 employed in breaking down the rocks. By the com- 

 bined efforts of the gases and moisture of the atmo- 

 sphere, by heat and frost, wind and rain, by rivers and 

 streams, by glacier and volcano, the rotks are gradually 

 split up, worn away, and reduced on the surface to a 

 condition of softness. 



In tropical regions, where the air is always moist, 

 and the rainfall large and violent, some rocks are 

 decayed and softened to a depth of a hundred or two 

 hundred feet. In drier climates, the work proceeds 

 more slowly and to a less depth, the rock beneath 

 'being to some extent protected from the weather by 

 the looser material above, when this is left to accu- 

 mulate. 



But the labourers which we have been thus briefly 

 considering are only pioneers. They accomplish only 

 the rougher work of preparation, and very much 

 remains to be done before anything that can properly 

 be called ' soil ' is ready for the crops. 



