20 Pioneer Labourers 



generally work singly and alone, but in union one with 

 the other, and the great ally of oxygen is moisture. 



Let us take basalt as an example, and see how this 

 rock is crumbled into soil. In perfectly dry air, at the 

 ordinary temperature, oxygen is powerless to do even 

 so much as tarnish iron in the mass, though it would 

 have no difficulty in reducing it all to oxide — that is, 

 rust — if the same mass of iron were exposed to its 

 action in the form of powder. Fireirons do not rust 

 in winter, or when in constant use, because the fire 

 keeps them dry ; they do rust when unused in summer, 

 because natural air is never perfectly dry, even on 

 the driest summer day, not even in the midst of the 

 parching desert. 



But, if iron quickly rusts when exposed to the damp 

 air of such a climate as ours, we all know how much 

 faster it does so when actually wetted ; and therefore 

 it is not surprising to find that basaltic and other rocks 

 containing much iron decay more rapidly on the side 

 which faces the rainiest quarter. Not that the force 

 of the rain makes so much impression on them as on 

 softer rocks, but that the wet enables the oxygen to 

 work faster. The decay is not confined to the surface, 

 moreover, for all rocks, even those which are most 

 close and compact and are called impervious, absorb 

 some amount of moisture, and this also finds entrance 

 through the cracks and joints, from which no large 

 mass of rock is ever entirely free. These joints are es- 

 pecially well developed in the basalt — an ancient lava — 

 which, in cooling down from the molten state, has 

 shrunk and contracted into columns having from three 

 to nine faces, and measuring from a few inches to 

 several feet across. The rain, of course, easily finds 



