1 8 The Great World's Farm 



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 con- 

 taining 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 especially well developed in the basalt — 

 an ancient lava — which, in coohng 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 its 

 way in between these columns; but patches of wet and 

 brown stains are also found actually inside the columns 

 themselves, when these are broken open, showing that 

 moisture has been sucked up by the rock. 



Now water in the natural state always contains some 

 amount of air dissolved in it, and wherever the water pene- 

 trates, there the oxygen of the air penetrates also, and 

 lays hold of any iron that comes in its way, as we see by 

 the stains that it has done in this instance. 



The iron of the basalt is not, indeed, pure iron, being 



