Piojieer Ladourcrs 21 



its way in between these columns ; but patches of wet 

 and brown stains are also found actually inside the 

 columns themselves, whep 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 penetrates, 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 

 already combined with some amount of oxygen, but it 

 does not acquire the reddish-brown colour of what we 

 familiarly call ' rust ' until it has absorbed as much 

 oxygen as it can hold. In this condition it is, of 

 course, heavier, and, as we have seen, softer than 

 before, and is therefore more easily washed or blown 

 away from the surface. But it is also more bulky, and 

 takes up more space than it did before, so that if it 

 be formed inside the rock where it has not room to 

 expand, the rock is cracked by it. This, of course, 

 opens the way for more water and more oxygen to 

 enter, and so the work proceeds, and the decay goes 

 deeper and deeper. 



We have chosen iron-rust as a sample of the way in 

 which oxygen works because it is one of which we all 

 know something, but it must not be forgotten that this 

 is only one of many oxides formed in the rocks ; and that 

 whenever oxygen combines with any other substance 

 in a rock to form an oxide, it makes that substance 

 take up more room than before, and so the rock is 

 cracked and crumbled. The other gas, carbon dioxide, 

 works in a different way, though it also helps the 



