RUSTED ROCKS. 31 



Now, many rocks contain a quantity of iron, and to them, 

 of course, oxygen is a great enemy. The very compact and 

 almost black rock, called basalt, which forms the columns 

 of the Giant's Causeway, contains much iron, and where 



Fig. 9. ROCK FORMATION, GIANT'S CAUSEWAY, IRELAND. 



exposed to the weather its surface is always found covered 

 with soft brown rust, which gradually eats deeper into the 

 mass (Fig. 9). 



Moreover, though compact, basalt is very absorbent ; 



