CHEMICAL WORK. 



127 



134. 



This oxidation process, and other methods of decay, go on with greatest 

 rapidity in the fissures of rocks, below a surface of soil, because the descend- 

 ing surface waters keep them almost continuously 

 wet ; and it is under such circumstances that a rock 

 which is much fissured or jointed becomes re- 

 duced to a pile of great bowlders with rusty earth 

 between, as illustrated in the figure annexed. The 

 balls of rock here represented are very common in 

 decomposing rocks from granites and trap to sand- 

 stones. They are simply a result of surface decay 

 along the many jDlanes of fracture (Fig. 134). The 

 decay or oxidation at first produces a thin discolor- 

 ing of adjoining surfaces, as in the lower part of 



the figure ; and this continues, eating off the angles, which are attacked from 

 three directions, until a bluff of solid rock becomes apparently a pile of great 

 bowlders. With the progress of the alteration, the discolored portion 

 becomes banded with yellow and brown ; and as it deepens, the outer part 

 of the spheroid sometimes separates in concentric shells, precisely corre- 

 sponding with the concentric structure of a concretion. But these concentric 

 shells are due to the decay that is in progress ; and apparently to alternations 

 in the work of decay dependent on climate and the capillary action above 

 explained. Rounded stones or bowlders are very often so made. After 

 separation from the pile, and therefore from exposure to almost permanent 

 moisture, the masses may decompose outside with extreme slowness. 



5. Constructive effects. — As the process is a means of reducing the hardest 

 rocks to earth and sand, it aids in preparing material for new rock-making, 

 and also in supplying earth and sand for soil and fertility. Without it, and 

 one other associated process mentioned beyond, the earth would have had 

 very scanty geological records and only low-grade life. 



This agency has produced, or aided in producing, a large part of the great 

 and valuable iron ore beds of the world's history, from Archaean time onward. 

 The limonite ore beds (often called by miners " hematite " beds) are among 

 the products. They occur of great size and value in West Stockbridge, 

 Mass., Salisbury, Conn., Amenia and elsewhere in New York, in eastern 

 Pennsylvania, western Virginia, and farther south to Alabama, as a result of 

 the oxidation chiefly of a ferriferous limestone, and of any iron carbonate 

 the limestone may contain. In the formation of the iron oxide, carbonic 

 acid is set free, and the weakened calcareous rock is hence readily removed 

 by percolating waters ; hence great cavities are made by the process, ready to 

 receive the ore as it is produced. Any slates or schists adjoining are also 

 destroyed by the action. 



Iron sulphides have been the source of similar beds, but such ore is 

 likely to contain some sulphur. The Amenia ore bed is a good place for 

 studying the formation of the ore from both a ferriferous limestone and 

 a massive iron carbonate. These ore-beds, although superficial, cannot be 



