26 ROCK-FORMING MINERALS 



A blue clay containing ferrous carbonate will, when fired in a 

 kiln, give rise to red bricks or pottery, by the conversion of FeC0 3 

 into Fe 2 3 . Exposure to moist air produces a similar effect in 

 nature, and the contrast in colour between the superficial and 

 deep-seated layers of the same rock is often as great as between 

 blue clay and red brick. 



Weathered blocks stained red on the outside are often blue, 

 grey, or nearly black on the inside, the change not having pene- 

 trated through the whole mass. Such changes are most con- 

 spicuous in the sandstones, because their porous character allows 

 a comparatively free circulation of air and water through them, 

 but similar effects are frequently to be observed in other rocks 

 also. 



