66 



When the hydrated peroxide is heated it loses its combined 

 water, and its colour deepens considerably. This action may be 

 noticed in brick-making, and in cases where rocks containing 

 peroxide of iron have been subjected to heat. In the case of 

 clay, which probably contains small quantities of the protocar- 

 bonate, the action of heat converts the protocarbonate into the 

 anhydrous peroxide,' the colour changing from grey to brick-red, 

 according to the amount of iron in the clay. 



The persilicate occurs chiefly in certain Felspars, in which 

 it is combined with other silicates : e.g., red granite owes its 

 colour to the red Felspar, which contains a certain quantity of 

 persilicate of iron. 



It is obvious, from what has been said, that iron compounds 

 could scarcely be naturally deposited under water as proto-salts or 

 proto-compounds, from the ease with which they are peroxidised. 

 The persalts and peroxides can however be reduced (i.e., deprived 

 of oxygen) by certain substances, such as decaying animal and 

 vegetable matter which have a stronger attraction for oxygen : 

 it is probable therefore that the proto-compounds of iron occur- 

 ring in sedimentary rocks have been reduced from the state of 

 persalts and peroxides, in which state they would naturally be 

 deposited, by the decay of the animal and vegetable matter 

 deposited with the sedimentary materials. 



"We see therefore, that in the cases where the sedimentary rock 

 would owe its colour to adventitious ingredients, if the iron com- 

 pounds be present as proto-salts or protoxide the colour of the 

 rock will be slight, or the iron compounds will have little or 

 no effect upon it except in cases where the iron compound is 

 present as protosilicate, in which case the rock will be coloured 

 more or less of a greenish colour. If, on the other hand, the 

 iron compounds are present as persalts or peroxides, the rock 

 will be coloured reddish-brown or deep red, according to 

 whether the peroxide is hydrated or not, and according to its 

 state of agregation, as in the Oolites of the neighbourhood of 

 Sherborne and the New Red Sandstone of South Devon. 



