238 ON THE DESTRUCTION OF ROCKS. 



analysis. But as far as the present object is concerned, 

 the subject is sufficiently understood. 



In the state of peroxyde, in which it communicates 

 a red or brown colour to rocks, iron is very little 

 susceptible of further changes from the action of water, 

 and, in general, it undergoes none. Hence stones of 

 this colour rarely experience any alteration of hue on 

 exposure; and, what i*> much more important to 

 architects, they are, in general, less susceptible of 

 decomposition than those which are dark or lead- 

 coloured. The red sandstones which are subject to 

 decomposition, owe that defect to the other causes 

 above enumerated. The existence of this oxyde of 

 iron in certain rocks, is often an interesting circum- 

 stance, as throwing light on the changes which they 

 have undergone ; but it is essential not to carry this 

 speculation too far. The clays found under the trap 

 rocks, appear unquestionably to owe their red colour 

 to the action of heat. Similar appearances are, in 

 some cases, observed in the argillaceous and micaceous 

 schists ; but it is not certain that, in all these, the 

 same cause has acted. In the red sandstone, if it does, 

 in some instances, appear to have produced the effect 

 in question, the colour, in the majority of these, has 

 been derived from that of the felspar which furnished 

 the clay in their composition, or else from the original 

 quartz. 



The next state in which iron exists in rocks, is in 

 that of the yellow, or the hydrated, carbonat ; forming 

 the rust which colours all the yellow rocks and clays. 

 In this case, however, it is even less a source of de- 

 composition than in the former, and is, indeed, in 

 general, perhaps in all instances, itself the produce of 

 decomposition. That some of the yellow rocks have 



