28 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 31, 



in the Silurian and metamorphic rocks, is that of the bisulphuret of 

 iron or iron pyrites, which is very abundantly contained in these 

 rocks, and to the decomposition of which I believe the red colours of 

 the derived deposits should be mainly attributed. 



It can scarcely be unfair to assume that the immense masses of 

 the older formations which have been worn down to furnish the ma- 

 terials of the carboniferous beds, contained proportionally as large 

 quantities of iron pyrites as those portions which remain. We may 

 therefore proceed to inquire respecting the changes which this mine- 

 ral probably suffered, before or during the degradation of these rocks. 

 Under ordinary atmospheric influences, iron pyrites passes by oxida- 

 tion into sulphate of iron and hydrous peroxide of iron, and much of 

 it is now, and probably has been in all past periods, undergoing this 

 change. Under the influence of heat, however, it is capable of under- 

 going other modifications, of more importance for our present purpose. 

 The formations in which the pyrites in question is contained, have 

 been greatly changed by igneous agents before the carboniferous pe- 

 riod ; and under such influences this mineral may have been changed 

 in three ways. First, in the deeper parts of the deposits it may have 

 remained chemically unaltered, but may have assumed a more cry- 

 stalline structure or aggregated itself into grains, masses and veins, as 

 it now appears in the remaining portions of the metamorphosed de- 

 posits. Secondly, in other circumstances, its sulphur might be sub- 

 limed, the iron remaining buried in the altered rocks, or, in low states 

 of oxidation, entering into the composition of molten masses. We 

 have no evidence of the occurrence of this change at the period in 

 question, though it probably occurs in many modern volcanos. 

 Thirdly, in the superficial parts of the deposits it would be converted 

 into peroxide of iron and sulphuric acid, or in parts less near the 

 surface, into oxides of iron and sulphuretted hydrogen, and the latter 

 might be subsequently oxidized and converted into sulphuric acid, in 

 passing through moist fissures*. Such processes, especially if car- 

 ried on in the presence of water, would produce large quantities of 

 the peroxide of iron, and would probably stain with it all the super- 

 ficial parts of the deposits subjected to their influence. It is not 

 always easy to understand the precise effects which may be produced 

 in nature on the large scale by the application even of familiar che- 

 mical agents ; but in this case, I think the results of the artificial oxi- 

 dation of iron pyrites, by the combined influences of heat and water 

 or heat and air, are sufficient to convince any person who can perform 

 even a few simple experiments upon this mineral, or clays in which it 

 is contained, that these processes are at least competent to produce 

 the required effects. 



It is evident that the oxide of iron produced in the manner above 

 stated, would naturally accompany the detrital deposits of the period, 

 and it is even possible that the chemical changes which produced it 

 would be accompanied by mechanical disturbances tending to produce 

 large quantities of fragmentary matter. It is also apparent that the 

 sulphuric acid resulting from the decomposition of pyrites would 



* This may be imitated by passing the gas through moist porous substances. 



