356 Sorby : The Origin of the CleveUind Iroiislone. 



I will not enter into a detailed description of its chemical com- 

 position, for that has been so ably treated of by others. I would 

 especially refer to the excellent analysis published on the Iron Ores 

 of Great Britain, part i of the Memoirs of the Geological Survey. 

 On comparing- together the amounts of the various constituents 

 there given, it may be seen that the rock consists chiefly of the 

 carbonate and some of the silicate and phosphate of the protoxide 

 of iron, along with a much smaller quantity of the carbonate of 

 lime and magnesia, and some alumina and peroxide of iron. 

 Independent, then, of the silica and alumina resulting from the 

 clay so commonly found in limestones, and the phosphate of 

 iron, the general composition is very similar to that of the 

 altered shell already described ; so that, as far as the chemical 

 composition is concerned, the same circumstances that must 

 have altered the shell, may have changed an ordinary limestone 

 into such a rock, in the manner indicated by the microscopical 

 structure to have really been the case. 



The silicate and phosphate of iron, to which the rock owes 

 its green colour, have been most probably formed by the same 

 process, from the decomposition of the phosphate of lime, so 

 often found in limestones, and the silicate of alumina of the clay 

 for phosphate of iron is produced by the action of bicarbonate 

 of iron on phosphate of lime, and many facts indicate that the 

 silicate of iron could be thus derived, either by the direct re- 

 placement of the alumina of the clay by the protoxide of iron, 

 or by the decomposition of silicate of lime. This does occur in 

 some limestones, and may have been formed from ordinary clay, 

 by the action of the sulphate or hydrate of lime, which are met 

 with in the recent limestones or coral reefs. 



The general conclusion that I therefore draw from these 

 facts is, that, at fifst, the Cleveland Hill ironstone was a kind 

 of oolitic limestone, interstratified wnth ordinary clays containing 

 a large amount of the oxides of iron, and also organic matter, 

 which, by their mutual re-action, gave rise to a solution of 

 bicarbonate of iron — that this solution percolated through the 

 limestone, and, removing a large part of the carbonate of lime 

 by solution, left in its place carbonate of iron ; and not that the 

 rock was formed as a simple deposit at the bottom of the sea. 



P.S., May, 1906. It may be well here to say that it seems 

 to me that the amount of iron oxide in the associated non- 

 calcarious beds would in all probability be quite adequate to 

 supply that now found in the ironstone. 



Since writing my original papers I devoted much attention 



Naturalist, 



