460 
I will not enter into a detailed description of its chemical 
composition, 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 chieflv 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 mag- 
nesia, 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 lime- 
stone 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 replacement 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 of coral reefs. 
The general conclusion that I therefore draw from these 
facts is, that, at first, the Cleveland Hill Ironstone was a kind 
of oolitic limestone, interstratified with ordinary clays con- 
