56 STOCKS: HYDRAULIC LIMESTONE CONCRETIONS. 
For comparison with my analysis, I quote one from the last 
edition of Muspratt's Chemistry. 
It will be seen from the above analysis, that the nodules consist 
essentially of a mixture of carbonate of lime and clay. The phosphoric 
acid most probablj^ comes from the shells which the nodules contain. 
The presence of phosphoric acid in these nodules does not seem to 
hav.e been noticed previously, I have discovered it in all the nodules 
that I have yet examined, and it has been found in the nodules of clay 
ironstone used for iron smelting, in the proportion of from a trace to 
1-86 per cent. 
It appears that many substances are capable of forming con- 
cretionary nodules, but only a few occur in large quantities ; these 
are carbonate of lime, carbonate of iron, iron pyrites, silica, and 
phosphate of lime. Iron pyrites occurs nodular in the chalk and in 
the clay of the coal measures. Silica occurs nodular in the chalk, as 
flint, jasper, etc. Phosphate of lime occurs in nodules in the 
greensand and other formations. 
The other concretions found in considerable quantities are : — 
Coal Balls consisting of variable proportions of carbonate of lime 
and iron pyrites. 
Baum Pots consisting of a mixture of carbonate of lime, iron 
pyrites, and silicate of alumina (clay). 
Acrospire, a curious concretion found in the millstone grit, 
consisting of carbonate of lime and sand (silica). 
Clay ironstone consisting of varying amounts of carbonate of 
iron and silicate of alumina. 
Chert consisting of a mixture of carbonate of lime with silica. 
Hydraulic Limestone as before stated, consists of a mixture of 
carbonate of lime and silicate of alumina. 
In these concretions the compounds composing them ma}^ 
replace each other to any extent ; thus chert passes into flint (silica) 
or into chalk (carbonate of lime), o^ving to one compound or the 
other being in excess. 
