SKEATS : CHEMICAL COMPOSITION OF LIMESTONES. fis) 
limits of variation in the amount of magnesium carbonate are very small, 
the minimum value being 34.6 and the maximum 39.1 per cent. In- 
soluble residue and calcium phosphate are present only in very minute 
quantities. 
Calcium |Magnesium| Insoluble | Calcium 
Carbonate.| Carbonate.| Residue. |Phosphate. 
NGILLANGILLAH. 
Microscopical. 475'.— The appearance of the section suggests that 
the rock was a fragmental limestone made up of Lithothamnion and 
other organisms, largely cemented by an incrusting deposit of radiating 
crystals of calcite. Subsequently the rock has been dolomitized. In 
some parts of the rock the matrix was originally “mud,” some of which 
has since altered to more or less clear granular dolomite. 
215’, A dolomitic limestone, in which the only recognizable organ- 
isms are Amphistegina, altered Lithothamnion, and “ ghosts” of corals, 
now represented by infillings of “ mud” between the septa of the vanished 
