SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 81 
ing the “centers of calcification” and spicular structure. Most of the 
cavities are empty, but some are filled with a dark “ mud,” while in others: 
long prismatic, rather muddy crystals of aragonite have arisen by altera- 
tion of the “mud.” These crystals of aragonite are seen to be crystal- 
lographically continuous with the spicules, making up the stereoplasm 
of the coral. 
Calcium Magnesium Insoluble Calcium 
Carbonate. Carbonate. Residue. Phosphate. 
120’, Second Terrace. — A rock made up of rounded or angular frag- 
ments of coral, Halimeda, Orbitolites, etc. The matrix consists partly 
of “mud,” but very largely of a deposit of radiating fibrous crystals of 
calcite, forming a concentric coating to the organic fragments. Any 
spaces left in the rock have been subsequently filled with a mosaic of 
calcite crystals. 
