SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 113 
eralogical change may be noticed. Since dolomite and calcite are 
regarded as not being strictly isomorphous, if the magnesium carbonate - 
occurred in the crystalline matrix of the rock it would be expected 
that crystals of dolomite should make their appearance. No such 
crystals are ever seen. It may be that an isomorphous mixture of 
calcium carbonate with magnesium carbonate may be possible up to 
about 15 per cent of magnesium carbonate, but if introduced beyond 
that amount, the stable compound dolomite is formed. A somewhat 
lps Ye 
Curistmas Istanp. No. 800. The matrix of the rock consists of dolomitic 
erystals, while a cavity is filled with clear dolomite. Some of the organisms have 
been incrusted with scalenohedra, while dolomite crystals have invaded some of 
them from without, inwards. X 30. 
analogous change is described by Mr. L. J. Spencer, in connection with 
mixtures of copper and silver iodides... On the other hand, it may be 
that the magnesium carbonate may be present in some form or another 
in the organisms. Unfortunately these are usually too small to be 
isolated for analysis. In this connection it may be mentioned that 
Hoégbom ? quotes a series of analyses of specimens of Lithothamnion 
in which magnesium carbonate has been found forming from 3 to 13 
1 Min. Mag., June, 1901, pp. 48-44. 2 N. Jahresbuch fiir Min., 1894, I., 262. 
