52 SCIENCE PROGRESS. 



ists have supposed the magnesia to be introduced in the 

 form of sulphate and chloride from sea-water, and the 

 frequent association of gypsum with dolomite has led to 

 various speculations on a possible reaction between mag- 

 nesium sulphate and calcium carbonate. The possibility of 

 such a reaction, resulting in the formation of calcium sul- 

 phate and calcium-magnesium-carbonate, raises an obvious 

 difficulty, since at ordinary temperatures and pressures the 

 opposite reaction takes place, gypsum and dolomite decom- 

 posing one another. 



Other writers, again, have inquired whether the mag- 

 nesia may not have been contained, wholly or partly, in 

 the rock itself from the time of its accumulation ; and there 

 are considerations which seem to leave room for difference 

 of opinion as to the extent to which the magnesia has really 

 been supplied from some definitely external source. Many 

 calcareous organisms at the present day contain a note- 

 worthy amount of magnesia, and in some of the calcareous 

 algae (Lithothamnion) the ratio MgC0 3 : CaCO, reaches 

 an average of 10 : ioo. Several geologists have drawn 

 attention to the large part that has been played by various 

 calcareous algae in the formation of what are now extensive 

 masses of dolomitic rocks. Von Wohrmann, for example, 

 has ascribed the thick formations of limestone and dolomite 

 which are so prominent a feature of the Alpine Trias to the 

 agency of algae of the families Codiaceae and Siphoneae (4). 



Now dolomite is much less readily soluble than calcite 

 and distinctly less soluble than aragonite ; and, if we may 

 assume the magnesia of calcareous organisms to exist in 

 the form of dolomite, it follows that anv removal of material 

 under ordinary conditions will leave the residue relatively 

 enriched in dolomite. It has been suggested that the dolom- 

 itisation of limestones may sometimes have been effected 

 in this way, rather than by an actual replacement of lime by 

 magnesia introduced in some form from without. 



This point has recently been taken up by Hogbom in 

 an interesting paper (5). He is dissatisfied with the ex- 

 planation of dolomitic rocks as ordinary limestones in which 

 magnesian solutions have effected a partial substitution of 



