Dr. D. Woolacott — Magnesian Limestone of Durham. 461 



(2) The second explanation is that the magnesium has heen 

 carried away as a soluble salt out of the dolomite, calcium meta- 

 somatically replacing it. It appears to me, that under certain 

 conditions, the reaction represented by the following reaction took 

 place : CaC0 3 MgC0 3 + CaS0 4 = 2Ca C0 3 + MgS0 4 . 



If dolomite was precipitated, as suggested in a former part of this 

 paper, by the formation of calcium carbonate and magnesium sulphate 

 in solution and their reaction on one another, then the process of 

 formation of dolomite has been reversed at the time of solution of the 

 magnesium carbonate. On this hypothesis the reaction represented 

 by this equation, 2CaC0 3 + MgS0 4 ^CaMg (CO s ) a + CaS0 4 , is 

 considered to be a reversible one. The reversibility will depend on 

 the temperature, pressure, and concentration of the solution. It is 

 impossible to be dogmatic on such changes, as little is known 

 regarding the exact way in which dolomite is produced in nature. 

 The reaction for the formation of dolomite by the action of 

 magnesium sulphate on calcium carbonate has been obtained at fairly 

 high temperatures (120°), but E. W. Pfaff has shown that when 

 a current of carbon dioxide is passed for a long time through a warm 

 solution of the sulphates and chlorides of magnesium and calcium, 

 and slowly evaporated at a temperature of 20° to 25°, a residue is 

 produced which contains the double carbonate, i.e. under conditions 

 approximately parallel to those which occur in the concentration of 

 sea- water, dolomite may be formed (Centralllatt. Min. Oeol. u. Pal., 

 p. 659, 1903). 



The same authority says that it is a well-known fact that dolomite 

 with gypsum solution is changed to magnesium sulphate and 

 calcium carbonate at ordinary temperatures and pressures. He 

 describes a simple experiment in which the reaction can be proved. 

 Powdered gypsum and dolomite are mixed with one another, placed 

 in a funnel, the stem of which is stuffed with cotton wool, and 

 water is poured on the mixture and allowed to trickle through. In 

 the clear filtrate magnesium can be proved in appreciable quantities 

 (E. W. Pfaff, " LTeber Dolomit und seine Entstehung": Neues 

 Jahrbuch fur Mineralogie, Geologie, und Palaontologie, p. 563, May, 

 1907). The experiment has been verified by Mr. A. D. IS". Bain, 

 B.Sc, who finds that the reaction can be proved to take place quite 

 easily if the temperature of the solution is raised. Snce arrivin g 

 at the idea of the reversibility of this equation I have noticed the 

 following statement: — 



"A. von Morlot by heating powdered calcite with magnesium 

 sulphate to 200° in a sealed tube, transformed the carbonate into 

 a mixture of dolomite and gypsum. This reaction has been suggested 

 by Haidinger in order to account for the frequent association of the 

 two last-named species. The process, however, is reversible, and 

 solutions of gypsum will transform dolomite into calcium carbonate 

 and magnesium sulphate. Efflorescences of the latter salt are not 

 uncommon in gypsum quarries, and H. C. Sorby has observed them 

 in Permian limestones. Because of this reaction, according to Sorby, 

 the upper beds of magnesian limestone are often more calcareous 

 than the lower. Their content in magnesia has been diminished in 

 this way" {Data of Geo. Chemistry, p. 535). * 



