54 SCIENCE PROGRESS. 



must have been introduced from some external source. It 

 is worthy of note, however, that the matrix of a shelly lime- 

 stone, which was originally in a finely divided state, seems 

 to be much more readily affected than the enclosed shells. 



As regards the possible processes by which calcite can be 

 converted to dolomite by magnesian solutions, various ex- 

 perimental investigations have until quite recently afforded 

 but little assistance. Morlot, in 1847, found that magnesium 

 sulphate and calcium carbonate reacted upon one another at 

 a temperature of 200° C. and a pressure of 15 atmospheres. 

 Marignac, with similar conditions, obtained a reaction 

 between magnesium chloride and calcium carbonate. Hoppe- 

 Seyler, in 1875, obtained a mixture of magnesite and dolom- 

 ite by the action of magnesium chloride or sulphate on 

 calcium bicarbonate, heated in a sealed tube; the action took 

 place at temperatures between ioo° and 160", but not below 

 ioo°. The special conditions necessary for success in such 

 experiments as these make their application to the origin of 

 dolomitic rocks in nature a matter of much doubt. Other 

 possible reactions have been suggested ; for instance, that 

 part of a limestone may be converted to magnesite by a 

 solution of magnesium bicarbonate, the excess of lime being 

 removed as calcium bicarbonate, and the magnesite sub- 

 sequently combining with the remaining limestone to produce 

 dolomite. 



More recently Klement (6) has given a discussion of 

 the whole subject of the origin of dolomitic strata, and has 

 made experimental researches which throw an interesting 

 light on some of the difficulties already alluded to in the 

 process of dolomitisation. A brief account of the paper in 

 which his results are embodied will therefore be of some value. 



After briefly noticing various former attempts to solve 

 the problem, Klement draws attention to certain ascertained 

 facts which have suggested to him the line taken in his 

 experiments. Typical dolomite, he remarks, is very often 

 found in the form of coral-reefs, and the most complete 

 dolomitisation in the case of atolls. Richthofen has noted 

 in the Trias of the Tirol that the atolls are of dolomite-rock, 

 the barrier-reefs of dolomitic limestone. Dupont, in the 



