160 THE CANADIAN NATURALIST. [Vo]. X. 



The obliteration of the ori^innl cnlcareous oriranisms was com- 

 pleted by the physical chanues which resulted in the combination 

 of the c.ilcareous matter, with the ma<rne8ian carbonate and the 

 Bubscqucnt re-crystallization in the form of the double salt. 

 According to the experiments of Mr. Sorby this was effected by 

 the magnesia replacing a portion of the lime. But Dr. Sterry 

 Hunt, many years ago, announced that, as indicated by his 

 experiments, all magnesian limestones are derived from the pre- 

 cipitation of both caibonates simultaneously in an inland salt 

 sea. At least as far as the Niairara dolomites are concerned, the 

 calcareous organisms have played a most important part iu fur- 

 nishing calcareous matter, although the magnesian s ilt n)ay have 

 been exclusively derived from the evaporation of the waters in 

 the immense inland Niagara waters, for at Grimbsv a bed of this 

 dolomite shows its derivation almost exclusively from crinoids, 

 and at Hamilton a similar bed in a more highly crystalline state, 

 and filled with pores from the shrinkage, forms a marked feature 

 of the series. 



In the molecular change a condensation in volume would occur, 

 thereby leaving the rock porous and permitting the carbonate of 

 lime of the calcareous fossils to be washed out; as illustrated in 

 the great bed of dolomite (No. 8 of sections) and some other 

 beds, where the cavities have not been subsequently filled with 

 aririllaceous mud. 



As a further illustration of the subsequent removal of the 

 material of the shells by water, we need only go a little beyond 

 the piesent region of study to the Guelph dolouiites, where are 

 numerous casts of shells in the porous stone, with the whole 

 shell and its filling removed, thus leaving numerous cavities in 

 the rock. 



Dr. Hunt has conducted a series of experiments which throw 

 light on the origin of dolonjites. In lake basins where there is 

 a considerable evaporation goiuL'' on. the waters containing bicar- 

 bonate of soda cause the separation of all the lime as carbonate, 

 and the foimation of soluble bicaibonate of mignesia, winch, 

 subsequently on evaporation, separates in the hydrated form. 

 The salts mingled togetjier under pressure and heat will combine 

 to form double caibon:ites. From the disintegration of feldsp<rs 

 and other rocks, an abnndmee of carbonates of sod i, lime and 

 magnesia, are constantly being brought down by streams and 

 emptied into the sea basins. These chemical precipitates mixing 



