158 THE GEOLOGY OF MINNESOTA. 



[Dolomites. 



potassa, when the magnesia is precipitated in the form of a hydrate. Mag- 

 nesia and lime differ in solubility in water. The former is almost insoluble 

 in water, while the latter unites with it with great avidity, the heat evolved, 

 as in slaking lime, being sufficient, sometimes for the ignition of wood. The 

 hydrate thus produced is soluble in 530 times its weight of cold water. 



Lime has a much stronger attraction for carbonic acid than magnesia, 

 and in calcination of their carbonates the magnesia parts with it sooner 

 than the lime. Non-air-slaked magnesian quicklime (i. e. a lime produced 

 by the calcination of a magnesiau limestone) remains in a caustic state much 

 longer than pure quicklime, when freely exposed to the air, and hence is 

 injurious to vegetation when mixed with soils, for several months. This is 

 owing to the slowness with which the magnesia extracts carbonic acid from 

 the air. But pure quicklime very soon becomes neutralized or "sets'" by 

 the absorption of carbonic acid from the air. It is for the slowness of set- 

 ting, and the gentler evolution of heat in slaking, that the magnesian quick- 

 lime is preferred by masons ; at the same time the cement is more perma- 

 nent. These distinctions all show the greater immobility and permanence 

 of magnesia as compared with lime. 



Carbonate of magnesia as compared with carbonate of lime. Magnesium 

 carbonate, or magnesite, which occurs in nature as a mineral, is infusible in 

 the blowpipe flame, and is nearly insoluble in cold dilute hydrochloric acid. 

 It is insoluble in water. On the other hand the carbonate of lime, which 

 constitutes the bulk of all limestones and marbles, is not only soluble in 

 cold acid with rapid effervescence, but also in water, making what is known 

 as " hard water." 



Dolomite compared with calcite. If a grain of pure dolomite be placed in 

 hydrochloric acid it will effervesce very feebly, if at all. On applying heat 

 the solution is more evident. Calcite effervesces rapidly in hydrochloric or 

 nitric acid. Dolomite has a hardness of 3| to 4, and calcite has a hardness 

 of 3. Water containing a small amount of carbonic acid derived from the 

 soil, passing into the earth dissolves carbonate of lime from the rocks and 

 becomes hard. When it evaporates again, as in caverns, it leaves a small 

 sediment which by long accretion forms stalactites. In regions of magnesian 

 limestones the incrustations and stalactites that are formed in caverns con- 

 sist almost wholly of carbonate of lime, containing only a mere trace of 



