DOLOMITE. 115 
2nd. Limestone containing Dolomite, in which the pro- 
portion of magnesia is of course less than in true Dolomite; 
possibly magnesite containing Dolomite may also occur, but 
I do not know of this being demonstrated. 
3rd. True Dolomite, as defined, namely, a chemical com- 
pound, and not a mere mixture of carbonates of lime and 
magnesia. 
It is possible that mixtures of these three divisions may 
occur, but the data for a complete classification of magnesian 
limestones are still, to a great extent, wanting. 
The physical characters of magnesian limestone may be 
summed up as follows: its fracture is occasionally earthy, 
more often microcrystalline or saccharoidal; it is rather 
harder than ordinary limestone; in colour it is sometimes 
white or grey, and frequently yellow or reddish. 
On account of its hardness and resistence to atmospheric 
influences magnesian limestone is much used as a. building 
stone. Its power of withstanding atmospheric influences is 
however variable, for pieces of rock of the same composition 
will sometimes behave very differently in this respect. A 
true dolomite is a far better material for resisting atmos- 
pheric degradation than is a rock consisting merely of a 
mixture of calcite and magnesite, even though in the same 
atomic proportions as Dolomite. For important structural 
purposes it would be desirable for engineers to investigate 
this point. 
The origin or mode of formation of magnesian limestone 
is an interesting and much discussed question. | Was 
Dolomite deposited as such?, or has it been formed by a 
process of pseudomorphism from a rock already deposited ? 
In the majority of instances the second view seems to be 
the more plausible, as it is supported by evidence showing 
that carbonate of lime is capable of conversion into Dolomite. 
Thus, Dr. Sterry Hunt quotes an instance of an Orthoceras 
occurring in the non-magnesian Trenton limestone of Canada 
