PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES. 505 
irreconcilable with his previously enunciated law. Nor certainly is this better— 
« At the same time, however, it must not be forgotten that the substitution of 
carbonate of magnesia for carbonate of lime (in the Dolomitic pseudomorph) is 
only partial.” 
It is any such substitution whatever, in opposition to his own law, that 
has to be explained ; and this cannot be regarded in any way as the “ proving 
exception;” nor is it so regarded by Biscuor himself, though he so trippingly 
here passes it by. 
His fifty page chapter commences—“ No rock has attracted greater attention 
than Dolomite.” In no part of his book does he, who tries to prove that water 
can do anything and everything, more firmly hold to its action than while show- 
ing that Dolomite has been formed from limestone, through the action of per- 
colating waters, which carry magnesia in solution as a sesquicarbonate, and 
which interchange that magnesia for lime ; riddling and rendering porous, in the 
so doing, the whole stratum from top to bottom ; and all this in direct opposition 
to his law—‘“ the displacing mineral is always less soluble than that which is 
displaced.” 
It is singular how closely Biscuor has, as it were, grazed the true explana- 
tion without hitting on it ;—a consideration of the precise extent of the “ only 
partial” replacement of carbonate of lime by the magnesia salt in Dolomite 
would have led to it. He has, perhaps, been even nearer when he writes—“ V. 
Mor.oT mentions a calculation made by ELIz DE BEAuMonrt, according to which, 
on the assumption that if of two equivalents of carbonate of lime, one equivalent 
was removed, and one equivalent of carbonate of magnesia deposited in its 
place, in the production of bitter spar, limestone would, in its conversion into 
Dolomite, have been reduced in bulk 12:1 per cent. He was induced by this 
calculation to estimate the actual proportion of hollow spaces in Dolomite to the 
entire mass. For this purpose he took Dolomite of average porosity, and found 
that the hollow spaces amounted to 12°9 per cent., or very nearly the calculated 
value.” He was, perhaps, nearer still when he says—“ Dolomite presents in 
its rent and fissured condition so great a resemblance to some hollow pseudo- 
morphs of bitter spar after cale spar, that this character alone seems almost 
enough to induce one to regard them as having been produced in the same 
manner. These pseudomorphs prove positively that crystallised carbonate of 
lime may, by combination with carbonate of magnesia, be converted into a double 
carbonate. If this is the case with crystallised carbonate of lime, it would be 
so likewise with amorphous carbonate of lime. Consequently, Dolomite would be 
produced wherever carbonate of lime in any state is brought into such condi- 
tions as are requisite for conversion into double carbonate of lime and magnesia.” 
Throughout this it must be evident that eqguivalentic replacement was mistily 
seen; though the force which produces and governs equivalentic interchange 
VOL. XXVII. PART IV. 6s 
