PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES. 503 
5th, While no structure whatever can with the lens be detected on the outer 
surfaces of these hollow crystals, their inner surfaces, whether a central core 
of calcite be present or not, are invariably studded with crystals of Dolomite, 
which are larger the greater the thickness of the walls, and largest when the 
calcite is altogether absent. 
In a word, the scalenohedron of Dolomite is not an individual crystal, 
built up through the action of crystalli-polar force depositing material upon 
a central nucleus, and so growing or incrementing from within outwards; but 
is produced by the juxtaposition of a multitude of individual crystals, which 
have cast or concreted themselves upon a mould, through replacement of its 
substance; and as that mould was removed by some solvent, have, by the 
aggregation of an ever-increasing quantity of their own material, continuously 
augmented in size, and so grown from without inwards. 
All specimens which have been through marine denudation dislodged, and 
are rolling on the shore, and also all specimens the walls of which have been 
crushed in, have the displacement complete—that is, the crystals contain no 
calcite ; while druses which are found and opened by quarrying into the rock 
still have the central calcitic core; and if they be deep-seated the Dolomite 
presents itself only as a thin layer, for it cannot be called a coating. 
Here then we unquestionably have pseudomorphism through true chemical 
replacement going on continuously almost under our eyes; and the question of 
deepest interest connected therewith is this,—How comes it that the resulting 
pseudomorphs are invariably wanting in solidity ?—the incompleteness of their 
filling up being greater in the ratio of the completeness of the chemical 
change. | 
Hollow pseudomorphs are far from uncommon ; BLum and BiscHoF mention 
many. The latter,—who hoped to solve the mystery of metamorphism solely by 
the application of Neptunian principles,—after the detail of a number of purely 
chemical experiments, at once opens his great work with pseudomorphic 
changes; and repeatedly and persistently returns to speak of hollow pseudo- 
morphs—founding upon them perhaps more than on any other bit of nature’s 
evidence. 
And yet his explanation of the mode of formation of these hollow pseudo- 
morphs—or, more precisely, of the cause of the vacuity thereof—does not 
satisfy. 
We find it first given at page 39 of the English edition; and it would 
appear that, read in the light of his law as afterwards enunciated, there is 
- —with all reverence be it spoken—some confusion of ideas in the way it is 
launched :—“ If during the metamorphism the specific gravity of the mineral is 
increased, this tends to give the mineral a porous structure.” Is this not con- 
founding effect with cause? In many cases of true chemical interchange the 
