Species of hard Carbonate of Lime, &c. 331 
Another circumstance which might prevent our recognizing, 
at the first view, the crystals of this substance, when placed 
among those of common carbonates of lime, is, that the crystals 
of the latter substance are sometimes found in the form of a 
hexaedral pyramid, nearly as acute as that of the crystals above 
described; but, in that case, they break with the greatest facility, 
and the fractures are always smooth, and in the direction of the 
planes of the primitive rhomboid ; a circumstance that is never 
observed in the crystals of the hard carbonate. 
The matrix of this substance, in most of the specimens I 
have seen, is a brown oxide of iron, mixed with a portion of 
argill, and also with a considerable number of calcareous par- 
ticles. In some of these may be observed the primitive rhomboid 
of the common carbonate of lime, grouped upon the crystals of 
the substance here treated of. 
It may perhaps be questioned, whether the hard carbonate of 
lime I am now describing, (the mineralogical characters of 
which seem so much at variance with the chemical ones,) ought 
not to be referred to that kind which mineralogists have been 
already obliged to separate from the others, under the name of 
Arragonite, or whether it ought to be considered as different 
from the latter substance, and forming an additional new species, 
among the combinations of the carbonic acid with lime. It ap- 
pears to me very difficult to determine the above question. The 
primitive crystal which I obtained from it, is not sufficiently per- 
fect to serve as an accurate criterion ; for, as we have seen, that 
crystal, which is a rhomboidal tetraedral prism, cannot be divided 
according to its basis, that is, in one of its three natural direc- 
tions ; and this is exactly similar to what happens with respect 
to the primitive crystal obtained from the Arragonite. I confess, 
however, that if I were obliged to adopt an opinion, I should 
