258 Count de Bournon’s Description of 
order to assume another ; this change caused the crystal to ter- 
minate by a pyramid less sharp; and, in many instances, it 
was evident that it had happened several times successively. 
These variations do not always take place in a regular order 
in the same crystal ; for it very often happens, that some of the 
faces have undergone two, three, or even four changes of in- 
clination, while others have not undergone so many ; and some- 
times, indeed, have not undergone any at all. I have seen some 
of these crystals, of which the irregularity was such that, upon 
some of the faces, the degree of inclination was changed from a 
greater to a less ; a circumstance which necessarily formed a 
depressed angle, and thereby produced a very irregular and 
even deformed shape, in the crystal itself. Among the very 
small number of crystals from the Carnatic which shew any 
disposition to assume the pyramidal form, I particularly observed 
one, in which this irregularity in the mode of decrease is very 
remarkable. This crystal, on three of its adjacent sides, appears 
to be a regular hexaedral prism ; but, from nearly the middle of 
two others, also adjacent, it becomes pyramidal, and of that 
modification in which the solid angle of the summit is of 50° ; 
and, from about one-third of the remaining side, it also assumes 
a pyramidal inclination, but of that modification in which the 
solid angle of the summit is of 40°. This crystal, which is 
represented in Fig. 26, is preserved in Mr. Greville’s collection. 
These pyramidal modifications also very frequently demonstrate, 
by the great number of transverse strias which are on their faces, 
and which sometimes resemble the steps of a staircase, the irre- 
gularity with which their decrements have taken place. 
Fourth Modification. The primitive rhomboid sometimes un- 
dergoes, in those acute angles which contribute to the formation 
