Species of hard Carbonate of Lime, See. 327 
of which measured 128° and 52. (See Plate VI. Fig. 1.) I found 
it, however, impossible to obtain any smooth and even fractures 
on the terminal faces of this prism. 
In some crystals, which were situated at the base of those 
which constituted the largest of the groups I examined, I ob- 
served perfeetty-formed hexaedral prisms, which appeared to 
me to have been occasioned by the two angles of 52 0 (belonging 
to the rhomboidal tetraedral prism of 128° and 52 0 , which I 
have just described,) having been replaced ; consequently, there 
were now two edges of 128°, and four others of 11 6°. See 
Fig. 2. 
The principal crystals which form the above group, although 
they are about three inches in length, do not show themselves 
distinctly, except at their upper extremity, being joined together, 
and entangled with each other, throughout the rest of their 
length. The upper extremity just mentioned, is a very sharp 
hexaedral pyramid, as in Fig. 3. The solid angle of its summit, 
taken upon two of the opposite sides, is of 15 0 ; and the sides of 
the pyramids, meeting together, form two angles of 128° each, 
and four others of 116 0 , as we have already seen to be the case 
with respect to the sides of the hexaedral prism ; consequently, 
the base of this pyramid is an irregular hexagon. 
Although the summit of this pyramid is sometimes formed in 
the manner above described, by the meeting of all its planes at 
the same point, (as is seen in Fig. 3,) yet it more frequently 
happens that the summit terminates in a ridge; the pyramid is 
then of a cuneiform shape, on account of the extension (which 
is sometimes very considerable) of two of its opposite sides, at 
the expense of the others, as in Fig. 4. Very frequently, indeed, 
the abovemen tioned extension is such as to cause the pyramids 
to be extremely thin ; they then appear in the form of a very 
U u 2 
