Species of hard Carbonate of Lime, &c. 329 
pentagons, meet together at the summit, in a ridge which is 
perpendicular to the axis, and form, with the edges of the py- 
ramids, on which they incline, an angle of 140°, as in Fig. 9. 
These planes are sometimes very distinctly striated, the striae 
being directed towards the summit; but, by following these 
striae over the whole surface of the crystal, it evidently appears, 
that they are occasioned by the aggregation of a greater or 
smaller number of very thin crystals, which are united by the 
broadest sides of their pyramids. 
Many of the crystals exhibit this variety combined with that 
represented in Fig. 8, as is seen in Fig. 10. The summit of the 
crystal is then terminated by a small tetraedral pyramid. 
It frequently happens, that there exists only one of the planes 
of the diedral summit, represented in Fig. 9. It nevertheless 
intercepts the summit of the pyramid, which then becomes 
terminated by a hexagonal plane, inclined upon one of the 
edges contiguous to one of the broad sides, in such a way as 
to form with it an angle of 140°. (See Fig. 11.) These crystals 
-are very often so considerably flattened, as to have nearly the 
appearance of very sharp isosceles triangular laminae, which 
have their summits truncated, in such a manner as to form with 
one of the broad sides an angle of 140°, as in Fig. 12. I have 
seen crystals of this variety, the thickness of which scarcely 
exceeded that of a sheet of paper; yet, notwithstanding their 
thinness, I found that they might easily be handled, without 
danger of being broken. 
Lastly, the summit of the hexaedral pyramid is sometimes 
terminated by a plane that is perpendicular to its axis. But I 
have never seen this last variety, except in combination with that 
represented in Fig. 9. This combination is shown in Fig. 13. 
