T" 
-= so deeply as entirely to: obliterate the 1 
_ mary faces P, at both extremities of the crystals; 
310 Minerals from New Holland. 
mary square prism is in élongated eryMtale, replaced 
on all the solid angles by triangular planes „and often 
ninal pri- 
thus tending to two four-sided pyramids having 
square bases, as in Fig. 1. But, as the crystals are 
usually implanted on the matrix in a vertical position, 
they commonly present only one of the pyramids, 
the apex being entire, or showing a portion (some- 
times only visible by the microscope) of the summits 
of the prism, as in Fig. 2: Sometimes one of the 
faces of the pyramids is considerably extended at the 
expense of the two adjoining ones. 
The lateral edges from a to a, Fig. 1, are kaij 
replaced, sometimes by a tangent plane, i inclining 
equally upon the two adjacent lateral planes, but 
most frequently by two planes, each of these being 
again followed by another very narrow plane, leaving 
scarcely any remaining portion of the primary faces ; 
thus imparting to the crystals an oval or cylindrical 
shape, and, in connexion with the low pyramids of 
the summits, rather a barrel-shaped appearance. 
Fig. 3 represents one of the crystals of occasional 
occurrence with the bevelment of the edges, the de- 
crement of the summit being complete and showing : 
the whole of the lower pyramid, where it unites with 
the matrix. Fig. 4 represents another of these crys- 
tals with the additional planes. 'The dotted lines on 
these figures are intended to show deep grooves, or 
strie, which extend longitudinally along the faces of 
the erystals, or parallel with their prismatic axes, an 
probably indicate the faces of cleavage in this direc- 
