16 
ON THE CRY^TAT.IJSATXONS 
and thus bodies are produced like that repre- 
sented fig, 32. 
The law expressed in I. 1 . is the same that occurs in 
the variety, fig. 5, considered above ; it is met with in many 
instances in those represented fig. 2, 7, 8, 9, 11 ? 14, 19, 
26. This sort of aggregation is very often repeated in 
faces parallel to themselves, from which, if it is in crystals, 
forms result like fig. 33. It extends, however, also to 
massive varieties, which then consist of alternating laminae 
of this mineral to be distinguished from each other by the 
different direction of the faces of cleavage. It is evident 
that the alternating laminae must be in parallel position 
with each other, on account of two revolutions of 180° 
being required in the same plane to determine the position 
of the third lamina in respect to the first. 
The law I. is not so generally found as the foregoing ; 
it gives rise to forms as fig. 34, where the individuals are 
of the variety fig. 6. The faces a and ; a" and a"' of the 
pyramid P+l fall into the same plane ; so do the faces 
b, y of the pyramid P. This is a necessary consequence 
of the face of composition being perpendicular to the ter- 
minal edge of the pyramid P+l ; which edge is parallel to 
a line drawn on the face of P from the apex, perpendicular- 
ly to one of the lateral edges of this pyramid. 
Some varieties, as that of fig. 12, are subject to the law 
II. 1., and then produce regular aggregations, as figg. 35, 
36, in the latter of which, as is' sometimes the case, the 
3 
faces of between the apices of the two individuals 
are wanting. The compound consists very often, by a re- 
petition of this law, like that occurring in I. 1, of alternat- 
ing laminae, parallel, now to one, now to the other indivi- 
dual. The variety of regular aggregation, or composition, 
