Count de Bo urn on on the Laumonlte. 85 



the other on their ends. This structure, if I may be allowed the 

 comparison, is like the bud of a flower, in which the form of the 

 petals cannot be seen except by tearing it open and unfolding them. 

 May it not be the same in many other substances which occur also 

 in fascicular masses of more or less considerable size, such as tour- 

 maline, thallite, prehnite, analcime, stilbite, &c. May not also 

 the irregularity of the interior of these masses be at the same time 

 'owing to the difference in the variety of forms which these aggre- 

 gated crystals present ? and if it were possible to separate in these 

 masses each of the component crystals, with as much facility as they 

 are separated in the masses of the laumonite, might we not obtain the 

 same result ? 



There is a striking irregularity in the manner in which the planes 

 of substitution are situated on the laumonite, an example of which 

 may be seen in fig. .3, with respect to those by which the edges of 

 92° 30 in the prism are replaced. The same irregularity occurs 

 in the position of the planes which are the result of the other 

 modifications ; for example, in the crystal delineated in fig. 28, 

 there occurs in the place of one of the two obtuse angles of the 

 terminal faces, a plane belonging to the 9th modification, whilst 

 the plane of subs.titution for the other angle belongs to the 8th. 

 In fig. 30. there exists a still greater irregularity in the planes 

 of substitution of these two angles, one of them being replaced 

 by two planes, which belong to the 8th and 9th modifications, 

 and the other by a single plane of the 10th. This fresh example, 

 if it were necessary, might serve to shew the difference which 

 exists betvs/^een the retrogradations on primitive crystals, where 

 the geometrical form is one in v.'hich all the parts are perfectly 

 symmetrical, and those on primitive crystals, in which the parts 

 are not so circumstanced. A symmetiy is established between 



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