Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/383



The only crystals of this substance in which I have found perfectly determinate forms, except the primitive prisms and its fragments, have occurred to me in the bardiglione from Hall, in specimens that included groups of these crystals. Most of them, though having secondary faces, are so united together, commonly in a direction parallel to their planes, that, till they were broken, and their planes thus exposed, they appeared absolutely to compose but one crystal, as is also the case in the laumonite. Owing to this their additional faces are most frequently destitute of that lustre, which commonly belongs to the crystals, and are even in general striated. The: fractures made in the direction of the diagonals, which are difficult to obtain, are likewise usually striated, thus exposing to view the edges of the laminæ placed on the planes of the prism, especially on those which have a pearly lustre.

I have represented at fig. 23, that which was already given at fig. 14, but of which, in the crystal that has furnished me with the variety I possess, as well as all those I have inserted in the plate annexed to this work, one part alone is in a state of perfect preservation. I placed this crystal so, that I could show the natural interior joints, as they are perceived in looking through its terminal faces. The lines that indicate these joints are very conspicuous in it. The two planes 1 and 3 are equally devoid of all lustre and striated, and both of them appear parallel to the interior joints. That marked 1, which I consider as belonging to a fracture, and which might have been produced by a retrogradation by a single row along the edges of the prism with square bases, makes with the faces of the primitive crystal adjacent to it an angle of 135°. That marked 3 appears to answer to fig. 24. pl. II. ef the Abbé Haüy's Tableau Comparatif; but instead of making with the primitive planes an angle of 129° 56′, as mentioned by that celebrated author, that