84 • JDr. Mac Culloch's Sketch of the 



As far as I have observed, the chabasite which I have described 

 is never, like the analcime, imbedded in the filamentous mesotype, 

 but is not unfrequently associated in the same cavity both with 

 stilbite and with analcime, nor is it unusual to find minute and well 

 formed crystals of this latter substance, imbedded in the crystal of 

 chabasite. In some cases perfect crystals of chabasite are lightly 

 sprinkled over the surface of crystals of stilbite, adhering so slightly 

 as to fall off on the slightest concussion : in other cases crystals of 

 this mineral, as well as of the analcime, are confusedly mixed with 

 rhomboids of carbonate of lime, hereafter to be described. 



The primitive crystal of this mineral is by much the most com- 

 mon, its modifications being rare and offering but few varieties ; it 

 is very frequently twinned, the angles of the one crystal appearing 

 on the faces of the other, nor is it uncommon to meet with it in 

 triplets, or even in more complicated groups, displaying an irregu- 

 lar mixture of prominent angles. The most common modification 

 consists in the truncation of one angle j sometimes two neighbour- 

 ing angles are truncated, and occasionally this defect extends to 

 three, the truncations being often so deep as to remove a third part 

 of the rhomb. In other modifications a single angle and a single 

 edge are removed, or the truncations extend to two angles and two 

 edges ; but I have not observed any specimens in which these de- 

 fects were extended to a greater number of edges. The edge is in 

 some cases replaced by two or by three planes, or even by a greater 

 number, so as to appear nearly rounded ; and each face of the 

 rhomb is also frequently replaced by two planes meeting in an edge 

 diagonally extended and sometimes rising by successive stages of 

 planes parallel to the original face. 



These crystals are sometimes opaque, more frequently transpa- 

 rent, but in by far the greater number of instances they acquire a 



