126 



sented by an outline on the left hand of the plate to show 

 its modification, and another more complicated on the 

 right. By examining these it will be found that they mo- 

 dify principally on one side of the crystal, leaving the other 

 sometimes unchanged. The right hand one is more com- 

 pound than any of Haiiy's crystals. 



The lower specimen has part of a pyramidal face exposed, 

 and the plated fracture is very distinct as well as the shining 

 lustre of the surface: this somewhat resembles the specimen 

 from the Isle of Man, and is the most usual form of large 

 ones. 



TAB. CLXVI. 



1 he rarity of this substance in determined crystals, espe-* 

 cially in Great Britain, is a sufficient excuse for figuring a 

 specimen of the present appearance, which has no preten- 

 sion at first sight to value. Experience, however, has 

 shown us that in some things that rarely crystallize a 

 characteristic specimen is a tolerable prize. The present 

 has some small but determined crystals when examined 

 carefully with a lens, and the little middle figure is a sin- 

 gular formation of hexangular plates, which being deposited 

 at the lower part regularly in equal-sized plates, form an 

 hexaedral column. Those immediately above the column, 

 diminishing by degrees, form part of a pyramid; and a few 

 plates of a little larger dimensions, hanging slightly over 

 at the apex, seem to explain by this small irregularity, that 

 the plates formed before they were deposited. The lower 

 middle outline explains the primitive rectangular figure, and 



