L. V. Pirsson — Remarkably developed Calcite Crystals. 61 



Art. IX. — On some remarkably developed Calcite Crystals / 

 by Louis V. Pirsson. 



Beautiful specimens of calcite are procured at G-uanajuato 

 in Mexico, and through the dealers have found their way into 

 collections. On these specimens, not only are simple scalen- 

 ohedrons of the ordinary form, 1 3 (.2131), common, often of great 

 beauty in the perfection of their crystal form, but also twins, 

 which are often developed in a most remarkable manner. As 

 these, so far as can be learned, have never been described, it 

 has been thought that an account of them with some figures, 

 would not be without interest. 



The twins consist in general of the ordinary l 3 scalenohe- 

 dron, twinned always upon 01l2, — -J-. By a development 

 of two faces of the scalenohedron and the corresponding 

 ones in twin position, the crystals are lengthened out into four 

 sided prisms, with a re-entrant angle at one end and a salient 

 angle at the other. They are usually attached to the rock at 

 the salient angle with the prism projecting and at first sight 

 present no appearance of hexagonal symmetry. They appear 

 rather like twinned monoclinic crystals, consisting of a prism 

 and pyramids, the latter forming the re-entrant angle at the end. 



