200 LIMESTONE FAMILY. lowish-white ; also pearl-grey; pale mounlain-grecn, and pale violet-blue. It occurs crystallised in the following figures : 1. Perfect equiangular six-sided prism, either equi lateral, or with two opposite sides broad, and four smaller lateral planes; or two opposite sides nar row , and four larger lateral planes. Sometimes the prism is so much compressed that it appears like a table *. 2. Six-sided prism, bevelled on the extremities, the bevelling planes set sometimes on the broader late ral planes, more rarely on the edges formed by the meeting of the smaller lateral planes, and the late ral edges and bevelling edges are sometimes trun cated. S. Oblique four-sided prism, bevelled on the extre mities, the bevelling planes set on the acute lateral edges, and the bevelling edge more or less deeply truncated. I. When the bevelling planes of the preceding figure increase very much, an acute octahedron is formed. I he crystals are middle-sized and small; they are ge nerally attached by their terminal planes, seidomer by their lateral planes; sometimes imbedded, and are to be observed intersecting each other. 1 he lateral planes of the crystals are sometimes smooth, more frequently more or less deeply streaked or grooved. The terminal planes are seldom smooth, generally uneven and rough, and sometimes also deeply notched. The * According to Bournon, the primitive form of this mineral is a rhom- hoidal four-sided prism, in which the lateral planes meet under ancles of tit 0 2' and 82° SH .