PHYSIOGRAPHY. 101 



Calcareous Spar. 



Fig. 83. The primary with its lateral edges replaced by 

 tangent planes, a: P on *-.= 135. The crystals of this 

 modification differ in the length of the new planes a. Fig. 

 84. The primary with its lateral angles replaced by tangent 

 planes, b. Fig. 85, has the planes elongated : this is the 

 most common modification of the species. The planes P 

 are often irregularly extended ; and sometimes one or two 

 of them are wholly wanting from each extremity of the 

 prism. P on 6 = 135. Fig. 86. The primary, having its 

 summit replaced by a tangent plane. Fig. 87 and 88, the 

 same, with the new plane more extended : these form the 

 basees of HAUY. P on c=135. The faces c are liable to 

 a pearly lustre. Fig. 89 unites the modifications of Figs. 

 84 and 86. Fig. 90, the same, with the faces b more ex- 

 tended so as to produce an elongated prism. Fig. 91, the 

 regular hexagonal prism, produced from Fig. 90 by the ex- 

 tension of c : the prismatique of HAUY. Occasionally, the 

 prism is reduced in length to a mere table. Hartz. Some- 

 times this modification is affected by the undue enlarge- 

 ment of certain of the lateral faces, so as to convert it into a 

 trihedral, tetrahedral or pentagonal prism. Fig. 92. Pri- 

 mary, with its upper edges replaced by tangent planes. P 

 on d=149 2'. Fig. 93, the same, more deeply replaced. 

 Fig. 94, d on c/=13426'. This is called the equilateral 

 rhomboid : it is the equiaxe of HAUY. It is a very abun- 

 dant form of Calcareous Spar. Fig. 95, the same, having 

 the lateral angles replaced by tangent planes. donbl=z 

 116 34'. Figs. 96 and 97, the same, in which b I is va- 

 riously produced. Fig. 98, the equilateral rhomboid, with 

 its lateral edges replaced by tangent planes. These prisms 

 are sometimes elongated. Fig. 99, the same as Fig. 97, 

 9* 



