1 3 4 Mr. Vanuxem's Description of a Lamellar Pyroxene. 



in the latter being infusible and different in its composition. 

 From amphibole, because there are but two cleavages in 

 this mineral, obtainable with the same ease, and both possess- 

 ing the same degree of smoothness and lustre, in short, abso- 

 lutely identical. The angles which they form are those of 

 124° 34', and 55° 26', angles which do not occur in the West 

 Point mineral. The chemical elements of amphibole and py- 

 roxene are the same, the difference being in the proportion 

 of their constituents. 



The essential character of pyroxene is derived from its 

 crystallization. The primitive form determined by its cleav- 

 ages, and with the aid of its secondary forms, is an oblique 

 prism with a rhombic base ; angles of the prism, by the com- 

 mon goniometer, 92° and 88°, or more accurately by the se- 

 condary forms, joined to certain theoretical considerations, 

 92° 18', and 87° 42'. The cleavages of pyroxene, as given by 

 its several varieties, are parallel to the faces of the prism and 

 diagonals of the base. One of the secondary forms of the py- 

 roxene is an octagonal prism, with angles of 136° and 134° 

 by the goniometer, or corrected in the aforementioned man- 

 ner 136° 09' and 133° 51'. The degree of smoothness and 

 facility of obtaining the different cleavages of pyroxene, vary 

 considerably in the different varieties of this mineral. Some- 

 times a cleavage which is very evident in one variety is indi- 

 stinct or scarcely to be perceived in another. Thus in certain 

 volcanic pyroxenes the cleavage parallel to the larger diagonal 

 is the most lamellar (according to Hauy), and none exists in 

 the direction of the base, whilst in other pyroxenes that of the 

 base is pre-eminent; so that this character, in all cases, must 

 be considered as very secondary in value to the character 

 which depends upon the angles of the cleavages and those of 

 the crystals. 



Thus the mineral of West Point and of Delaware corre- 

 sponds with pyroxene in the, form and value of the angles of the 

 primitive form, and those of the octagonal prism ; in the cleav- 

 ages parallel with the s-ides of the primitive form, and diago- 

 nals of the same : in hardness, action with the blowpipe, specific 

 gravity and chemical composition. The only difference being 

 this, that its lamellar structure is parallel to the smaller and 

 not to the larger diagonal, as in volcanic pyroxene, a circum- 

 stance which cannot be considered of specific importance. 



XXV. No- 



