1824.] Mr. Levy's Description of a new Mineral. 61 



The form M. Rose has taken for the primitive of cleavelandite 

 differs only in its angles from that T had assumed. He gives for 

 the incidence of T on M 117° 53'. I have constantly found it 

 upon brilliant cleavage planes 119° 30', or between 119° 30' and 

 120°, which makes a difference of 2° between our measure- 

 ments; mine agrees with that obtained by Mr. Brooke, and I 

 believe also by Mr. W. Phillips. This difference will of course 

 change most of the angles calculated by M. Rose, but not so 

 materially as to make it necessary to trouble you with the result 

 of my own calculations. 



The flat crystals from St. Gothard are, as M.Rose had suspected, 

 cleavelandite. It was indeed the observation of specimens of 

 that locality which are not hemitrope, as most crystals of that 

 substance are, that led me to the distinction of the two substances, 

 and which gave me the best data for the determination of the 

 primitive form. Mr. Turner's collection contains a great variety 

 of forms of that locality ; one of the most complicated I have 

 represented in tig. 10. In some of the crystals, the planes I 



3 



have marked d~-' and d? are wanting, and then the crystal has 

 precisely the same form as some of the varieties of felspar. In 

 the same collection are found crystals which are not hemitrope, 

 from the Tyrol and from Siberia. Those from this last locality 



are very large, and contain only the modifications p m t and o a 

 or o 2 , and the figure of the plane m is triangular. However, most 

 of the crystals are hemitrope, but their form is generally much 

 more complicated than those M. Rose has figured. He says in 

 his paper this substance is never found laminar, but from North 

 America, and from Silesia. I have seen specimens in large 

 laminae, each of which is formed by the juxta-position of two 

 laminae parallel to the face T of the primitive, so as to present, 

 when cleaved parallel to P, the same re-entering angles offered 

 by the hemitrope crystals of that substance. 



I shall feel obliged if you can spare room for a short descrip- 

 tion of, I believe, a very scarce and new mineral from Vesuvius. 

 I have observed it upon a specimen Mr. Heuland purchased at 

 the sale of Mr. Desse, to add to his private collection. This 

 substance occurs in small brilliant colourless and translucent 

 crystals. They are sufficiently hard to scratch rock crystal. 

 Mr. Children, who kindly undertook to examine a small quan- 

 tity of it, found it to be mostly composed of silex and mag- 

 nesia. The only form I have observed is represented by fig. 12, 

 and the crystals cleave easily in the direction of the plane p. 

 The angles I have measured with the reflecting goniometer led 

 me to adopt for the primitive form of this substance, a right 

 rhombic prism, fig. 11, the lateral planes of which corre- 

 spond to the planes marked m in fig. 12, and the base to the 

 cleavage. The incidence of the two lateral planes of the primi- 



