190 Mr. Brooke on Sarcolite and Woliastonite. 



ceived from Dr. Donati, have enabled me to give the annexed 

 figure and measurements of this substance; the primary form 

 of which is a square prism, whose terminal and lateral edges 

 are in the ratio of 62 to 55 very nearly. 



oAo, 



i2, 



d. 



oAj, B, G, 



2 



G. 



The planes al, a2, 



3 1 



are produced by the laws A, A, 



P,al = 157° 19' 

 P,a2 = 128 33 

 V,d = 90 

 F,c — 138 25 

 M,6l = 153 20 

 M,a2 = 123 34 

 M,J2 = 102 28 

 M,e = 153 26 

 M,ri = 135 



The plane b% as the symbol denotes, and as appears in 

 the figure, occurs singly on each angle, instead of being ac- 

 companied by a corresponding plane on the other side of c. 



Woliastonite from Vesuvius. 



This is the same mineral as was, according to the ticket 

 accompanying specimens from Vesuvius, formerly named 

 Zurlo7iite or Zuriitc. The crystals are generally very impei'fect, 

 and the surfaces very dull ; a specimen, however, in the pos- 

 session of Dr. Somerville presented some bright and well- 

 defined crystals, an examination of some of which has en- 

 abled me to give the annexed figure and measurements. The 

 primary form is an oblique rhombic prism. There is one 

 cleavage parallel to P, and three others parallel to the three 

 planes a3,h,c2; that parallel to h is the brightest, and I 

 have supposed it parallel to the edge of the prism, and have 

 thus assumed a different cleavage angle from that usually 

 assigned to tabular spar. I have, however, found a cleavage 

 corresponding to the plane //, in specimens from the Bannat, 

 in which the cleavages usually observed and hitherto quoted 

 are those parallel to P and c 2. The ratio of the terminal 

 and lateral edges of the prism which I have taken as the 

 primary is very nearly as 25 to 40, and the obtuse angle of 

 the terminal plane is 91°'56'. The planes are more than 

 usually perfect, several of the measured angles having agreed 

 exactly with those given by calculation. 



Planes 



