Grah am — Pseudomorphs in Mc Gill University Collection . 53 



sawn in half at right angles to the prism ; it was found that 

 the tourmaline forms a thin outer shell (except on one face, as 

 noted above) whose thickness is very uniform and seldom 

 exceeds l mm , and that no visible trace of the original corundum 

 exists. The interior is filled with a pale green material, hav- 

 ing a greasv feel and a hardness between 2 and 3, which is, in 

 fact, identical with that which forms the outer surface on one 

 side of the crystal ; dispersed through this are a few patches 

 of white to pink calcite, and numerous minute scales of 

 pennine and of a silvery white mica (damourite) ; there are 

 also one or two small patches of the granular black tourmaline. 

 For the most part, the green material appears quite compact 

 on the smooth surfaces obtained by cutting through the crystal, 

 but on a broken surface it has a more definite prismatic struc- 

 ture, with a pair of fair cleavages inclined at about 90°. The 

 cleavage angle of these prisms could only be determined 

 approximately by maximum illumination measurements owing 

 to its irregularity and the dull lustre. Thin fiakes are trans- 

 lucent but exhibit no definite optical characters under the 

 microscope. The specific gravity is about 2"6 and the streak 

 white. The mineral fuses easily and quickly in the Bunsen 

 flame to a white glass, and gives off Avater when heated in a 

 closed tube; it is insoluble in acids. Owing to the intimate 

 mixture with calcite, and especially to the presence of the 

 small pennine and damourite flakes, it is difficult to select a 

 pure sample for analysis, and the material used may possibly 

 have contained small amounts of these minerals. A single 

 analysis gave the following : 



SiO„ 43-05 



Al o q 30-28 



Fe^CT 3-30 



MnO 0-49 



CaO 1-85 



M 2 6-70 



Kfi 3-68 



Na 2 0-92 



Ignition 10-70 



100-97 



The above analysis serves at least to show the approximate 

 composition of the pale green material which constitutes the 

 main portion of the interior of the crystal, so far as can be 

 judged from the surface exposed by sawing through the speci- 

 men. This appears to be a mineral of indefinite composition 

 belonging to the pinite group. In view of its (probable) 

 rectangular cleavages, it may be noted that this mineral 



