52 Graham — Pseudomorphs in Mc Gill University Collection. 



2. Pseudomorph after corundum from Perth, Ontario. 



The specimen here described is an unusually large and well 

 developed crystal of which a photograph is reproduced (fig. 2); 

 it is about 5 inches in length and the prism has a diameter of 

 2 inches. At first sight it somewhat resembles a crystal- of 



black quartz with rough surfaces, the predominant forms being 

 the hexagonal prism and bipyramid, but it is terminated at 

 one end by a fairly flat basal plane. It is also found, on closer 

 inspection, that the edges between the prism and pyramid are 

 truncated by faces belonging to a steeper pyramid, although 

 these can scarcely be distinguished in the photograph ; the 

 lower end of the crystal presents a somewhat blunted appear- 

 ance. The various angles were measured by means of the 

 hand goniometer, and were found to be those of a corundum 

 crystal, the forms present being : — a { 1120 \ , n \ 2213 \ , c \ 0001 \ , 

 and v )4483 \ (fig. 3). 



The crystal is almost entirely coated with black tourmaline, 

 usually as a compact granular layer forming a fairly smooth 

 surface ; but sometimes the grains are more distinct and have 

 bright facets, by which the mineral can be identified, as well 

 as by the strong pleochroism under the microscope. On one 

 side of the specimen (the left hand of fig. 2), however, the 

 crust consists of rounded grains of a soft, pale apple-^reen 

 mineral, associated with some pink calcite. From the low 

 specific gravity of the crystal (Gr= 2*6 about) it was evident 

 that very little, if any, of the original corundum remained 

 unaltered in the interior, and the specimen was accordingly 



