132 The Ottawa Naturalist. 



show a slight undulatory extinction thus indicating that the 

 rock has been subjected to pressure since their formation. 



SilUmanite presents some of the largest individual crystals 

 of the section. It occurs in longitudinal sections some of which 

 show parallel cleavage, and all, parallel extinction. Both the 

 single and the double refraction are high, which give it a 

 prominent appearance in the slide. No distinct pleochroism has 

 been observed. Several of the crystals are broken. One 

 basal section is present with a nearly rectangular outline and 

 showing distinct diagonal, (?'. e. parallle to the macropinacoid), 

 cleavage. 



A few grains of red garnet and pyrite are enclosed by 

 biotite. 



The structure of the rock is schistose and it may be termed 

 a sillimanite gneiss. 



A rock 'of this class from St. Jean de Matha, Que., recently 

 described by Dr. F. D. Adams, (Am. Jour. Sci. July, 1895), has 

 been determined by him to be in all probability an altered 

 sedimentary rock of a very old formation. It differs from the 

 present specimen chiefly in having a greater amount of garnet 

 and less biotite. 



This rock is also a common constituent of the Grenville 

 and other metamorphic series. 



NO. 3. OLIVINE DIABASE. 



This rock consists essentially of plagioclase, augite and 

 olivine, with a few grains of iron ore as an accessory constituent. 

 While apparently semi-crystalline it probably possesses an 

 ophitic or true diabasic structure. 



The feldspar is for the most part fresh and well preserved 

 and occurs both as lath-shaped crystals enclosing triangular 

 areas of augite and other minerals, and as minute individuals of 

 the same form in a dark groundmass. A few of the larger 



