270 ROYAL SOCIETY OF CANADA 



mon in the alteration rims of the pyroxenes. îsTosean is fairly common 

 in dusty grains which are nearly always isotropic. Sphene is very 

 abundant in the usual wedge shaped crystals. Apatite, ilmenite and 

 pyrite are frequent accessories. The order of crystallisation has been, — 

 apatite, ilmenite or pyrite, augite, hornblende, sphene, nosean, felspar, 

 nepheline. This rock is called a nepheline aplite by Mr. Allan. The 

 other patches and dykes of breccia have this same base. Slides Nos. 1, 

 6, 7, 16 and 18 agree very closely with this description. Nos. 19, 21, 

 and 23 are characterized by the addition of segirine-augite, with this 

 exception being otherwise the same. 



Medical Building, McGill University. 



A small occurrence of breccia was found when excavating for the 

 foundations of the Medical Building at McGill University.^ Here a 

 fissure in the Trenton limestone held a breccia principally of limestone, 

 but containing in addition a fragment of typical Laurentian gneiss. 



A thin section shows the material to be made up of fragments of 

 limestone and shale, surrounded by an abundance of quartz and felspar 

 grains, the latter being much kaolinized. The grains are seen in various 

 stages of brecciation, from unopened cracks to those with the fragments 

 actually separated. Several flakes of biotite are present, most of them 

 showing bending and folding from the pressure of the surrounding 

 grains of quartz and felspar. (See photomicrographs, figs. 15 and 16.) 

 The paste is entirely decomposed only calcite being now present. The 

 fragments and general appearance of the materials are very similar to 

 those of the Ste. Helen's Island breccia. 



Ste. Helen's Island. 



As has already been mentioned, this occurrence has been the subject 

 of numerous investigations, the most recent and the most complete of 

 these being embodied in the paper by Miss Nolan and Miss Dixon. In 

 the present instance no further field work has been done, but a more 

 thorough study has been made of the petrography of the breccia. The 

 following outline of the geology of the island is largely quoted from the 

 paper cited above. 



Two geological formations are present on Ste. Helen's Island, and 

 these have had a marked influence on the topographic features. The 

 southwest half of the island, underlain by Utica shale, is low and level, 

 while the northeast area underlain by breccia, is rolling and compara- 

 tively hilly. Both the shale and the breccia are cut by dykes and sills 

 which are genetically connected with the Mount Eoyal intrusion. 



1 Dr. Deeks, Can. Rec. Sc, LV, 1S90, p. 105. 



