246 THE CANADIAN NATURALIST. [Vol. ix, 



Silurian limestones, but also the dolerite of Mount Royal. 

 Rounded masses of the diorite of precisely similar character 

 occur in the Lower Helderberg conglomerate or breccia of St. 

 Helen's Island. Those, therefore, who would classify eruptive 

 rocks according to age, would say that Mount Royal is a diabase 

 and not a dolerite. Admitting such to be the case, how is it, 

 the question may be asked, that dykes of jjJionoUte are abruptly 

 cut oif by the diabase, when phonolite, according to the chrono- 

 logical theory, ought to be of Tertiary or more recent age ? It 

 may be that future investigations will solve the difficulty, but, in 

 the meantime, the eruptive rocks of Montreal do not seem to fall 

 into their proper place in a classification based upon age. 



II. PYROXENE AND URALITE. 



Of all the mineral associates of apatite in the Ottawa region, 

 pyroxene is the most constant and the most abundant. In one 

 form or another it is probably present in all the apatite deposits, 

 excepting, perhaps, some of the calcareous veins with imbedded 

 apatite crystals. The most common variety appears to be an 

 aluminous sahlite or lime-magnesia-iron pyroxene, but a light- 

 coloured variety, probably diopside or malacolite, is also common. 

 Less frequently a beautiful black kind maybe observed, excellent 

 examples of which have been obtained from the thirteenth lot of 

 the eleventh range of Templeton. It is here associated with 

 green apatite, white orthoclase, scapolite, graphite and small grains 

 of titanite. The pyroxene crystals often contain little round or 

 irregular mases of the orthoclase as well as scales of graphite, 

 and their surfaces are sometimes coated by broad plates of the 

 last-named mineral. The crystals differ from those of the more 

 ordinarily occurring sahlite not only in colour, but also, to a 

 certain extent, in chemical composition and form, having the 

 faces of the inclined rhombic prism usually much more fully 

 developed than the clinopinacoid, and presenting rather different 

 pyramidal terminations. The observed planes are those of the 

 inclined rhombic and rectangular prisms go P. oo Pco . [x Pco ], 

 combined with the pyramidal faces P. 2 P.-P. and the clinodome 

 [2 P oo]. The faces of the rhombic prism are often developed 

 almost to the exclusion of the ortho- and clinopinacoid. In some 

 crystals the pyramidal planes are pretty equally developed, but 

 in others much distorted. In the specimens examined the basal 

 plane oP. is absent, but there is a very distinct basal cleavage. 



