lO ROYAL SOCIETY OF CANADA 



quartz seems to^ be entirely lacking, but an examination of thin sections 

 under the microscope often reveals this mineral, sometimes in no incon- 

 siderable amount. 



The rocks of these several groups pass into one another by imper- 

 ceptible graduations. The magmas of all four types were in places 

 supersaturated with alumina, this excess crystallizing out as free alumina 

 in the form of corundum. In those varieties of nepheline syenite which 

 are unusually rich in nepheline and in the rocks of the Urtite group, 

 the corundum is only developed Avhen the iron magnesia minerals do not 

 occur in any appreciable amount. 



Intimately associated with the rocks of these several types and form- 

 ing part of the same igneous complex are certain abnonnally coarse 

 phases which are their pegmatitic equivalents. These may occur ^as 

 parallel or intercalated bands or they may cut across the foliation of 

 the rock in the form of dyke-like masses. The contact of these peg- 

 matites with the parent or nonnal plutonie rock is sometimes quite sharp, 

 especially in the case of those which intersect the foliation. They 

 usually, however, present a rather abrupt, though quite perceptible, 

 transition into the ordinary medium grain type. The nepheline syenite 

 pegmatite is usually composed altogether of nepheline and albite. Some- 

 times very large individuals of biotite and occasionally of hornblende, 

 apatite and magnetite are present. The pegmatitic form of the red 

 syenite is made up almost exclusively of microperthite, consisting of an 

 irregular intergrowth of orthoclase and albite. 



The term S3'enite as applied to the nepheline syenite, in the north- 

 eastern portions of the syenitic band, is in some cases somewhat of a 

 misnomer, for plagioclase varying from albite through oligoclase to 

 andesine is the prevalent and often the only feldspathic constituent. 

 Dr. Adams^ in his paper announcing the discovery of these occurrences 

 in the townships of Dungannon and Faraday, made reference to this as 

 follows. — " If the distinctive character of the nepheline syenite named 

 Litchfieldite by Bayley be the replacement of the orthoclase by albite, 

 this rock is a more typical Litchfieldite than that from the original local- 

 ity. The propriety of defining nepheline syenite as a rock composed 

 essentially of nepheline and an alkali feldspar, instead of one composed 

 of nepheline and orthoclase is rendered evident, as otherwise it would bo 

 necessary to classify this rock as a theralite from typical specimens of 

 which it would differ greatly in composition.'' 



These various rocks, while sometimes quite massiv<\ possessing a 

 true hypidiomorphic granular structure, usually have a more or less per- 



' American Joiirnal of Soi.nco. vol. XKVIH. 1S94. ]t. in. 



