178 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



dyke there is first 'observftd a differentiation of the rock into con- 

 stituents of different periods of crystalization, the order being fix'st 

 plagioclase in more or less idiomorphic* lath-shaped individuals lying 

 in all positions, then augite generally allotriomorphic,* sometimes 

 hypidiomorphic* and finally a base or matrix of both these minerals 

 in a very much more finely crystalline state together with magnetite. 

 The structure of the base is rather obscure, the chloritic substance 

 usually present in diabase rocks being more prominent here than in 

 the coarser grained part of the dyke when it is almost or perhaps 

 entirely wanting. Quartz is present but in smaller quantities than 

 in the coarser grained portions of the dyke. The augite occurs both 

 in simple individuals and in polysomatic masses. The uralitization of 

 the augite, which is generally observable, is much more pronounced 

 in the irregularly bounded polysomatic masses than in the simple 

 allotriomorphic development of the same mineral. A few garnets 

 are present as inclusions in the feldspar but were not identified with 

 certainty. In this respect this portion of the dyke differs mai-kedly 

 from the moi-e central portions examined. Thg most interesting 

 constituent of this portion of the dyke remains, however, to be 

 mentioned. It is the non-pleochroic colorless rhombic pyroxene, 

 enstatite; it occurs in idiomorphic development showing the char- 

 acteristic obtuse domes in some of the sections. It shows regular 

 cleavage parallel to xi P (110), upon which the angle of extinction 

 is zero, and characteristic cross parting along which partial alteration 

 of the mineral to bastite or serpentine is apparent. This enstatite 

 is not abundant and plays the role of an accessor}' mineral. Its 

 occurrence in a rock of well mai'ked diabase structure is interesting. 

 Rosenbusch remarks that it is yjresent in only a few diabases which 

 have a gabbro-like structure,+ and Teall has recorded the occurrence 

 of the allied rhombic pyroxene bronzite in the Whin-Sill of the 

 north of England as an accessory. :j: Enstatite also occui's in a 

 variety of the allied rock diabase jiorphyrite from Schaumberge, 

 which has been described by Laspeyres and Streng under the name 

 Palatinite. This enstatite was not observed in the coarser parts of 

 the dyke but occurs, as will be noted, in the still finer grained diabase 

 at the contact. 



* Terms introduced by Rosenbusch. Cf. op. cit. p. 11. 

 t Mik. Phys. der.Massigen Gestine, 2nd Ed., 1886, p. 188. 



♦ Q. J. G. S.,1884, p. 652. 



