204 Lawson on diabase dykes cf the (Rainy lake region. 
At 6 feet from the contact the rock is fine grained and the 
ophitic structure of typical diabase is much more characteristic- 
ally developed than in the coarser grained parts of the dyke. 
In this part of the dyke there is first observed a differentiation of 
the rock into constituents of different periods of crystallization, 
the order being first plagioclase in more or less idiomorphic^ 
lath-shaped individuals lying in all positions, then augite gen- 
erally allotriomorphic,^ sometimes hypidiomorphic' and finally 
a base or matrix of both these minerals in a very much more 
finely crystalline state together vith 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 where 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 inpolysomatic 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 min- 
eral. A few garnets are present as inclusions in the feldspar 
but were not identified with certainty. In this respect this por- 
tion of the dyke differs markedly from the more central portions 
examined. The 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 id- 
iomorphic development showing the characteristic obtuse domes 
in some of the sections. It shows regular cleavage parallel to 
CO P (no), 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 accessory mineral. Its 
occurrence in a rock of well marked diabase structure is inter- 
esting. Rosenbusch remarks that it is present in only a few 
diabases which have a gabbro-like structure,^ and Teall has re- 
corded the occurrence of the allied rhombic pyroxene bronzite 
in the Whin-Sill of the north of England as an accessory.' En- 
1 Terms introduced by Rosenbusch. Cf. op. cit. p. ii. 
- Mik. Phys. der Massigen Gesteine, 2nd Ed., i8S6, p. 188. 
° Q.J. G. S., 1884, p. 652. 
