PETROGEAfHY OF ONTARIO. S3 



the margin iu its angle of extinction, the light or shade beginning in the centre and 

 flowing wave-like to the circumference on revolution between crossed niçois. This 

 probably results from a vague zonal structure, the centre differing chemically from the 

 exterior. 



Hornblende. — In the specimens examined, this mineral is generally in tolerably compact 

 crystalline masses. It is highly pleochroic, changing from blue-green or brownish-green 

 to yellow or yellowish-green. Occasional twins are seen, having the orthopiuacoid as 

 twin plane. 



Biolite. — A little biotite usually occurs, generally brown or dark green, sometimes 

 with parallel plates of light grass-green mica on the outside, perhaps a result of 

 weathering. 



Magnetite is uniformly present, unless replaced by titanic iron ore or ilmenite with 

 a wide rim of leucoxene. Titauite is sometimes found, and in a few cases, augite is 

 suspected. Epidote and other decomposition products of the felspar and hornblende are 

 of course very often met. 



With few exceptions, the drift rocks of this group belong to the diorites proper. In 

 one specimen, however, the biotite is important enough iu amount to warrant us iu 

 setting off the rock as biotite-diorite. Another may be named diorite-porphyrite, since 

 reddish and greenish plagioclase crystals are thickly disseminated through a dark green 

 magma, consisting of minute strips of plagioclase and hornblende, with grains of magne- 

 tite. This rock is decidedly handsome. 



Diabase. 



The rocks in which some species of augite is an essential ingredient are not very 

 numerous in the drift, but are interesting from the variety of minerals they contain. 



Macroscopic. — These are generally rather coarse-grained rocks, differing from the 

 diorites in having a darker, often purplish, grey colour, and in wanting the light-colored 

 felspars, which are apt to give a spotted look to the latter when weathered. 



Microscopic. — Quartz is rare in the drift diabases, having been found with certainty in 

 only two of the sections examined. Orthoclase is found, more or less doubtfully, it is true, 

 in about half the sections studied, Ijut iu small amounts. 



Plagioclases. — These are found in considerable quantities in almost all sections. 

 The twin structure is characteristic, and there are frequently two systems of striations 

 crossing at an angle of 86°. In addition to this Carlsbad twins are sometimes fouud. 



In composition they appear to belong chiefly to the varieties rich in calcium. The 

 angle of extinction on each side of the twin plane was found in a few cases to be about 7°, 

 corresponding to audesine. It was generally, however, in the neighborhood of 19°. In a 

 few crystals, where the angle could be read for both sets of twins 24J' and 31° or 32° 

 were observed, indicating labradorite. 



Most of the larger crystals are rendered more or less greyish by innumerable fine dots 

 and minute dashes, often arranged iu rows, sometimes so as to look curiously like the 

 short hair of an animal. These inclusions maybe arranged iu two or even three planes, 

 forming triangles iu the latter case by their intersections. Oblong brown plates with 

 clean-cut angles and others of round or oval forms are also seen. , Apatite crystals are 



