634 DR. J. A. THOMSON ON THE [DeC. I913, 



(c) The Fine- Grained Greenstones. 



Generally speaking, these rocks are most abundant in the 

 southern part of the ' North End,' passing gradually into the amphi- 

 bolites on the north and into the calc-schists on the south, -without 

 being positively confined to this area. They are dark-green 

 massive or slightly-schistose rocks when mined, but frequently 

 develop a latent schistosity after exposure for some years in dumps. 

 Microscopically they are exceedingly variable in appearance, but 

 agree in consisting of fine-grained aggregates of carbonates, 

 chlorite, sericite, quartz, and rutile, to which should probably be 

 added untwinned felspars, although these have escaped identi- 

 fication. There is seldom any well-defined structure, except in 

 some cases a tendency to parallel arrangement in the sericite and 

 chlorite, or the development of large rhombohedra of carbonates. 

 All gradations from the fine-grained amphibolites to perfectly 

 structureless greenstones can be traced, and it may be observed 

 that hornblende disappears before zoisite or epidote. 



In the eastern part of the ' North End ' are several occur- 

 rences of an apparently concretionary greenstone with small 

 light-coloured ellipsoids distributed in a dark-green dense matrix. 

 These whiter patches differ from the rest of the rock only in the 

 greater abundance of calcite, with a corresponding diminution in 

 the amount of chlorite. In all probability, these rocks are derived 

 from contact-altered amphibolites similar to those of Monument 

 Hill described above ; and it is found, on careful mapping, that 

 they are distributed around the margin of the great intrusion of 

 peridotite that lies within the greenstones of the ' North End.' 



Another type of greenstone showing abnormal characters may be 

 described here, although its position among the older greenstones 

 is not unequivocal. On the east side of Trafalgar Township, east 

 of ' The Mile,' are several dumps containing a schistose rock in 

 which small, black, lustrous crystals of chloritoid are plentifully 

 developed along planes oblique to the schistosity. This mineral 

 has sharper outlines and higher refringence than the chlorite 

 also present in the rocks, possesses a well-marked pleochroism in 

 blue-green to yellow-green tones, shows polysynthetic twinning 

 with oblique extinctions up to 28° measured from the trace of the 

 twin-plane, and has a high birefringence and strong dispersion. 

 A blowpipe examination failed to give a reaction characteristic of 

 manganese, so that ottrelite is excluded. The rocks consist 

 chiefly of calcite, forming broad bands of coarse mosaic alternating 

 with narrower bands of chlorite (pennine), and it is within or near 

 the last-named that the chloritoid is developed. Rutile is very 

 abundant in slender prismatic crystals showing knee-shaped and 

 heart-shaped twins, and is freely enclosed by the chloritoid. A 

 small amount of quartz and muscovite sometimes accompanies the 

 calcite. The rocks are often brecciated and recemented by a 

 matrix in which chloritoid and pyrite play a considerable part. 

 Chloritoid is usually regarded as a mineral characteristic of contact- 



