no THE ROYAL SOCIETY OF CANADA 



schistose. The more massive phases often exhibit the ellipsoidal or 

 pillow structure so common in basic rocks of this age. 



Examined in thin sections under the microscope this basic rock 

 usually shows traces of ophitic structure, and appears to have been 

 originally diabase, oi similar igneous lock, which has been very highly 

 altered, the alteration consisting in the formation of secondary horn- 

 blende and chlorite from former pyroxenes, and in the decomposition 

 of the plagioclases with the foimation of saussurite and the introduc- 

 tion of carbonates, the latter being probably derived, in part at least, 

 from the decomposition of feldspar rich in lime. Quartz, usually 

 showing strain shadows, is a secondary constituent in some of the rocks 

 examined, while pyrite and magnetite are also both common secondary 

 constituents, some of the smaller grains of magnetite having a surface 

 covering of leucoxene. 



This greenstone is remarkably similar throughout, both in hori- 

 zontal and vertical extension, the alteration from its original character 

 being just as far advanced 200 ft. below the surface as on the surface 

 itself. 



Diorite-porphyry or Metagahro-porphyry. Intruded into the 

 greenstone is a diorite- or metagabro- or feldspar-porphyry, which, 

 on the surface, weathers to a dirty white colour, but on fresh exposure 

 varies in colour from light green to red. It is usually distinguished 

 by the presence of light-coloured phenocrysts of plagioclase, often 

 altered to saussurite, with occasional crystals of biotite, imbedded in 

 a finer grained matrix of plagioclase, hornblende and chlorite. Quartz 

 is also often present, and in some places to such an extent that the 

 rock becomes a quartz-mica-diorite. In other places the ferro- 

 magnesian constituents may be absent and the rock then partakes of 

 the character of aplite. 



As a general rule the contact of the diorite-porphyry and the 

 older greenstone is not a simple plane, but is rather a zone of varying 

 width in which the rocks are mixed together in a very irregular man- 

 ner. As originally formed, the contact was doubtless characterized 

 by fragments of greenstone included in the porphyry, and probably 

 also by tongues of porphyry extending into the greenstone. This 

 original complexity has since been greatly accentuated by the squeezing 

 and crushing to which both rocks have been subjected, accompanied 

 by the formation of a number of small faults in and along which the 

 rocks have moved in a very irregular manner. 



The porphyries may be correlated with the acid rocks which 

 Dr. Coleman has called Wawa tuffs in the Michipiroten district. 



