23 



There is every reason to believe that these quartzites 

 represent, in most cases at least, altered sandy sediments. 



(c) Gneises of Sedimentary Origin (Paragneisses) . 



These rocks differ distinctly in appearance from the 

 foliated granite- gneisses already described as constituting 

 the batholithic intrusions. They are fine in grain, and 

 show no protoclastic or cataclastic structure, the original 

 material having been completely recrystallized. They 

 have, therefore, an allotriomorphic structure with a 

 tendency for certain of the constituent minerals to be 

 elongated in the direction of the original bedding. While 

 quartz, feldspar and biotite are among the constituents 

 present, the mica is usually more abundant than in the 

 granite-gneisses. In addition to these, garnet, sillimanite, 

 graphite and pyrite are very frequently present, the 

 oxidation of the last mineral giving rise to a prevailingly 

 rusty colour on the weathered surface. These gneisses occur 

 in well-defined beds, and are usually found intimately 

 associated with the limestones. They resemble in many 

 lespects the hornstones which are found in granite contact- 

 zones, but are rather more coarsely crystalline than is 

 usual in this class of rocks. 



III. — Amphibolites. 



Intimately associated with these sedimentary gneisses 

 and the limestones on the one hand, and with the gabbros 

 and diorites on the other, is another class of rocks which 

 is grouped under the name of amphibolite. While many 

 varieties of these rocks occur in the area, differing con- 

 siderably one from the other in appearance, they have 

 in common a dark colour and a basic composition. Quartz, 

 one of the commonest constituents in the gneisses, is 

 absent or is present only in very small amount; while 

 hornblende and feldspar, the latter chiefly plagioclase, 

 are the main constituents of the rock. Pyroxene and 

 biotite often replace the hornblende in part. 



These rocks underlie large areas, as represented on 

 the Bancroft sheet. They also occur as interbedded 

 layers so intimately associated with certain developments 

 of the limestones that these limestone-amphibolite occur- 

 rences have been mapped separately. In places the sedi- 



