75 

 Carboniferous f 



The rocks tentatively grouped under Carboniferous 

 correspond to those of the Rossland group, subdivided 

 at Phoenix into the Knob Hill group and the Attwood 

 series. Their broad structural relation suggests a synclinal 

 fold. 



Knob Hill Group — ^The Knob Hill group consists of a 

 complex of highly altered rocks of igneous origin with 

 minor developments of sediments. The more common 

 types are massive breccias, tufifs and cherts with small lens- 

 like masses of argillite and limestone. Hornblende and 

 augite porphyrites, as dykes and sheets, are less 

 common than in the Rossland area. The most marked 

 feature is the widespread silicification and extensive 

 development of cherty types. The absence of definite 

 structural features prevents an estimate being made of 

 the thickness of the group, but it is known to have a 

 vertical depth of over i,ooo feet (304-8 m.). 



Attwood series. Brooklyn formation — ^The Brooklyn 

 formation overlies the Knob Hill group without any 

 marked stratigraphical break. Originally consisting of 

 limestones with some tuffs and calcareous argillites, the 

 formation now admits of a threefold division based on 

 lithology. The lower zone consists of jasperoids and the 

 upper is composed essentially of lime-silicates (garnet, 

 epidote, etc.), while the limestone forms residual lens- 

 like bodies in both of the above zones. 



The jasperoids are derived from limestone tuffs and 

 fine breccias by replacement along planes of bedding, 

 jointing and fracture. They consist of oval, rounded, 

 oblong and subangular pebble-like individuals of crypto- 

 crystalline quartz varying in size from almost microscopic 

 grains to those six inches (15 cm.) or more in diameter. 

 The color is usually dull white or grey, but pink, brown 

 and bright red (jasper) types are locally prevalent. The 

 matrix consists of calcite and shreds of chlorite. Pyrite 

 is usually present. The jasperoids originating from 

 coarse tuffs or breccias contain fragments of different 

 types of volcanic rocks. All transitions are to be seen 

 in the field from the normal limestone to jasperoid in 

 which the original limestone has entirely disappeared. 



