The Petrography of the Quartz-porphyry, etc. 



63 



In places rocks of this class occur which contain much higher 

 proportions of gold than are usually present. These are found in the 

 immediate vicinity of intrusive dykes of diabase, and the metal, proljalily, 

 was impregnated into the acidic rocks by percolating waters at the time 

 of the intrusion of tlie diaV)ase. Some of the rocks obtained from the 

 tunnels at the Ironside placer in the Minnehaha District of the Potaro- 

 Konawaruk goldfield gave a mean contents of 65 dwts. of gold and 

 23 dwts. of silver per ton of the rock ; the contents of gold in the 

 various samples examined varying from 31 grains to 300 dwts. 



The schistose rocks contain small quantities of gold, the average 

 result of my assays showing them to yield at the rates of from less than 

 1 to about 3 grains of gold per ton of the rock. In places, however, 

 the quartz-sericite-schists have yielded from 24 to 36 grains of gold to 

 the ton. These have not been included in the above average. 



Mineraloyical Composition. — The following are the minerals and 

 aggregates of minerals which have been found to be present in the 

 rocks of this group : — 



1. Essential. 



Orthoclase, plagioclase, quartz, micropegmatite, biotite, muscovite, 

 augite and hornblende. 



2. Accessory. 



(a) Original. — Epidote, apatite, sphene, titaniferous iron, 



magnetite and garnet. 



(b) Secondary. — Epidote, sericite, chlorite, chloritoid, viridite, 



hematite, magnetite, leucoxene, sphene, zoisite, pyrite, 



gold and calcite. 

 The probable mineralogical compositions of the rocks of this series 

 which I have examined both microscopically and chemically are as 

 follows : — 



MASSIVE ROCKS. 



