[TYRRELL] PRE-CAMBRIAN GOLDFIELDS OF CENTRAL CANADA 111 



An analysis of a specimen of this porphyry given by Mr. Bruce^ 

 contained about 50 per cent, of feldspar, chiefly Albite. 



Near the porphyry-greenstone contact narrow dykes of felsite 

 were seen not far from the Railway station at Swastika. They are 

 very similar in composition to the Metagabro-porphyry and may be a 

 later phase of the same magma. 



Iron formation or Jaspylite. I did not see any Iron Formation 

 in place in this district, but pebbles of Jasper derived from it are abun- 

 dant in the overlying conglomerate of the Temiskamian series, and Mr. 

 Burrows has recorded its occurrence at several places in the vicinity. 



TEMISKAMIAN. 



Conglomerate and Greywacke. Unconformably overlying the 

 greenstone and diorite-porphyry are conglomerates, greywackes, etc. 

 They consist of a green fine-grained ground mass, the particles of which 

 are in places rounded and waterworn, while in other places they are 

 sharp, crystalline and angular, suggesting a volcanic or tufaceous 

 origin. Some beds are fine-grained throughout, while others are 

 packed with well-rounded, waterworn pebbles or boulders of the 

 older rocks, such as greenstone, diorite-porphyry, red jaspilite, etc. 

 In the finer varieties of these sediments it is often difficult to distin- 

 guish them with the naked eye from altered igneous rocks, but the 

 presence of particles of jasper usually furnishes a distinguishing 

 characteristic. 



All have been deposited in water in a horizontal or approximately 

 horizontal attitude, but since their deposition they have been squeezed, 

 folded, and upturned, so that the beds are often highly inclined or 

 vertical. Many of the pebbles are shattered and broken, and in some 

 cases the green matrix has been squeezed in between separated portions 

 of pebbles of such distinctive rock as red jasper. 



POST-TEMISKAMIAN OR ALGOMAN ? 



Lamprophyre. In the railway cutting east of Swastika Station 

 dykes of minette are intruded into the Keewatin greenstone. The 

 rock is characterized by slender hornblende needles averaging one- 

 eighth of an inch in length, and a few plates of chlorite. In a ground- 

 mass of orthoclase and plagioclase are crystals of hornblende, biotite 

 (now chlorite) and diopside, while apatite is an abundant accessory. 

 While the biotite has been altered, the other mineral constituents are 

 quite fresh, in which particular it differs strongly from the older Kee- 

 watin rocks through which it cuts. 



1 E. L. Bruce. The Swastika Gold Area. 21st Rep. Ont. Bur. Min. 1912, p. 259. 



