PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 777 



Conglomerate. Diabase.] 



Mic. This does not differ essentially from the foregoing, but shows pennine as 

 well as the other alteration products. One (thick) section. 



Age. Cabotian(?) N- H w _ 



No. 1757. CONGLOMERATE. 



West Twin peak, southeast from Ogishke Muncie lake, northern slope; about section 33. 

 Ref. Annual Report, xxi, page 158. 



Meg. Gi-eenstone, pebbly. 



Mic. The rock shows the characters of much of the Ogishke conglomerate. 

 With a green tint, due to the fine dissemination of honillem/ic shreds, modified by 

 leucoxene and hematite, there are numerous large quartzes, angular and sub-rounded, 

 and remnants of fragmental grains of feldspar. The most interesting feature is the 

 different degree of alteration that these old feldspars have suffered. Some are 

 wholly lost and some still show the albite twinning, while many are preserved 

 sufficiently to serve barely for proof that they once were perfect. This alteration 

 consists chiefly in the development within the areas of the feldspars of a micro- 

 granulitic mass which encroaches from the borders toward the centre. This micro- 

 granulitic substance consists largely of new feldspars and of quartz. It is nearly 

 free from hornblende or other coloring matter, but the areas are set in a surrounding, 

 greenish mesh which contains much debris of hornblende. Hence there is an aspect 

 as of pebbles. In some cases this micro-granulitic structure is very fine, but in others 

 it can be separated into its constituents by the higher power of the microscope, when 

 it appears to consist very largely of granular interlocking feldspars. One section. 



Age. Keewatin (Ogishke). N. H. w. 



No. 1758. DIABASE, frrn/ific.) 



Northern slope of West Twin peak. 

 Ref. Annual Report, xxi, page 158. 



Meg. Shows evident igneous characters. 



Mic. This rock was originally semi-ophitic, but now consists almost wholly of 

 urnlite arid plagioclase, with a small amount of magnetite and less of hematite. One 

 section. 



Age. Archean (Keewatin). 



Remark. It is only rarely that a true igneous rock is encountered among the 

 clastic strata of the northern slope of this mountain. N. H. w. 



No. 1759. DIABASE. ( Uralitic. ) 



North slope of West Twin peak. 

 Ref. Annual Report, xxi, page 158. 



Meg. A coarse uralitized diabase, similar to, but coarser grained than, No. 1758. 

 No section. 



Age,. Archean (Keewatin). u. s. G. 



