LITHOLOGY AND AGE OF THE SERIES 369 



This vein is cut by dikes, showing its relatively great age. It is be- 

 lieved to extend past Barnum and to reappear on Kettle river, where a 

 similar quartz vein has been explored for gold west of Sturgeon lake. 



In the Blackhoof valley are several interesting veins. Quartz is the 

 dominant constituent still, but associated with it is siderite, somewhat 

 coarsely crj^stalline, which alters easil}^ leaving a soft, hydrous oxide 

 of iron. The siderite is not confined to veins, but is scattered exten- 

 sively through the neighboring schists in crystals and crystal clusters, 

 weathering easil}^ leaving the iron-rusted pits more or less thickly scat- 

 tered through the rocks. There is usually a film of dark green to black 

 talc like material enveloping the veinstuff and separating it from the 

 rock mass. 



West of Sturgeon lake there is an enormous number of quartz veins. 

 They are of the lenticular and gash- vein type. The rock becomes thickly 

 studded with garnets; its texture grows considerably coarser; the veins 

 are not wide, a few inches being the greater thickness. They frequently 

 anastomose in a very complex manner. 



Passing farther southwestward into the central Minnesota area the 

 veins are partly quartz and partly of the granitic type. The latter are 

 locally pegmatitic, with coarse and well developed feldspar individuals 

 imbedded in a matrix of hornblende and biotite, while elsewhere they 

 are finely textured, x)ossess a reddish color, and are highly silicious in 

 composition. 



The veins are thus noted because their lithology and distribution are 

 closely associated with the petrographic characters of the rocks under 

 discussion. They carry evidence which, taken in connection with other 

 lines, confirms in the writer's mind the close genetic relationship of the 

 entire series. 



Age of the Series 

 earlier views 



The age of these rocks has always been considered with reference to 

 the occurrences at Thomson or where the staurolitic biotite schists cross 

 the Mississippi river around Little Falls. 



The Thomson series were regarded as Animikie, Upper Huronian, 

 until Spurr announced in 1894, on what he regarded as sufficient evi- 

 dence, his belief that they were at least as old as the Keewatin.* - The 

 grounds on which the earlier correlations were based were partly theo- 

 retic and partly lithologic. General composition and structural habit 

 constituted the basis of determination. 



When T. Sterry Hunt, in 1883, found the concretions of the slates and 



*Amer, Jour. Sci., vol. 148, 1894, p. 162. 



