DESCRIPTION OF THE AREAS 285 



then disappears under a heavy overburden of clay. The succession is as 

 follows : 



Granite. 



(Intrusive contact.) 



Volcanic flov^s and pyroclastics. 



Mica scliist. 



Paragneiss. 



The oldest rock is a finely banded, gray gneiss which in places carries 

 garnets. In its upper part it is interbanded with a black mica schist. 

 In places the contrast between the gray or reddish gray bands and the 

 black schist bands is very marked. The bands vary in width from less 

 than an inch to over a foot. Above the interbanded zone the series con- 

 sists of black banded mica schist. The whole was once clearly a series of 

 clastic and shaly beds which have been highly metamorphosed. 



The volcanic rocks consist of andesitic greenstones which in places 

 show flow structure. Certain parts are composed of large oval masses 

 composed of the same material as the flows and considered to be pyro- 

 clastics. Dikes of porphyry lamprophyre were found cutting the vol- 

 canics. 



Structurally, the schists and paragneisses everywhere dip underneath 

 the greenstone belt. The dips average around 45 degrees. The series 

 forms a monocline cut off on either side by intrusive granite. 



LAKE OF THE WOODS AND RAINY LAKE AREAS ^^ 



The Lake of the Woods and Rainy Lake region is one of the classic 

 areas of pre-Cambrian geology, and the succession determined there has 

 played an imjjortant part in pre-Cambrian nomenclature. It was there 

 that the term Keewatin was introduced by Lawson, and the intrusive 

 relationship of the Laurentian granite to the Keewatin volcanics was 

 shown by him in 1885. In 1911 Lawson restudied the Rainy Lake re- 

 gion. The pre-Cambrian succession, according to his latest work, is as 

 follows : 



Keweenawan Diabase dilies. 



Algoman Porpliyroid gneiss, banded and streaked 



gneiss, granite and granite - gneiss, 

 syenite gneiss, basic phase of syenite. 

 Huronian. 



Seine series (Upper Huronian) . Lamprophyre rocks. 



Quartzite and slate. 

 Conglomerate. 

 Laurentian Granite and granite-gneiss. 



" A. C. Lawson : The Archean geology of Rainy Lake restudied. Geol. Surv. Can., 

 Memoir 40. 



