151 



developed duiing a state of plasticity. Soma of the inclu- 

 sions retain their angular shapes, some are rounded, and 

 others have been drawn out into thin ribbons by magmatic 

 movements. They are always notably crystalline, being 

 chiefly hornblende gneiss, amphibolite, or biotite gne^'ss. 

 The "transition zone" thus developed in the Laurentian 

 near its contacts with the older rocks varies in width from 

 a few yards to five miles (8 km.) and merges by reduction 

 of the inclusions into ordinary Laurentian gneiss. 



KEWEENAWAN. 



Near Lake Nipigon the greatly dissected Keewatin- 

 Lower Huronian and Laurentian surface is unconformably 

 overlain by a sedimentary series consisting of a thin basal 

 conglomerate, sandstone, and impure ferruginous dolomite. 

 The dolomite is usually bright red in colour and contains 

 disseminated gypsum and traces of salt crystal casts. 

 These formations, which are Keweenawan in age, are 

 almost flat-laying and little metamorphosed. They have 

 been intruded by somewhat later dykes and thick sills 

 of diabase similar to the ore-bearing diabases of Cobalt 

 district and the south shore of Lake Superior [7; 12.]. 



ORDOVICIAN. 



The eroded Laurentian surface of the western part of the 

 region is also unconformably overlain by undisturbed 

 sandstone and impure limestone of Ordovician age [9]. 



PLEISTOCENE. 



All of the preceding solid rocks are glaciated and are 

 overlain by a thin mantle of unconsolidated gravel, sand, 

 boulders and boulder clay, materials which were laid down 

 in association with the Pleistocene continental glaciers. 

 These glacial and fluvio-glacial deposits are in turn over- 

 lain, by stratified clay, sand and gravel deposited in the 

 previously mentioned glacial lakes, Agassiz, Warren and 

 Ojibway. All three of these lakes are believed to have 

 been dammed on the north by the waning ice sheet. 



Lake Agassiz was a vast body of water which extended 

 throughout nearly the whole of southern Manitoba as well 

 as the adjacent portion of Minnesota, North Dakota, 



