4 A. P. COLEMAN 



into greenstones and green schists; but there are acid rocks in 

 important amounts — quartz porphyries, felsites, etc., and their 

 schists. In a number of places these eruptives were lava flows 

 showing pillow and amygdaloidal structures, and often pyroclastic 

 materials accompanied the outbreaks of lava. It is probable 

 that most of the characteristic Keewatin eruptives were volcanic, 

 though the squeezing and shearing they have undergone often 

 obscure their origin as lavas or ash rocks. Undoubted plutonic 

 rocks occur among these surface eruptives, but often they can 

 be proved to be much later in age, since they have penetrated the 

 other rocks and carried off fragments of them. 



There are also many dikes of both basic and acid rocks cutting 

 the volcanics, and there were probably laccolithic sheets and 

 masses invading them; but later mountain-building processes, 

 connected mainly with the elevation of granite batholiths, have 

 greatly obscured the relationships. While terrestrial lava flows 

 and falls of bombs and ashes played the most prominent part in 

 the formation of the Keewatin in many places, submarine lava 

 flows may have taken place also, since the pillow structure is 

 generally regarded as resulting from the action of water on hot 

 lava streams. There is every reason to suppose that then as now 

 there were volcanic eruptions both on land and from the sea bottom. 



Through what substratum these volcanic rocks came to the 

 surface is unknown. At present they commonly rest on the 

 gneiss and granite of the Laurentian — deep-seated eruptives of a 

 later age, which have invaded and swept off fragments of the 

 Keewatin rocks in ways showing that they were cold and solid at 

 the time. The floor on which the lavas flowed and the volcanic 

 ashes were rained down has generally vanished, though in places 

 the volcanics rest on sedimentary schists or gneisses of the 

 Couchiching, which will be described later. 



In most cases the old volcanoes themselves have disappeared, 

 but the base of one of them, consisting of gabbro, anorthosite, and 

 granite, has been described by Lawson, from Shoal Lake east of 

 Rainy Lake. 



It was an age of intense volcanic activity, and the results were 

 just such as we find in the Keweenawan and more recent eruptive 



