126 THE JOURNAL OF GEOLOGY. 



The Keweenawan is the thickest of the series about Lake 

 Superior, its maximum being estimated by Irving at the Montreal 

 river to be fifty thousand feet. From this thickness it varies to 

 nothing. This vast quantity of material does not, however, of 

 necessity mark a period longer or perhaps even as long as the 

 Lower Huronian or Upper Huronian, for the greater part of it 

 is of igneous origin. The lava flows in their extent and thick- 

 ness are to be compared with the great volcanic plateaux of the 

 far West, rather than with local volcanoes such as Vesuvius, or 

 the local volcanoes of the Upper Huronian and Lower Huronian. 

 Associated with the lavas no volcanic fragmental material has 

 been as yet discovered. 



The source of the lavas of the Keweenawan is beyond the 

 scope of this paper. It was, however, suggested that the fusion 

 of a portion of the Basement Complex, and even Lower Huron- 

 ian, may have in part produced the deep-seated magmas, the 

 extrusion of which produced the Keweenawan lavas. 



In large measure the sandstones and conglomerates derived 

 their materials from the volcanics of the series, but a lesser quan- 

 tity came from earlier series. This latter is particularly true of 

 the great detrital formation constituting the topmost member of 

 the Keweenawan. Partly because fragments derived from the fel- 

 sites and porphyries are more resistant than those from the basic 

 rocks, acid pebbles are relatively abundant in the conglomerates. 



The fact that erosion was contemporaneous with eruption for 

 much of Keweenawan time is to be noted. Certainly, when the 

 period was well inaugurated, most of the Lake Superior basin 

 was normally below the sea or near tide water. Many of the 

 eruptions may have been sub-aqueous. Here and there volcanic 

 masses of such magnitude were built up as to rise above the 

 water, and upon such areas, the sea at the base, and the air and 

 rain above, immediately began their course of destruction. The 

 acid and more viscous lavas may have formed the more promi- 

 nent elevations, and thus the attack was here more vigorous. 

 This may partly explain the predominance of the acid pebbles 

 in the conglomerates. 



