HISTORICAL SKETCH^LAKE SUPERIOR REGION. I 25 



rocks constituting the series are mainly surface flows. The gab- 

 bro flows are often of immense thickness. The diabase flows 

 are usually much thinner, and frequently pass in their upper parts 

 into porphyrites and amygdaloids. Many flows are porphyritic 

 or amygdaloidal throughout. The beds of quartz-porphyry and 

 felsite are abundant in certain districts, but usually have no great 

 lateral extent, but while a single flow may be traced but a little 

 way, frequently a group of flows of the same general character 

 may have a great extent and thickness. But even the groups of 

 flows cannot be regarded as general formations for the whole of 

 the Lake Superior basin. 



Since the number and thickness of the volcanic beds as well 

 as the detritals vary greatly, the Keweenaw series as a whole 

 is widely variable in different districts in its character and thick- 

 ness. Structurally, Irving has divided the series into two parts, 

 a lower division, in which eruptives are present, and an upper 

 division, in which eruptives are absent. In any one section of 

 the Keweenawan, at the lower part of the lower division, are 

 generally found numerous volcanic flows, with few or no detrital 

 beds. In passing toward the middle of the series the sandstones 

 and conglomerates become more and more numerous and of 

 greater thickness. Still higher the sandstones and conglomerates 

 become predominant, and finally volcanic products disappear, 

 the upper ten or fifteen thousand feet of the Keweenaw series 

 being wholly composed of mechanical detritus. A given detrital 

 bed varies from a mere seam of narrow local extent to thick beds 

 of sandstone and conglomerate, one of which has been traced 

 by Marvine for more than one hundred miles. The most gen- 

 eral detrital formation is the upper sandstone and conglomerate. 



The Keweenawan rocks extend about the entire area of the 

 Lake Superior basin. They appear upon the east shore of Lake 

 Superior, cover a large area of Keweenaw Point, northern Wis- 

 consin, eastern and northeastern Minnesota, and a a^reat area 

 about Lake Nipigon. A similar set of volcanics, occupving a 

 like stratigraphical position, is also known adjacent to Hudson 

 Bay, and this may be a contemporaneous series. 



