74 THE GEOLOGY OP MINNESOTA. 



[The Manitou epoch of eruption . 



the north. A single conglomerate on the south side has been found to have a thickness 

 of about 2,000 feet, and the total thickness of coarse conglomerate on Keweenaw 

 point has been estimated at 8,000 feet. The beds of this horizon on the north side of 

 the lake do not probably exceed 200 feet in thickness. The igneous lava flows that 

 followed the great conglomerate epoch are also much thicker on the southern side 

 than on the northern. In the former they are estimated by Irving at 33,000 to 35,000 

 feet at Montreal river, whereas in Minnesota they probable do not exceed 1,000 feet; 

 while, if the Puckwunge conglomerate and all the other fragmentals, together with 

 the Beaver Bay diabase and red rock, be united under the term Keweenawan, the 

 total thickness in Minnesota would probably not exceed 3,000 feet, and certainly 

 would not reach 5,000 feet. It is to the eruptives of the epoch following the Puck- 

 wunge conglomerate that is applied the name Mauitou. 



These lava sheets extend along the lake shore from near Baptism river to near 

 Grand Marais, except where they are replaced by the Beaver Bay diabase, or 

 by some of the intersheeted fragmentals. It is uncertain how much of the imme- 

 diate shore line east of Grand Marais is occupied by these traps. There is some 

 reason for believing that the so-called " black traps " of Irving, seen eastward from 

 the mouth of the Brule, are a portion of the Beaver Bay diabase. Some part of the 

 shore west from Grand Portage bay, as far as to near Red Rock bay, is made by trap 

 sheets of later date than the Puckwunge conglomerate. The geographic distribution 

 of the parts of the Keweenawan at Grand Portage, and for some miles westward, 

 cannot be said to be sufficiently studied. The question is complicated by the occur- 

 rence of diabase dikes of great thickness cutting even the latest of the known trap 

 sheets, as may be witnessed at Grand Portage island. 



As to the source of the diabase forming these later sheets, and also the later 

 dikes, it may have been from the same great gabbro mass, which must have extended 

 laterally, beneath the later rocks, to great distances, and which must have cooled 

 with great slowness. Such dikes and sheets may also have originated from the 

 Beaver Bay diabase itself before it became solidified. Whether from one or the other 

 they are of later and later dates toward the east and may be, at the last, as late as 

 the dikes cutting the Trenton limestone at Montreal, as suggested by Lawsou. 



