220 PROCEEDINGS OF THE CAN'ADIAX INSTITUTE. 



cold occurred where Lake Superioi" is, and that jjreat icebergs 

 streamed from that point into a southern sea. The great de))th of the 

 lake, even now, gives color to this supposition. With thousands of 

 feet of newer formations distributed over its bed, it is yet the deepest 

 ■depression on the continent, and the belts of greatest depth — sur- 

 vivals, so to speak, of its foi-mer greater profundity — run southerly 

 and south-westerly. Along its westerly shores, from Thunder Cape to 

 near Duluth, embracing the Isle Royale, the present depth is from 

 800 to 1000 feet, and as the level of the lake is 6U2 feet above 

 tide water, its floor is here from 200 to 400 feet lower than the level of 

 the sea. At the early epoch we are now considering, the lake was per- 

 haps many thousand feet deeper still, and there was an uninterrupted 

 stretch of ocean all the way to the present Gulf of Mexico. As the 

 detritus from the primitive formations settled in this sea — a-s the 

 Alleghanies gi'ew and the Rocky Mountains developed — this ocean 

 contracted into a gulf, running south from Lake Superior. As the area 

 of the gulf became restricted by the continuance of these processes, it 

 would assume a shape not unlike that of the Baltic of to da}' ; one arm 

 running up from the present position of St. Louis, by Duluth, to Black 

 and Thunder Bays ; the other from the same point up the valley of the 

 Ohio, and by Chicago north and east. Then might well follow the 

 period spoken of in "The Geology of Minnesota," Vol. I., p. 35, 

 when the lake region was cut off from the salt water, when Lake 

 Superior was 500 feet above its present level, but the water still ran 

 by the St. Louis valley (Duluth) to the Mississijipi. Afterwards the 

 southerly out-flow stopped, the regions between the lake and the gulf 

 rising further yet, and a new eastward channel to the Atlantic gi'ew 

 into being — this compai'atively recent system now under many names 

 having an outlet to the ocean by the noble St. Lawrence. Gen. 

 Warren's map, showing a stream connectmg Lake Winnijieg with the 

 Mississippi, while an arm of the Gulf of Mexico brings salt water as 

 far up the gi-eat valley as the parallel of Chicago, the Ohio and the 

 Missouri being extensions of this arm, represents a very probable late 

 stage of the tran.sfoi'mations referred to. The scenes are still shift- 

 ing, the secular change now going on is doubtless on the lines of 

 the past, the basin of Lake Superior is still filling up, though as ever, 

 very slowly ; soundings almost everywhere disclose a bottom of 

 clay, brought down by the rains and rivers from its rim, still being 



