222 The American Geologist. April, 1891 
crystals it is inferred that the rock was related to the norites. In 
one case strongly marked transitions from the ordinary diabase to 
serpentine was observed. Olivine has been detected only in very 
small amount. 
HISTORY OF LAKE AGASSIZ. 
By Warren Upham, Somervilln, Mass. 
FromPartE. of the annual reportof the Geol. and Nat. Hist. Sur., Canada, 1888-89. 
II. 
The foregoing observations show that the ice-sheet was melted 
away from at least half of the area of lake Agassiz during its 
Herman stages. During the ensuing Norcross, Tintah, Campbell, 
and McCauleyville stages, through which the lake continued to 
outflow southward by the river Warren, the recession of the ice 
doubtless permitted it to extend north and east beyond lake Win- 
nipeg and along the lower valley of the Saskatchewan. Each of 
these stages is represented by two or three beaches in northern 
Minnesota and North Dakota and in southern Manitoba, which, 
with the seven beaches of the Herman series, make seventeen 
shore lines recognizable in that part of the lacustrine area belong- 
ing to the time of its southern outlet. Between the Herman and 
Norcross beaches the channel of the river Warren was eroded 
about 25 feet; it was deepened 15 to 30 feet more at the time of 
the Tintah beaches; 10 to 20 feet farther down to the Campbell 
beaches ; and again 10 to 20 feet to the McCauleyville beaches. 
In all, the mouth and southern end of the lake were lowered about 
100 feet between the highest Herman beach and the lowest Mc- 
Cauleyville beach. Proceeding northward, the vertical distance 
between these beaches gradually increases to 240 feet on the inter- 
national boundary, the difference of 140 feet more than the de- 
pression caused by erosion of the outlet being attributable to the 
northward rise of the land and subsidence of the water-level. 
Before lake Agassiz could obtain an outlet to the northeast, the 
thick ice-sheet that had filled the basin of Hudson bay was so far 
melted, as to admit the sea, which at first covered the land west of 
James bay 350 to 500 feet above the present sea level. Eleven 
stages of lake Agassiz are marked by beaches that lie below the 
beds of lakes Traverse and Big Stone, which were the channel of 
the river Warren when the lake ceased to outflow to the south. 
