INTERPRETATION OF PHENOMENA AS TO UPPER GREAT LAKES. 73 
outlet the water was conducted, after passing over lake Nipissing and 
the col east of it, down the Mattawa valley eastward to the Ottawa. The 
measured discharge of the several lakes shows that lake Erie contributes 
on the average only about one-ninth of the present volume of Niagara 
river.* It follows that if at any time since Niagara first began at Lewis- 
Ficure 2+.—Hydrography of the Lake Region during the Time of Nipissing Great Lake. 
ton any circumstance led the three upper lakes to discharge by one of 
the other outlets, then so long as that condition lasted Niagara was de- 
prived of eight-ninths of its water; and if this condition, being once 
established, lasted for any considerable time it would seem certain that 
*L. Y. Sechermerhorn: ‘t Physical Characteristics of the Northern and Northwestern Lakes,” 
Am. Jour. Sci., iii, vol. xxxiii, April, 1887. 
+The heavy line surrounding the three upper lakes marks the boundary of their watershed, and 
the heavy line around lake Erie bears the same relation to it. The conditions shown are those of 
the time of Nipissing Great lake with the Nipissing-Mattawa river as its outlet. Lake Erie was then 
independent with the Erie-Niagara river, carrying its unaugmented overflow, as its outlet. The 
lake shores of that time were not exactly the same as now, but the difference was so slight that they 
haye not been represented separately. They are now aboye the present lake level north of the ap- 
proximate nodal lines VV, and beneath it, south of them, the level of lake Superior being 20 feet 
higher than lake Huron. The shaded area in the Saint Lawrence and Ottawa valleys and the basin 
of lake Ontario represents the approximate extent of the contemporary marine Champlain sub- 
mergence, ; 
