CORRELATION SCHEME. ie) 
Niagara has not been ashort one. Turning to Niagara, we find that the 
Upper Great gorge stands out clearly and strongly as the correlative of 
the present episode of the upper lakes. The making of this gorge could 
have begun only when the outlet had been changed from North Bay to 
Port Huron. Hence it is plain that the lower end or starting point of 
the Upper Great gorge is the precise correlative of that change. 
Going back one step farther in the reverse order of events, the critical 
episode of Niagara history comes clearly into view. In the upper lakes 
the next preceding episode is Nipissing Great lake, with the Nipissing- 
Mattawa river as its outlet. This leaves lake Erie as a perfect correla- 
tive, separate and alone, giving its relatively small and unaugmented 
overflow to Niagara. In the gorge of Niagara the episode next preced- 
ing the Upper Great gorge is the gorge of the Whirlpool rapids. If the 
relatively small Erie-Niagara river accomplished any gorge-making on 
its own account, its work must have been done in that section of the 
gorge which lies next below and contiguous with the Upper Great gorge, 
and the gorge which it would make would be relatively narrow and shal- 
low. The gorge of the Whirlpool rapids has these two characteristics 
clearly and strongly marked, and it lies in just that position—joining 
onto the lower end of the Upper Great gorge —which it would have if it 
were made by the Hrie-Niagara as the correlative of Nipissing Great lake 
and the Nipissing-Mattawa river. : 
Passing on in backward order of time to the next preceding episode 
of the lake history, we find that before the beginning of Nipissing Great 
lake the deep, narrow part of the Ottawa valley east of Mattawa was 
blocked by an arm of the receding ice-sheet. At that time a great gla- 
cial lake called lake Algonquin occupied the three upper lake basins 
and discharged its waters through the Saint Clair and Detroit rivers and 
lake Erie to Niagara.* Looking again for a correlative in the Niagara 
gorge, we find, next below the gorge of the Whirlpool rapids, the Eddy 
basin and the Cove section probably constituting together one continu- 
ous division interrupted only by the Whirlpool washout and the mak- 
ing of the two shoal ledges as explained above. ‘The section as a whole 
is wide and deep and has been called, provisionally, the Middle Great 
gorge. This episode of the gorge history is the correlative of the latter 
part of the lake Algonquin episode of the upper lakes. Lake Algonquin 
came to an end and ceased to discharge its waters through Niagara river 
when the ice-dam gave way in the Ottawa valley. The same event was 
the beginning of lake Hrie’s independence and of the making of the gorge 
of the Whirlpool rapids ; hence the contraction of width and depth at 
*F. B. Taylor: ‘‘A short history of the Great lakes,” as above. 
J. W. Spencer: ‘‘ Deformation of the Algonquin beach, and birth of lake Huron,” as above. 
