(Jj'iffill (1.11(1 A(/(' (>!' flic hd II re iilid II T.dl.cs. I'plKllli. 170 
reduce the river and to make it erode more slowly during 
portions of its histoiy than at present; but other ciianging 
conditions liave witli equal certainty promoted its erosion and 
so have tended to shorten the time required for the excavation 
of its gorge. With one exception, the conditions of variabil- 
ity appear to have tended, in the aggregate, to sul)tract from 
tile time indicated l)y the present rate of erosion, rather tlian 
to add to it. 
TflK HyPOTHKSIS OF THE XiPISSING AND MaTTAWA OUTLKT 
FROM Lakes Huron, Michigan and Supkriok. 
Among the conditions which have been supposed to cause 
Niagara river to vary from its ])resent size, only one would 
produce a great and long continued diminution of the river, 
so giving for a large part of its history only very slow erosion 
and recession of the falls. This hypothetical factor in our 
l)roblem, which has been assumed by Gilbert, Wright, Sjien- 
cer, and Taylor, to considerably prolong the time of the gorge 
erosion, is the diversion of the outtlow from the three lakes 
above lake Erie to forsake its present course and pass east- 
ward from Georgian bay by the way of lake Nipissing and 
the Mattawa river to the Ottawa. On the other hand, Bell 
and Barlow, of the Canadian Geological Surve}^ oppose this 
hypothesis, and state that the}' find no evidence of a former 
great river there. 
An examination of the relationship of tlic scvei-al glacial 
lakes in the St. Lawi-ence basin, and of the appioxiniate out- 
lines of successive stages of retreat of the ice-sliecl wliicii was 
their barrier (as shown in figure 1), convinces nie that tliis 
Mattawa outlet from lakes Huron, Michigan and Superior 
was obstructed by the receding ice-front until after the land 
there had risen from its Late Glacial orGhaniplain depression 
to such altitude that the St. (Jlair and Detroit i-ivers contin- 
ued to be constantly the outlet from those lakes, sending their 
waters to the Niagara river and falls during all their histiu'y. 
Lake Algonquin was at first held on the Mattawa district l>y 
an eastern barriei- of th(^ waning ice-sheet, as is known l)y the 
Nipissing beach far above the present lowest ])oint of tlie wa- 
tershed. But difi'erential elevation of the land, as soon as it 
was unburdened by the glacial retreat, took place here, as on 
the area (If lake Agassiz. while vet the ice barrier remained. 
