J. W. Spencer — Duration of Niagara Falls. 463 



gan and Huron) with an outlet by way of the Ottawa valley, 

 and Lundy Lake (occupying the Erie basin and) extending 

 into the Ontario valley. These two bodies of water appear to 

 have had a common level as if connected in some way across 

 the Ontario basin, but their northeastern extensions are not 

 known and involve unsettled questions that do not effect the 

 history of Niagara. Again the waters were lowered so that 

 the Niagara River emptied the overflow of the Erie basin, 

 without a fall into the Ontario valley. This condition did not 

 last long, for the waters sank to a level (Iroquois Beach) of 

 300 feet below the Lundy (and also Algonquin) plain and the 

 falls commenced their descent with the waters of the Erie 

 basin alone. The subsidence was accompanied by slight 

 pauses, but waters remained for a long time at the level of the 

 Iroquois beach, which is now about 135 feet above Lake 

 Ontario at the end of the gorge. Again the waters subsided 

 to the level about 80 feet beneath the present level of the 

 head of Lake Ontario and thereby lengthened the river to 12 

 miles beyond the end of the chasm. At this time the descent 

 of the river after passing the rapids at Johnson's ridge was 120 

 feet. By the continued northeastern terrestrial elevation the 

 waters of the Huron basin were turned from the Ottawa 

 drainage into the Erie basin, whose northeastern rim was ele- 

 vated so as to flood the lake. Later, the waters at the head of 

 Lake Ontario were raised 80 feet to the present level. This 

 differential movement was at zero at the head of Lake Erie ; 

 2*5 feet per mile in the Niagara district ; 1 feet northeast of 

 Lake Huron, and 5 feet per mile at the outlet of Lake Ontario. 

 At the nativity of the Niagara River, there was no fall. 

 A little later in the Iroquois episode the falls were very much 

 like the modern American cataract both in height and volume, 

 but afterwards it increased in magnitude and went through 

 the changes noted later. 



8. Laws of Erosion. 



When erosion is considered from a theoretical point of view 

 and the whole energy of the water is supposed to be expressed 

 in the erosion, it varies as the mass of the water into the 

 square of the velocity (wv~). Hence for a given river increase 

 of the amount of its water or increase of the velocity along 

 its course should be expressed by greater erosion. But erosion 

 is not the only expression of the theoretical value of the 

 energy of the river. Again it is well known that the more 

 rapid the descent of the stream the more the erosive 

 effects are expended on the floor of the channel, in deep- 

 ening and forming U-shaped valleys or gorges. On the other 



