THE HISTORY OF THE NIAGARA RIVER. 239 



Perhaps a word of general explanation is necessary in speaking of 

 tlie attitude of tiie laud. Geologists are prone to talk of elevation and 

 subsidence — of the uprising of the earth's crust at one place or at one 

 time, and of its down-sinking at another place or another time. Their 

 language usually seems to imply the rise or fall of an area all together, 

 without any relative displacement of its parts; but you will readily see 

 that, unless a rising or sinking tract is torn asunder from its surround- 

 ings, there must be all about it a belt in which the surface assumes an 

 inclined position, or, in other words, where the attitude of the land is 

 changed. If the district whose attitude changes is a lake basin, the 

 change of attitude will cause a change in the position of the line marked 

 about the slopes of the basin by the water margin, and it may even 

 cause the overflow of the basin to take a new direction. 



The Ontario basin has been subjected to a very notable change of 

 attitude, and the effect of this change has been to throw the ancient 

 shore line out of level. When the shore line was wrought by the waves, 

 all parts of it must have lain in the same horizontal plane, and had 

 there been no change in the attitude of the basin, every point of the 

 shore line would now be found at the level of the old outlet at Rome. 

 Instead of this, we find that the old gravel spit near Toronto — the 

 Davenport ridge — is 40 feet higher than the contemporaneous gravel 

 spit on which Lewiston is built; at Belleville, Ontario, the old shore is 

 200 feet higher than at Rochester, New York ; at Watertown 300 feet 

 higher than at Syracuse; and the lowest point, in Hamilton, at the 

 head of the lake, is 325 feet lower than the highest poiut near Water- 

 town. From these and other measurements we learn that the Ontario 

 basin with its new attitude inclines more to the south and west than 

 with the old attitude. 



The point of discharge remained at Rome as long as the ice was 

 crowded high against the northern side of the Adirondack Mountains, 

 but eventually there came a time when the water escaped eastward 

 between the ice and the mountain slope. The line of the St. Lawrence 

 was not at once opened, so that the subsidence was only partial. The 

 water was held for short times at various intermediate levels, recorded 

 at the east in a series of faint shore lines. Owing to the attitude of the 

 land, these shores are not traceable all about the basin, but pass be- 

 neath the present water level at various points. 



Finally the ice blockade was raised in the St. Lawrence Valley, and 

 the present outlet was established. During the period of final retreat 

 the attitude of the land had slowly changed, so that it was not then so 

 greatly depressed at the north as before; but it hatl not yet acquired 

 its present position, and for a time Lake Ontario was smaller than now, 

 its western margin lying lower down on the slope of the basin. 



An attempt has been made in PI. iii to exhibit diagramatically the 

 relations of ice dams and basin attitudes to one another and to the river. 

 The various elements are projected, with exaggeration of heights, on a 



