Science, Geology and Physics 



through the American channel, so that the conditions affecting 1907 

 erosion were somewhat similar to those illustrated by the Ameri- G,lbert 

 can Fall. For this reason it is worth while to inquire at what 

 rate the American Fall has receded since the first precise observa- 

 tions on its position and contour. 



• • • • • 



There is good reason to question the accuracy of the map of 

 1842, especially in the vicinity of the northern shore. The area 

 there indicated outside the line of 1 875 and later maps is 1 1 

 feet broad. As its position is close to Prospect Point, which 

 has been a popular view point through the entire period under 

 consideration, the falling away of such a body of rock, either 

 gradually or all at once, could not have escaped notice, but (so 

 far as my reading goes) current literature, including the litera- 

 ture of the guide-books, is silent in regard to it. In addition to 

 this negative evidence, there is positive information in the Basil 

 Hall sketches. Comparing his sketch from Goat Island (PI. V) 

 with my photograph made from approximately the same point 

 in 1895 (PI. VI), it will be seen that there is essential corre- 

 spondence in the distant headlands along the river. By means 

 of these headlands I was enabled not only to establish a definite 

 relation between the two views, but also to correlate the sketch of 

 1827 with the map of the gorge made in 1875, and by the aid 

 of that map with the various charts of the crest line. . . . 



As the Basil Hall sketches have thus served to discredit a 

 portion of the map of 1 842, it is in order to inquire whether they 

 afford a substitute for the evidence ruled out. Once more using 

 the vesta down the gorge as the basis of correlation, and applying 

 measurement to points recognized as identical, I have ascertained 

 that the sketch of 1827 and the photograph of 1895 give to the 

 extreme salient of the American Fall almost identically the same 

 position. At that particular point the recession appears to be 

 zero. Nearer than the salient, and appearing about one-fourth 

 inch to the right of it, is a peculiar configuration of the crest line 

 which seems to be common to the two views. In the photograph 



665 



