356 



EARTH FEATURES AND THEIR MEANING 



if the volume of water has varied during the process of recession, 

 these changes in volume will be registered in the width and also 

 in the depth of that section of the gorge which was drilled at the 

 time the cross section of the gorge at any place is proportional 

 to the volume of the water falling in the cataract which produced 

 it, modified, however, by the competency to handle the joint blocks 

 of definite size (Fig. 381). 



The present rate of recession. There are various sketches, 

 more or less accurate, made in the early part of the nineteenth 



FIG. 382. Comparison of a sketch of the Canadian Fall made with the aid of a 

 camera lucida in 1827 with a photograph taken from the same view point in 1895 

 (after Gilbert). 



century, and from the later period there are daguerreotypes, photo- 

 graphs, and maps, which refer especially to the Canadian Fall; and 

 which, taken together, render possible a comparison of the earlier 

 with the later brinks. By comparing the earliest with the recent 

 views it is seen at a glance that the Falls are receding, and at a 

 quite appreciable rate (Fig. 382). A careful comparison of the 



