+ POPULAR SCIMNCE MONTHLY. 



ago Prof. James Hall laid the foundation of all i utnre ' calculations 

 when he made the first instrumental survey of the crest of the falls. 

 The changes in the crest have heen measured three times since, 

 and from these surveys the mean recession for nearly half a cen- 

 tury is now known and has heen found to he much more rapid 

 than was formerly supposed. If the whole history of the falls of 

 N ,,,,,,, wen thui told, then it would appear thai their age is 



Fig. 3. Falls of Montmorency. (About 275 feet liigh.) 



about nine thousand or ten thousand years. Indeed, some writers, 

 among others Mr. Gilbert, took this reduced estimate and minim- 

 ized it to seven thousand years, not knowing or overlooking the 

 history of the river which tells of the changing conditions ; but 

 these views he now abandons. The chronometer needed correc- 

 tion. During a term of several years of actual work, but ex- 

 tended through a decade and a half, for the investigations were 

 often blocked with difficulties, the writer has been able to decipher 



