FORMER HIGHER LEVEL OF THE GREAT LAKES. 245 



of cubic yards, and weighed nearly seven billions of tons. 

 The time consumed in the execution of this stupendous 

 piece of engineering may be roughly calculated from the 

 observed rate of recession of the falls. In 1842 Professor 

 Hall executed a careful trigonometrical survey of the shore- 

 lines and landmarks of the falls. In 1855, thirteen years 

 later, M. Marcou made careful re- examinations, which he 

 reported to the Geological Society of France. From these 

 data it appears that the Canadian Fall, over which the 

 largest body of water is discharged, has receded, by the 

 wearing of the rocks, to the extent of twelve feet, or a lit- 

 tle more than eleven inches a year. With this clew, we 

 determine that the time required for the excavation of the 

 entire distance from Lewiston is over thirty-five thousand 

 years. This presumes the rate of recession has always 

 been the same. The more I consider this subject the more 

 I am impressed with a conviction that the rate of recession 

 was formerly more rapid than during the last one hundred 

 years. I am willing to reduce the time consumed to twen- 

 ty thousand, or even to ten thousand years. Geologists 

 most greedy of time ought to be satisfied with this when it 

 is considered that this interval is but the unit in the arith- 

 metic which calculates the time consumed in the revolu- 

 tions of the globe. Before the beginning of the excavation 

 of the great gorge, geological agencies had strewed the sur- 

 face with drift-deposits, some of which had been transport- 

 ed hundreds of miles. Before the transportation of the 

 drift, the basin of Lake Ontario had been scooped out, and 

 the vast erosion of the escarpment at Lewiston had. been 

 effected. Before the period of the erosion was that of the 

 solidification of the sediments ; and still farther back, the 

 incalculable intervals during which the sediments were ac- 

 cumulating five miles of thickness. At the commencement 

 of the excavation of the gorge, the fauna which populated 



