SURFACE GEOLOGY. 19 



Mary's river — between Lake Erie and Lake Ontario*, through Canada — 

 between Lake Ontario and the Hudson by the valley of the Mohawk — 

 between Lake Michigan and the Mississippi, somewhere along the line 

 I have indicated before. 



The channel of the lower Tennessee must have been excavated when 

 the southern portion of the Mississippi valley was higher above the Gulf 

 level than now, and Prof. Hilgard has shown that at a subsequent period, 

 probably during the Champlain epoch, the Gulf coast was depressed 500 

 feet below its present relative level. This depression must have made 

 the lower Mississippi an arm of the sea, by which the flow of the Ohio 

 and Tennessee was arrested, their channels filled up, terraces formed, etc. 

 If the upper Tennessee has, as appears, a channel lower than the Muscle 

 Shoals, it must be somewhere connected with the deep channel of the 

 lower river. 



It should be said, however, that it by no means follows that when an 

 old earth-filled channel passes around the rocky barrier by which the 

 navigation of our rivers is impeded, it will be most convenient and eco- 

 nomical to follow it in making a canal to pass the obstacle ; as the course 

 of the old channel may be so long and circuitous that a short rock cutting 

 is cheaper and better. The question is, however, of sufficient impor- 

 tance to deserve investigation before millions of dollars are expended in 

 rock excavation. 



If it is true that our great lakes can be connected with each other and 

 with the ocean by ship-canals-pin making which no elevated summits 

 nor rock barriers need be cut through — the future commerce created by 

 the great population and immense resources of the basin of the -Great 

 Lakes may require their construction. 



THE DRIFT DEPOSITS OF OHIO. 



The area over which the Drift is spread in Ohio corresponds in a gen- 

 eral way with the area of glaciation, but through the influence of ice- 



* When the water in the lake-basin had subsided to near its present level — the old 

 avenues of escape being all silted up by the Drift clays and sands — the surplus made 

 its exit by the line of lowest levels, wherever that chanced to run. That happened 

 to lie over the rocky point that projected from the northern extremity of the Alle- 

 ghanies into the lake-basin, and the line of drainage was established there, in what 

 is now known as the Niagara river. 



Though among the most recent of the events recorded in our surface geology, this 

 choice of the Niagara outlet by the lake waters was made so long ago that all the 

 erosion of the gorge below the falls has been accomplished since. The excavation of 

 the basin into which the Niagara flows — the basin of Lake Ontario, of which Queens- 

 town Heights form part of the margin — belongs to an epoch long anterior. 



