

Prof. J. W. Spencer — High Continental Elevation. 209 



in sculpturing its surface, there would be little interest as to what 

 was its relative height, before the commencement of the Pleistocene 

 period. But we find valleys vastly greater than meteoric agents 

 could have produced under existing circumstances. Thus, there 

 are not only deep canons, but also vast depressions, descending 

 to levels far below the sea, which are now filled with the earlier 

 drift accumulations, or form channels submerged beneath ocean 

 waves, or constitute basins occupied by lakes. Hence, in the study 

 of the drift itself, in the investigation of the lake history, or in the 

 research upon the growth of modern rivers, we necessarilj' inquire 

 what was the altitude of the continent that would permit of the 

 mouldings and channellings of the original rock surfaces. 



Following the period of high continental elevation, the geologist 

 sees in the valleys and old channels, still below the level of the sea, 

 and in the high level beaches, an extensive submergence, succeeded 

 by a re-elevation, but not to the original height, when the continent 

 was being chiselled out by the ancient rivers. That this re-elevation 

 is still going on is shown by the northward tilting of the compara- 

 tively recent marine accumulations, along the St. Lawrence valley 

 and gulf coast, and the raised beaches in the Lake region, as well 

 as by the shoaling of the waters of Hudson Bay during the present 

 period of observation. 



As general statements do not satisfy investigation, it becomes 

 necessary to search for definite measurements of the former height 

 of the continent among the archives of the geological past. Let us 

 first seek for the testimony recorded by the Mississippi River. 



For the distance of eleven hundred miles, measured in a direct 

 line, above the mouth of the " Father of Waters," the modern valley 

 is merely maintaining its own size, or more generally is being slowly 

 filled by the deposition of river alluvium upon its floor. There are 

 only two exceptions, of a few miles each, where the river is scouring 

 out the rocky floor, and these are over barriers recently exposed, 

 during changes of the Pleistocene period. To such an extent has 

 the ancient valley or canon been filled, first with drift, and this 

 covered with river alluvium, that its original rocky floor is now 

 buried to a depth of 170 feet, 1 even at La Crosse, a thousand miles 

 from the Gulf of Mexico. Farther south, the depth of these loose 

 deposits increases, until at New Orleans a boring of 630 feet 2 below 

 sea-level does not penetrate the southern drift, nor even reach to its 

 lowest members. The lower 500 miles of the ancient Mississippi 

 were excavated out of Eocene or Cretaceous deposits, whilst the 

 valley above the mouth of the Ohio has the form of a canon exca- 

 vated out of Palasozoic rocks, varying in width from two or three 

 to ten miles, and having a depth (exclusive of the nowfilled portion) 

 of from 150 to 550 feet. 3 



From this inspection of the river, it is easily seen that no natural 



1 Geol. "Wis. vol. i. p. 253. 



2 Prof. E. W. Hilgard, Am. Journ. Sc, Nov. 1869, p. 333. 



3 Val. Min. and Miss, to Junct. of Ohio ; Gen. G. K. Warren, Kept. Eng., 

 U.S.A., 1878. 



DECADE III. — VOL. VII. — NO. V. 14 



