18 GEOLOGY OF OHIO. 



The change of place of the delta was caused, however, for the most part, 

 by oscillation of the sea level, and not, as Ellet supposed, by the simple 

 filling of the channel with the materials transported by the river itself 

 without change of bed. 



Prof. E. W. Hilgard, in his interesting report to Gen. A. A. Humphreys 

 on the Mississippi delta, states that he found true northern Drift 354 feet 

 below the surface in Calcasieu district, Louisiana ; and he cites evidence 

 that, during the early part of the Drift period, the country about the 

 mouth of the Mississippi was at least 600 feet higher than now. During 

 the subsequent period of submergence it was, as he states, much lower 

 than at present. It will be noticed that these facts accord precisely with 

 those observed in the upper Mississippi valley and lake-basin, where, in 

 the period of excavation of the buried channels, the country must have 

 been high, and the drainage free. Afterward a great submergence oc- 

 curred, which has left its indubitable records in the stratified Drift over- 

 lying the Forest Bed and in the Loess. The locality where Prof. Hilgard 

 found northern Drift in Louisiana was undoubtedly in, though not in the 

 bottom of the old trough of the Mississippi, as I have noted elsewhere. 

 I regard this as valley Drift, swept down the Mississippi from its northern 

 watershed, when the continent was higher, and its current more rapid 

 than now. 



On the west coast of North America evidence of a subsidence of the 

 continent is afforded by the deeply excavated and partially silted-up 

 channels of the Golden Gate, the straits of Carquinez, the trough of the 

 lower Columbia, the Canal de Haro, Hood's Canal, Puget Sound, and all 

 the net-work of channels in that vicinity. As Dana first pointed out, 

 years ago, the systems of inlets or fiords on both sides of our continent- 

 channels which must have been excavated by subserial erosion — afford 

 additional proof of modern continental subsidence. 



The importance of a knowledge of these old channels in the improve- 

 ment of the navigation of our larger rivers is obvious, and it is possible 

 that it would have led to the adoption of other means than a rock canal 

 for passing, the Louisville falls, had it been possessed by those concerned 

 in the enterprise. 



I ventured to predict to General Warren that an old, filled-up channel 

 would be found passing around the Mississippi rapids, and his examina- 

 tions have confirmed the prophecy. I will venture still further, and 

 predict the discovery of buried channels of communication between 

 Lake Superior and Lake Michigan, probably somewhere near and east 

 of the Grand Sable, at least between the Pictured Rocks and the St. 



