1871. 



113 [Shaler. 



along the line of the Manassas railroad, indicate that extensive reliefs 

 once existing in that region have been worn away since the time of 

 their upheaval. This erosive force I conjecture to have been 

 brought into action by the successive periods of glacial activity 

 which have probably operated in this region. It is in the highest 

 degree improbable that the period of wide spread glaciation which 

 has just passed away, was an unexampled accident in the history of 

 our earth. It is, on the contrary, becoming more and more probable 

 that we must accept glaciation as a recurring phenomenon, giving 

 alternations in the action of the erosive force at work upon the sur- 

 face of the continents ; at one time the streams acting in the local 

 manner in which water works in its channels, again working over 

 the whole surface of the country when its surface is ice covered to 

 profound depths. 



The glacial streams, which have dug out the vast excavations of 

 the Chesapeake and Delaware Bays, must have been accompanied by 

 lesser streams, or more likely by an almost continuous sheet of ice, 

 pouring down the flanks of the mountain range along its whole ex- 

 tent. Supposing such repeated ice action, we should naturally expect 

 to have many of the lower outlines of the range of mountains over- 

 ridden by the rising streams and much ground by their action. The 

 energy of the glacial action on the borders of a range is naturally 

 much greater than in the central parts thereof; generally speaking, 

 the weight of the ice sheet, and therefore its cutting power, increases 

 from its sources towards the border. We should, on this as well as on 

 other accounts, expect glaciation to be most destructive of reliefs,, 

 not in the centre of a chain but rather upon its borders. During 

 the last glacial period in Switzerland, the great peaks of the inner 

 region probably lost little, if anything, in height. Their summits 

 covered with incoherent snow, or with neve, would be protected 

 from external accidents; the lower regions alone, where the snow 

 had become compacted, where it had gained its real glacial charac- 

 ter, would be eroded. 



It is not to glaciation or other erosion alone that we owe the 

 disappearance of this, and probably other of the eastern outlines of 

 the Alleghanies- There is abundant evidence of a considerable 

 subsidence which has carried this and other summits below the bend 

 which at one time they had. To see this point more clearly it will 

 be necessary to look at some of the singular results obtained during 

 the exploration of the Gulf stream by this survey. In the course of 



PROCEEDINGS B. S. N. H.— VOL. XIV. 8 AUGUST, 1871. 



