664 THE POPULAR SCIENCE MONTHLY. 



The occurrence of ancient beaches above the terraces on Iloosac 

 Mountain, and among the White and Green Mountains 2,200 and 2,600 

 feet above the ocean, proves its former presence and the movement 

 of its currents and waves. 



At that period the site of the present rivers was the bottom of an 

 ocean. It was during the progress of that period of continental de- 

 pression and submergence that the glacial drift was modified and re- 

 distributed, forming enormous deposits, filling old basins and river- 

 valleys, so that when the land emerged from the waters it was com- 

 paratively level, a few mountain-peaks rising above the plain. 



It is at this point, as we have seen, that the present river-system 

 with its terraced valleys begins, and the phenomena may occur in the 

 following order : 



1. Elevated beaches, indicating ancient sea-shores. 



2. The highest river-terraces. 



3. Continuous excavation of river-valleys, and formation of flats, or 

 flood-plains. 



4. Elevation of those plains above the overflow of the river, form- 

 ing terraces. 



The process of formation we have already described ; nor does it 

 appear that any dynamic agent was then in operation which is not in 

 operation now. The work has been continuous. 



The superposition of terraces early suggested the idea that their 

 origin was due to a succession of sudden elevations of the land rather 

 than to a continuous movement, and such, indeed, may have been the 

 case in some instances. But their usual want of uniformity through 

 long distances and of correspondence on opposite sides of the valley 

 induces the conclusion that their immediate distribution is controlled 

 by local circumstances, while the general cause has been a continuous 

 and gradual elevation of the land, and the equally continuous action 

 of running water. Currents of rivers are thrown from side to side by 

 ice-borne bowlders and accumulations of debris, and pebbles will be- 

 come so adjusted in the river's bed as to resist erosion, as shown in 

 Fig. 3. 



Fig. 8. 



Showing the Position assumed by Pebbles in the Bed of a Stbeam. 



This is one of the causes of the sinuosities of rivers. The water, 

 as Sir Charles Lyell observes, is thus frequently forced to cut new 

 channels, by which means new terrace flats may be formed by redis- 

 tribution of materials. 



The transporting power of running water depends on its velocity. 

 Hopkins, cited by Prof. Dana, says that its force varies as the sixth 



