THEORY. 187 



5. River terraces usually slope towards the mouth of the stream by the same amount 

 as the current descends, and sometimes more. 



It is on the smaller and more rapid streams that we see this slope most conspicuously ; 

 indeed, on these it is so obvious that measurements were unnecessary. No measurements 

 of this slope haVe been taken in Vermont. 



6. Terraces are usually very high about gorges and obstructions in river courses. Such 

 is the fact at Bellows Falls, on the Connecticut, and at Johnson on the La Moille River, 

 besides many other examples. We have already shown that in consequence of the re- 

 tardation of the current by a semi-obstruction, terrace materials would be deposited upon 

 or near the obstruction. The same principle will apply to gorges, except that often the 

 accumulations are found at the lower end of the gorges as well as the upper. They occur 

 at the lower end of the gorges, because there the waters would spread out laterally and 

 produce eddies or ponds, where detritus would accumulate. 



7. The chief agent in the formation of terraces and beaches appears to have been 

 water : for, 1st, the materials have been comminuted and rounded as no other agent but 

 water can do; 2d, the materials have been sorted and arranged in different layers; 

 3d, the horizontal arrangement of the strata can be effected only by water. Having 

 established this point, let us see in what condition the continent was, in this period, and 

 for what reasons the detritus assumed the terrace form. 



The continent is now supposed to be raised high enough to bring nearly all the surface 

 above the water, which is now above the level of the highest terraces. The valleys are 

 occupied as arms of the sea, in the forms of friths, estuaries and bays, or bodies of water en- 

 tirely disconnected with the sea. In Connecticut River there would be a chain of lakes for 

 the different basins. The rivers beginning to exist, produce a current in these estuaries 

 towards the ocean. Their waters acting on the drift over which they run, would commi- 

 nute and carry into the estuary the fine sediment, and thus form shoals or banks along 

 their mouths. Meanwhile the ocean is sinking, and at length these banks will come to 

 the surface, as small deltas to the rivers. The streams, too, will wear down their beds, 

 as the estuary sinks, and hence they must cut passages through their deltas, and urge 

 foward a new mass of sorted materials into the now diminished estuary. Thus another 

 delta may be formed, and a third and fourth in process of time, even though the vertical 

 movement be uniform. The delta terraces are formed everywhere, and in every period, 

 essentially like the beaches, but more regular upon their tops, and perhaps of a different 

 shape, as seen by protraction upon a map. 



An example of the formation of deltas at the present day may be seen in Switzerland. 

 In the canton of Unterwalden, the lake of Lungern has been artificially lowered within 

 the last sixty years. Where the head of the lake formerly was, and into which a num- 

 ber of small streams formerly emptied, several deltas were laid bare by the draining off of 

 the water, and they are cut through by the streams, which have worn deep chasms 

 through the loose materials, and are still wearing them backwards towards the Alps. 



8. Next let us consider how, in like circumstances, lateral terraces may have been 

 formed. 1. The various Streams might bring into the lake, or estuary, so much detritus 

 that the whole valley below the surface of the water would be filled up. When the land 

 had risen, the stream would cut a passage for itself through the deposit, leaving terraces 



