56 SURFACE GEOLOGY. 



beyond, and there begin to form a new bank, which in like manner, would at 

 length reach the surface; and then a third bank would be formed, all the while 

 the vertical movement proceeding without pause or paroxysm. 



It may be thought that in such a case the sediment would be deposited in one 

 continuous slope or talus : and it would be without a current along the coast to 

 wear away the successive banks on the outer margin; and thus, it seems to me, 

 the result might be terraces, or rather successive beaches, at different levels. And 

 thus might the lower beaches, that now fringe the coasts of North America, have 

 been formed by a secular and perfectly uniform elevation of the continent. Until 

 rivers existed, however, I should expect the beaches to be very irregular and 

 indistinct, unless there were pauses in the upward movement : and so I do find 

 them near their upper limit, while the lowest beaches on our present shores, are 

 almost as perfect as river terraces, especially at the mouths of rivers, where per- 

 haps, they should be called terraces. 



17. Let us now take a bird's-eye view of the continent, raised high enough to 

 bring nearly all the surface above the waters, which is now above the level of the 

 highest terraces. We see the valleys occupied as arms of the sea, in the forms of 

 friths, estuaries, and bays, and in some places, bodies of water exist, cut off entirely 

 from the ocean. Some of the estuaries, too, are so narrowed in particular places, 

 by the approach of barriers on opposite sides of the estuary, as to form, as it were, 

 a chain of lakes, connected by straits. Such would be the aspect at the time 

 supposed of the Connecticut valley. Along the shores, we see on a diminished 

 scale, those rivers Avhich are now its tributaries, emptying into the lake-like 

 estuary, and thus producing a current towards the ocean. Their waters, acting on 

 the drift over which they run, would comminute and carry into the estuary the 

 smaller particles, and thus form shoals, or banks, along their mouths. Meanwhile 

 the ocean is sinking, and at length these banks will come to the surface, and con- 

 stitute 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 

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

 another delta may be formed, and even a third, or fourth, in the same manner; 

 and even though the vertical movement be perfectly uniform, the current towards 

 the ocean, produced by the tributaries, will so act upon the outer margin of the 

 embankments, as to form terraces, rather than a simple talus. 



In this manner, it seems to me, may the delta terraces have been formed by the 

 slow drainage of the country, and without supposing pauses in the vertical move- 

 ment. These are in fact, among the most usual and striking of the terraces. 



Though formed in essentially the same manner as the beaches above described, 

 they would be more regular on their tops, because not exposed as the beaches were, 

 during their emergence, to the action of the breakers. 



Mr. Charles Darwin, I believe, first suggested the mode in which delta terraces 

 were formed, as described above, in his paper on the Parallel Eoads of Glen Boy. 

 Mr. Kobert Chambers, however, has pointed out a case in Switzerland, which fully 

 confirms these views. In the canton of Unterwalden, the lake of Lungem has 

 been artificially lowered within the last sixty years. Where the head of the lake 



