50 SURFACE GEOLOGY. 



unmodified drift and the modified materials of beaches and terraces. The gradua- 

 tion of one into the other is often so insensible, that we cannot tell where the one 

 ends and the other begins. But I shall refer to this again in another place. 



2. The successive beaches and terraces, as we descend from the highest to the 

 lowest, in any valley, seem to have been produced by the continued repetition of 

 essentially the same agencies by which the materials — originally coarse drift — 

 have been made finer and finer, and have been more carefully sorted and arranged 

 into more and more perfect beaches and terraces. 



This seems to be the general law : at least such is the conviction produced in 

 my own mind. Yet occasionally we meet with limited deposits, as already 

 remarked, of fine materials in the midst of, or beneath those very coarse. This 

 only shows that in certain places the comminuting and sorting processes were 

 carried on at an early date as perfectly as afterwards when they were extended to 

 large areas. 



3. By far the largest part of the materials constituting the beaches and terraces 

 is modified drift, in other words, fragments torn from the rocks in place by all the 

 eroding agencies down to the close of the drift period. 



This position is proved by the occurrence of drift scratches and furrows over 

 most of the rocks in place, in the valleys as well as on the hills. Indeed, I expect 

 to show in my paper on Erosions, that some rivers have made deep and long cuts 

 through the rocks since that period : for instance, the gorge of Niagara river, from 

 the Falls to Ontario, and the still deeper cut between Portage and Mount Morris, 

 on Genesee river. But in the valley of Connecticut river no such gorges have 

 been worn, since we find the drift strise in many places almost as low as the 

 surface of the present stream, even at those points where once gorges were worn 

 out. Thus, at Bellows falls, the rocks at the top of the falls, even to the 

 water's edge, exhibit distinct and beautiful examples of furrows and protuberances 

 produced by the drift agency, although the cataract has undoubtedly receded con- 

 siderably at this spot since that force acted. At Brattleborough the slate on the 

 west side of the river shows drift furrows only a few feet above the river. Here 

 too was once a gorge : but it was worn out earlier than the drift period. At Sun- 

 derland, where Mettawampe and Sugar Loaf, between which the Connecticut now 

 runs, were doubtless once united, drift scratches now show themselves almost on 

 a level with the stream. The same is true on the trap rocks at Titan's Pier, in 

 the gorge between Holyoke and Tom, which were once still more certainly united. 

 As to the gorge a little below Middletown, I am not able to speak certainly, yet 

 so far as I could judge, in passing upon a steamboat, I do not doubt the occurrence 

 of drift erosions at a low level. 



4. Hence on such rivers as the Connecticut, wherever, indeed, we can find marks 

 of drift agency low down on the rocks at gorges, we cannot suppose that rocky 

 barriers closed those gorges during the period when the terraces were forming; and, 

 therefore, we cannot call in their aid to explain the formation of the terraces. 



5. The highest distinct terraces which I have measured above the rivers on which 

 they occur, are as follows : On Connecticut river, at Bellows Falls, 226 feet ; on 

 Deerfield river, 236 feet; on Genesee river, at Mount Morris, 348 feet; on the 



