N.S. Shaler—F luviatile Swamps of New England. 219 
It is clear that the precise amount of this dislocation cannot 
be determined from a study of the streams themselves ; we can 
only set it in general terms as equivalent to a diminution in 
the rate of descent to the amount of from one to two feet to the 
mile. It is, however, clear from the evidence afforded by the 
valleys that the change was in effect a downward tilting of their 
southern ends. This is shown by the fact that the encumber- 
ing by marshes often extends to the head-waters of the streams. 
If the subsidence had consisted in an even lowering of the 
whole shore the result would have been the checking of the 
current of the rivers in the lower part of their course alone. As 
it is, it has affected the movement of some of their tributary 
streams up to a height of one hundred feet above the sea. 
There is a chance that the amount of the dislocation may be 
determined in the manner pursued by Gilbert in his admirable 
work on the lake basins of the centra] and western portions of 
the continent, viz: by tracing the elevated shore lines of 
shrunken fresh water areas. In the essays which I have made 
towards the use of this method I have as yet attained no suc- 
cess, for the reason that over New England elevated shore-lines 
are rare and extremely indistinct in their outlines. It is likely, 
however, that a careful study of the numerous lakes of New 
England which have been partially drained since the Glacial 
period will afford some data for the accurate determination of 
this question. 
An inspection of the best accessible maps seems to show that 
this encumbering of the valleys of the streams which flow from 
south to north, becomes less evident as we recede from the 
shore. It is but slightly shown in the hill districts of southern 
New Hampshire, or of central Massachusetts, as well as in the 
Berkshire hills. It will not do, however, to conclude that this 
tilting action did not take place in those districts. The only 
streams which in that region have a course from south to north 
are of small size, and rapidly descend from considerable heights. 
If, as seems likely, the amount of down sinking of the level to’ 
the south did not exceed two or three feet to the mile, the 
original slope of the stream beds may not have been so far 
overcome as to lead to the formation of swamps in their 
valleys. 
It need hardly be said that this tilting movement was most 
likely a change which involved a large part, if not the whole, 
of the glaciated district of the continent. The occurrence of 
such a movement is sufficiently proved by the position of the 
post-glacial beds containing marine fossils along the shore from 
northern New England to Greenland, as well as by the evidence 
obtained by Gilbert from the old shores of Lake Ontario. 
These observations above recounted on the northward-flowing 
