i89i.] 173 [r>avis. 
is quite possible that the plains were built up at a time when the 
streams were surcharged with glacial waste ; and that at a later 
stage, when the load of the streams had decreased, they cut down 
valleys in the plains that they had before built up. Their action 
changed from aggradation to degradation. Cases of this kind 
are well known now in Alaska. Whether this change of activity 
was sufficient in quantity, as well as in kind, cannot now be 
determined. But the change should be considered, and until 
shown to be insufficient, a change in the level of the land does 
not seem to be essential in explaining the excavation of the 
valleys. 
In view of the evidence of wind action during the growth of 
the gravel plains, we need not postulate a depression of the land 
during their accumulation, as might at first sight be thought nec- 
essary on account of their distinct stratification, which is so com- 
monly associated with subaqueous })rocesses. In view of the 
possible change from the ;iggrading to the degrading action of 
streams, it does not seem necessary to call u]»on an elevation of 
the land after the plains were formed, in order to explain the 
excavation of the valleys below their general surface. Indeed, 
the only change of level absolutely demanded in the region of the 
Cape since the ice lay along its backbone is a slight depression by 
which the lower parts of its valleys have been drowned into bays. 
This conclusion would accord well with the results obtained else- 
where by De Geer and Upham. 
In tliis connection, I wish to put on record certain notes 
regarding the position of tree stumps below salt-water level, 
outside of the present shore line of Cape Cod. Such stumj^s 
may be frequently seen at low tide along the shore at various 
places ; and the fishermen of the Cape know of many tree stumps 
in deeper water. It was from one of these self-trained observers 
that I first heard of the following exjjlanation of their origin, 
independent of changes of level of the land with respect to the 
sea. Imagine a pond or swamp, a little distance inland from the 
shore line. In the course of time, as the basin is silted up, vege- 
tation of different kinds will occupy the depression, finally includ- 
ing certain species of trees. While this change is going on, the 
sea may be pushing back the shore cliff and approaching the old 
pond. Many an example may be found now in which swampy 
basins, overgrown with trees, stand practically at tide level. 
