Prof. O. P. Hubbard on Erosion in New Hampshire. 169 
Willey House, and some twelve miles from where they began the 
descent—I learn that “two or three miles above the mouth of 
Dry river there are several remarkable trap dikes at right angles 
“even with the rock enclosing it, and the stream makes a nearly 
perpendicular plunge at this point, of eight or ten feet,”—thus 
presenting such phenomena as are common in other similar places, 
r 
cess of excavating a valley by a running stream. 
The facts which I have recorded, showing the relations be- 
tween trap dikes, which have been denuded and eroded to con- 
siderable depths by running streams, will need no comment or 
illustration. 
In regard, however, to such cases as Mount Pleasant and Moose 
Mountain, where no running water is found, we must recur to 
the periods when the relations of land and water were very dif- 
ferent from the present. 
We are too much disposed to look upon the great features of 
this group of mountains as fixed so long ago as to have no con- 
nection with the minor ones now presented, and which are in 
process of increase. ‘The agencies of decomposition are now at 
in a generation, and the accumulation of debris since August, 
1826, at the gorge back of the Willey House, and in the outlets 
of the two below it, if we could add also the large amount of 
fine materials carried down stream by the Saco from these sour- 
ces, are well calculated to prevent our underestimating their 
time when it has become a separating valley two or three thou- 
valleys of erosion. 
Srconp Serres, Vol. IX, No. 26.—March, 1850. 22 
