ven 
In all cases the upper courses are more mature, both as regards slope 
of the bed and with regard to steepness of banks and presence of water- 
falls, ete. 
The profiles to the right of the map are accurately drawn to scale 
from the data of the U. S. Topographic sheet. It will be seen that there 
is a remarkable uniformity in one particular, namely, the points (M, N) 
where the prolongations of the upper slopes intersect. <A line coinciding 
with xy, the upper slopes of A and D, meets the prolongation of a line 
coinciding with HW’; the upper course of HE, at M, and a line coinciding 
with the upper course of IY (FN) meets the line coinciding with the upper 
courses of G and H (GG’, HH’) at N (nearly). 
This state of affairs is not due to structure, for, as will be seen by the 
geologic sections (XY and W V), the formations and structure encountered 
by the several streams vary to a marked degree. G and H flow over hard, 
arenaceous limestone (Calciferous and Trenton in part)); I flows over the 
soft Utica shale; A flows over the even more yielding Hudson shales, and 
D and B over Utica. A and F are determined by a fault line. The lower 
courses, where not in glacial drift (fF, D and A), are in limestone (G and 
H) and Utica shale (KE). 
There are three possible explanations of the peculiarity in question. 
(1) These mature upper gradients represent a period of base-leveling and 
subsequent elevation which has rejuvenated the streams, allowing them 
to re-excavate their beds; (2) the Mohawk Valley was plowed out to a 
depth of 240 to 260 feet by the Mohawk Valley glacier*; (3) the water of 
the Mohawk was dammed back to a level of 240 to 260 feet above the 
present river level for a length of time sufficient for the streams in ques- 
tion to mature. 
Of these possible explanations the first is the more probable inasmuch 
as the stream, H, is manifestly preglacial and has been modified in its 
upper course to some extent by drift, nevertheless the upper gradient of 
this stream conforms distinctly to a river at a level of 500 feet (A. T.), 
instead of at a level of 240 feet as in the present Mohawk River. We 
must, therefore, believe that this stream reached grade before it was inter- 
fered with by the presence of the glacier. 
As for the hypothesis of a plowing out of the Mohawk Valley, this 
seems hardly probable in view of the fact that the Hoffmans Ferry fault 
*See Dana, A. J. S. (2) Vol. 35, pp. 243-249: Brigham, Bull. Geol. Soe. Amer., Vol. 9, pp. 
183-210; Chamberlain, U.8.G.S. Third Ann. Rep., pp. 360-365. 
12—Science. 
) 
