Inter cjlacial Chronometer. — WinclieU. 71 
Minnesota river eastward through some valleys which lead to the 
Mississippi river at points south of 8t. Paul, thus possibly remov- 
ing the Minnesota river from the elements of the problem, has 
been described both by Mr. Upham and by the writer in reports 
on Martin, Blue Earth, Rice, Goodhue and Dakota counties. 
Some data from deep wells and from topographic levels have been 
obtained bearing upon the probability of a continuous old rock- 
cut valley extending from the mouth of Rice creek, near Fridley, 
to St. Paul. 
More recently the attention of the writer has been given again 
to this subject,* and he has attempted to illustrate on plates iv 
and y and vi some of the data on which the conclusions of this 
paper are based. That the valley of the upper Mississippi is 
very old is evident on a moment's reflection.! That the main ar- 
tery of its drainage must always have had a channel, more or less 
wrought in the rock}- crust, is equally evident. It is also very 
plain that that old valley, and that old excavated channel, must 
have dated from the uprising of the rocks that formed the surface 
in the vicinitj^, from below the ocean's level. Man}- changes, 
perhaps such as to cause the shifting of the actual line of erosion 
by such drainage from place to place, within certain limits, may 
have taken place since the Archean rocks of the central region of 
Minnesota first began to shed the continental waters. Some of 
these maj' have occurred since the Cambrian and Lower Silurian 
rocks, which are the only ones (with one non-important exception) 
which now exist in the region concerned later than the Archean, 
rose above the ocean and added their quota to the land area of 
the region. Allowing for all these shiftings, which are entirely 
hypothetical and have no claim to be allowed, in any exact state- 
ment, yet it will be at once admitted that there was sufficient of 
quiet in the interval from the Lower Silurian to the present, to 
permit the early Mississippi to excavate what might be styled a 
' ' base-leveled" gorge through the rocks over which it flowed. We 
will not here enter upon the evidences that the present gorge is 
the oldest, dating from Silurian times, at least between St. Paul 
and the southern boundary of the state, but will simply call atten- 
tion to the fact that the immediate source and the mouth are most 
'^Op. cit. The Geology of Hennepin County. 
tE. W. Claypole. The Story of the Mississippi-Missouri. A.mer. 
Geologist. Vol. iii., pp. 361-377. 
