248 The American Geologist. April, isoa 
both ciises. North of the Ohio and south of the St. Lawrence 
basin the direction is south in both instances. In Wisconsin the 
(h'ainage is radially out from the central Archean area. The pres- 
ent drainage of the eastern Mississippi basin is therefore in the 
main that which ma}" be assumed as the original consequent drain- 
age of the region when the land was first elevated above the sea. 
In the absence of any evidence to the contrary-, the most probable 
explanation of the coincidence is that the river courses in these 
parts of the Mississippi basin have remained in general unchanged 
since Permian time. Thev are here considered original or conse- 
quent streams. 
2. Adjustments of the post- Carboniferous drainage. There are 
however several important diflferences between the h^-pothetical 
post-Carboniferous drainage and the present drainage, in which 
the present strefims cannot be explained as original streams. The}' 
are as follows : — (a) The St. Lawrence drainage, (b) The drain- 
age of the Silurian areas of Tennessee and of Ohio, i. e. of the 
Tennessee and Cincinnati anticlinals. (c) The course of the upper 
Mississippi, (d) The course of the lower Mississippi. These de- 
partures will now be considered. 
(a) The St. Lawrence drainage. The most marked departure 
from the presumed post-Carboniferous drainage, is the drainage 
of the St. Lawrence Basin. The earliest consequent drainage of 
this area must have been southward from the crj'stallines of Can- 
ada to the Ohio. But the great drainage lines of the St. Law- 
rence basin, which are occupied b}' the great lakes and the rivers 
connecting them, are structural valleys developed along the strike 
of the softer members of the Paleozoic formations and at right 
angles to the slope of the original surface. This structural con- 
trol of the drainage shows of itself that the present arrangement 
s not the consequent arrangement of the streams, but that it is 
an arrangement which was effected after the land had been long 
elevated above the sea. The streams had then had opportunitj' to 
cut down into the underlying strata and had slowly adjusted them- 
selves to the differences in hardness and structure which they 
there discovered. 
The cause and date of this change from the early consequent 
to the later subsequent drainage in the St. Lawrence basin have 
not been definitely determined. It is probable that the adjust- 
ment was effected during that long interval between Permian time 
