7t) The American Geologht. Aupust, isoa 
proachcd the region of Minneapolis it must have found the Mis- 
sissippi running in its most direct course southward, for it can 
only be supposed that the ordinary events of ordinary seasonal 
changes, and ordinary erosion of the surface of the country, prior 
to that event, would have acted to determine the location of the 
stream. Archtean highlands existed toward the east, in Wiscon- 
sin, and toward the west, in Minnesota. When the land rose 
from the Cretaceous baptisrn, those highlands must have shed 
their surface waters toward this valley. The resultant main 
stream, taking the easiest course toward the sea, excavated its 
channel first through the Cretaceous sediments and then into the 
Silurian or other strata, guided only by the accident of pose of 
the surface. If we know of no cause that could have diverted 
it, as we do not, we must suppose, a priori^ that its course Avonld 
be the most direct and shortest to reach sea level. We take no 
account here of the possible, and even probable, existence of a 
pre- Cretaceous river and hence a pre-Cretaceous gorge, for had 
such existed it would, in the first instance, have been governed in 
its location by the same influences. We might add, that had 
such pre-Cretaceous gorge existed, its effect on the contours of 
the surface after the mantle of the Cretaceous sediments had been 
spread over the region, would have been favorable to the re-loca- 
tion of the stream, when the land became dry again, in the same 
gorge as it had excavated in pre-Cretaceous time, and hence that 
it matters but little whether we discuss here the pre-Cretaceous 
or the post-Cretaceous drainage. In either case the shortest, and 
the lowest line of drainage was chosen. A glance at the map, 
and a knowledge of the geology of the region would at once indi- 
cate that the Mississippi, on the advent of the first glacial epoch, 
was probably running in the valley which has lieen referred to, 
from the mouth of Rice creek to the mouth of Trout creek, thus 
cutting out both of the right-angled turns that it now makes be- 
tween those points. This valley it must have occupied during 
Tertiar}^ time at least, and probably since Lower Silurian time. 
Where was the Mississippi running on the withdrawal of the 
ice of the first glacial epoch? As has been stated, it was shown 
that when the second glacial epoch approached, the Mississippi was 
running in a channel excavated through the Trenton-St. Peter 
along the western suburbs of Minneapolis and reached the Minne- 
sota valley at some point above Fort Snelling, and it is fair to as- 
