Problems, Middle Mississippi River Region, Pleistocene Time 205 
on pre-I[Ilinoian drift in Illinois fails to reveal any similarity 
between the pre-IIlinoian drift from the vicinity of St. Louis 
and either the Kansan or Nebraskan drift of Illinois. 
In all cases, the per cent of foreign pebbles among the in- 
soluble ones in the pre-Illinoian drift in and near St. Louis is 
considerably lower than the per cent of foreign pebbles, if based 
on the insoluble ones, found by MacClintock in either Kansan 
or Nebraskan drift in Illinois. On this basis it would appear 
probable that the pre-Illinoian drift of St. Louis and vicinity 
is not from the same ice sheet that produced either the Kansan 
or Nebraskan drift that MacClintock studied in Illinois. 
For future work an improvement on the method of analysis 
it is recommended, namely to divide the quartzite pebbles into 
two groups, reddish ones similar to either Sioux or Baraboo 
quartzite, and all others. It is apparently impossible to distin- 
guish between the Sioux and Baraboo q:tartzites; so in this 
study, when either name is used, it is intended to refer 
to the reddish, pinkish, or purplish quartzite which is highly 
silicified and usually conspicuously banded, that is found in the 
neighborhoods of Baraboo, Wisconsin, and Sioux Falls, South 
Dakota. While it is recognized that pebbles of quartzite of quite 
similar appearance are found in Labradorean drift sheets occa- 
sionally, they are not usually as common nor as large as the 
cobbles and boulders of this very resistant and characteristically 
colored quartzite in the Kewatin drift, especially near St. Louis. 
In every instance where the evidence in the field made it 
clear that the drift was Illinoian or pre-Illinoian, the pebble 
analysis completely agreed with the field observation. There 
were several instances in which the field evidence was not con- 
clusive. In these cases, the pebble analysis fell so far on one 
side or the other of 15 per cent, which seems to be the point 
of demarcation between the Illinoian and pre-IIlinoian tills, that 
the identification of the drift was clear. It is to be hoped that 
some satisfactory method of distinguishing between Kansan and 
