24 The Aiiierican Geologist. juiy, is^a 
of tlu^ white clay district, the kite Dr. L. B. Welch found in many 
wells, at a depth of about ;}0 feet and beneath the till of the newer 
drift, a loess-like loam containing fossils. As yet it is not certain 
that these overridden silts should be correlated with the white 
clays, but they are of much significance in showing that the ice in 
the later invasion encroached upon silt covered districts. 
The fact that the clay deposits l)ecome thinner toward the 
south and are gathered into the valle3's, strengthens the view that 
the condition which obtained at the time of their deposition was 
one of overflow rather than of general sulimergence. In both 
Indiana and Ohio, the thickness at the border of the moraine is 
four or five feet, occasionally greater, and a gradual decrease is 
found in passing southward, the thickness on the interfluvial 
tracts in southern Ohio, 20-30 miles from the moraine, being 
little more than half as great. In Indiana I have made no exami- 
nations so far south of the moraine but am informed by Pres. 
Chamberlin, who has explored the drift covered region to some 
extent in the vicinity of the Ohio river, that there is scarcel}' anj' 
white clay on the interfluvial tracts for many miles north of the 
river. There is, however, loess-like silt, in small amount, along 
the Ohio below Louisville which he thinks may be correlated with 
these white clays. It is not improbable that the East White 
river carried a large amount of water from southeastern Indiana 
at the time the white clays were being deposited. The white 
clays have been traced down its valley to the point where it 
enters the driftless portion of southern Indiana, and they have 
there as strong a development as anywhere in that part of the 
state. 
From facts presented in the above discussion it appears that 
the white clays and the moraines with which they are associated 
have a distinct chronological position, being separated from the 
earliest glaciation 1)V an interval during which the till sheet was 
undergoing oxidation and leaching, and from the last glaciation 
by an interval during which a marked change in altitude occurred. 
Equally strong evidence of the occurrence of these intervals is 
found in the character of the drainage and the degree of erosion 
As to the relative length of the intervals, we can at present offer 
no opinion, little being known as to the length of time involved 
in the changes by which they are denoted. 
