1890 .] 
455 
[Leverett. 
been produced on lake Michigan, cannot be much more, according 
to the investigations of N. H. Winchell, Andrews, Gilbert, and 
Wright, than 7,000 to 10,000 years. Only a minor fraction of 
this time could have been occupied in the retreat of the ice from 
lake Traverse, at the mouth of lake Agassiz on the west side of 
Minnesota, to Hudson ba}', finally allowing this glacial lake to be 
drained by the Red and Nelson rivers flowing northward as now 
to the ocean. During this short time, however, several prominent 
moraines were formed on the country adjoining lake Agassiz. It 
thus appears scarcely less difficult for the meteorologist to explain 
the climatic causes of the rapid departure of the ice than of its be- 
coming accumulated so thick upon so large areas of the earth’s 
surface. 
CHANGES OF CLIMATE INDICATED BY INTERGLA- 
CIAL BEDS AND ATTENDANT OXIDATION AND 
LEACHING. 
BY FRANK LEVERETT. 
A study of the climate of the glacial period involves not only a 
consideration of the conditions necessary to produce glaciation, 
but also of the changes of climate that occurred. Owing to the 
complexity of the phenomena that evidence these changes, their 
extent has not been demonstrated to the satisfaction of all students 
of the subject. As is well known, opinion is divided on the ques- 
tion of the unity of the period, many now considering the evidence 
to be conclusive that there were two distinct invasions of the ice, 
with an interglacial period of great length ; others still maintain- 
ing the occurrence of one continuous glacial period. 
The main evidence brought in support of the former theory is 
(1) a sheet of carbonaceous soil, called by Dr. Newberry the forest- 
bed, and fossiliferous interglacial sands and clays, all of which 
are found extensively between sheets of till, both in America and 
in Europe ; (2) the great erosion of the older drift (,i. e., of the de- 
posit belonging to the so-called first glacial epoch) previous to the 
deposition of the newer, shown by the unconformity of the newer 
drift sheet, covering as it does great valleys and eroded ridges in 
the older drift ; also a more eroded and more aged surface in the 
uncovered portion of older drift lying south of the margin of the 
newer. 
