64 BOTANICAL GAZETTE [AUGUST 
superimposed layers of colloidal type of peat material in certain 
deposits probably indicate another kind of evidence of former 
relatively dry and warm climatic conditions. 
A third form of evidence which may aid in the interpretation 
of the age of deposits and the climate which characterized their 
development consists in the seams of clay found between layers of 
peat material. These are often found with an admixture of organic 
matter, but rarely laminated in a manner similar to the seasonally 
laminated glacial clays described by DE GEER (10) and SAURAMO 
(32). Interstitial clay seams appear to be coincident with the 
earlier portion of the Wisconsin group of moraines. They have 
been noted especially in connection with peat deposits located in 
areas where readvances of the ice sheet are displayed in the drift. 
The investigations, however, are not sufficiently extensive to show 
whether the clay seams would be prominent also in morainal 
systems which are free from a surface cover of wind-blown loess. 
The fourth form of evidence which seems very promising is that 
of the marked structural differences found in certain peat deposits 
over a wide extent of country in which a series of moraine systems 
is the time factor of distinction (fig.1). Leverett (19) has shown 
that the Wisconsin drift displays moraines which are distinctive 
and well preserved. They are more or less concentrated in groups 
which permit of much greater detail of correlation than is possible 
in connection with the glacial stages in Europe (12, 18, 27). 
The morainic systems of the Wisconsin ice sheet mark halting 
places in the recession of the ice front. They obviously represent 
climatic pulsations, for the evidence seems clear that the ice sheet 
was subject to increase or decrease in response to climatic variations. 
Periods of warmth during which the ice sheet retreated somewhat 
rapidly, leaving nearly level tracts of drift, must have alternated 
with periods in which the climate ceased to be mild, and either 
remained nearly uniformly colder for a time or else reverted toward 
the conditions which induce glaciation. 
For the study of past climatic changes and plant migration 
since the culmination of the last stage of glaciation, a comparison 
of the stratigraphic features of peat deposits should bring out 
evidence of great value. By actual test borings of peat deposits 
