Correspondence. 251 
graphic and structural arguments on which these were based obtained 
the endorsement of all of them. This later confirmation by special 
study of a part of the field then not explored, is interesting in itself and 
in relation to the geological history of the Continent. 
Dr. Bergt criticizes unfavorably Gabb's geology of San Domingo. 
Peksifor Prazer. 
The Interglacial Deposits of Northeastern Iowa. [Ab- 
stract.]* Ihterglacial deposits occur at two horizons in the glacial 
series of northeastern Iowa. The first is the peat and forest bed 
which, so far as this region is concerned, was first brought to the at- 
tention of science by the writings of McGee. The second is the Bu- 
chanan gravels of Calvin. 
Owenf was the first geologist to refer to the drift of northeastern 
Iowa. He was much impressed by the great bowlders strewn over the 
surface, and expressed the belief that they had probably been trans- 
ported to their present position by floating ice. White discussed the 
drift more fully, and recognized its glacial origin, but the time had not 
yet come for recognizing the complex nature of the Pleistocene deposits 
and hence the numerous problems with which more recent investigators 
have been chiefly concerned were not considered. 
It remained for McGee to introduce methods of investigation that 
finally furnished the key to the interpretation of the complex Pleistocene 
system. He pointed out in numerous contributions to geological liter- 
ature that the drift was certainly not single, but that it embraced 
at least two distinct sheets ot till.§ He insisted that the in- 
terval between the two glacial invasions was one of enormous 
length. He regarded the forest bed as lying between his lower and up- 
per till. He furnished criteria for discriminating the two till sheets by 
their color and contents. He it was who led the way to a satisfactory 
classification of the Pleistocene bed of this part of the Mississippi val- 
ley. 
l^ecent investigations show that McGee's lower till embraces two dis- 
tinct drift sheets, and that it is between these two that the forest bed 
invariably lies. Three drift sheets, therefore, are recognized in north- 
eastern Iowa, and in recent literature referring to Pleistocene geology 
they are known respectively as Sub-Aftonian, Kansan and lowan. No 
forest material has been observed between the Kansan and lowan, but 
in this situation there occur extensive beds of stratified sand and gravel. 
The forest bed between the first and second drift sheets is frequently 
accom])anied by beds of peat from an inch or less to three or four feet 
in thickness. The peat beds often cover areas of considerable extent. 
*Read before the Iowa .\cademy of Sciences, Dec, 1897. 
+Rpport of a (i(M)lofrical Rpconnoissanco, etc., \t. 69, 1848. 
Report of a (li'ol. Sur. of Wis.. Iowa and AJiiin., p. 144, IS.")'-'. 
^Report on Gool. Sur. of Iowa, pp. 82-102, 1.S70. 
SMcG^e's obtiorvations on tiio drift of tlii.s rcfriou is well summed up in "Tiic Pleis- 
tocene History of Northeastern Iowa," U. S. (ieol. Sur., Klcvi-uth An. Rpt., pp. 18SI- 
.')77, 18!»;{. 
