Shaler.] 
258 
[March 4, 
tain in some cases remains of the original concretion. A large 
portion of the manganese of Vermont is of this latter variety. 
By this process of concentration the percentage of manganese is 
increased in the “kidney ore” as compared to the earthy varieties, 
and the percentage of phosphorus and iron is decreased, while in 
the veins proper the oxide of manganese exists in nearly its pure 
state but as the sesquioxide. 
In a future paper several important deductions resulting from 
the recognitions of the character of the beds associated with the 
manganese ores will be brought out. 
General Meeting, March 4 , 1891 . 
Mr. S. H. Scudder in the chair. 
The following communication was presented. 
THE ANTIQUITY OF THE LAST GLACIAL PERIOD. 
BY N. S. SHALER. 
Published w by the permission of the director of the U. S. Geol. 
Survey. 
Certain geologists are now disposed to consider the end 
of the glacial period as very near to the present day. They base 
their conclusions upon the rate of recession of waterfalls, the des- 
sication of our dead seas, the measure of decay which has been 
brought upon the drift materials and upon the rock scored by 
the passage of the ice. The unmodified nature of the more 
delicate tojiography remaining from the glacial period in our 
Indian ridges and kettle kames has also been regarded as evidence 
that the glacial age was not very far in the past. Although the 
array' of this evidence appears at first sight strong, there are cer- 
tain manifest imperfections in the general nature of the proofs 
which it affords. In the first place, nearly all the groups of purely 
physical phenomena which find a place in the arguments concerning 
the lapse of geological time have the defect that they represent ac- 
tions which may have been so far discontinuous that they do not 
