THE DATE OF THE GLACIAL PERIOD. 605 



and the wood (a small specimen of a knot of some coniferous 

 tree) is remarkably fresh in its whole appearance. It is 

 scarcely possible that it should have remained in such a posi- 

 tion during the immense period supposed by Mr. Croll to 

 have elapsed since the glacial age. 



Many of these cases of subglacial vegetable accumula- 

 tions are beneath or in deposits of the very earliest portion 

 of the glacial period. Unquestionably of this age are those 

 found in Jackson, Jennings and Jefferson counties, Indiana, 

 and those found by Professor I. C. White in the terraces of 

 the Monongahela River, which are now correlated with the 

 earliest stages of the ice advance to the water-shed between 

 the Great Lakes and the Ohio River. 



Farther north, notably in Mower County, Minnesota, a 

 stratum of peat from eighteen inches to six or eight feet in 

 thickness, with much wood, is very uniformly encountered 

 in digging wells, the depth varying from twenty to fifty feet. 

 "From all accounts it (the peat stratum) appears to be em- 

 braced between glacial deposits of gravelly clay, and it 

 seems to mark a period of interglacial conditions when 

 coniferous trees and peat-mosses spread over the country . . 

 There are extensive marshes now existing in northern Min- 

 nesota, mainly covered with ericaceous plants, with some 

 cedar and tamaracks that are forming immense peat deposits. 

 With an increase of the amount of moisture in the air such 

 peaty accumulations would spread over much higher levels. 

 A return of glacial conditions would bury such marshes be- 

 low the deposits that are known as drift."* 



The observations of Professor Tarr upon the burial of 

 forests and peat bogs by the recent advance of glaciers in 

 Alaska are deserving of the most careful consideration in our 

 interpretation of the significance of the facts which are being 

 here detailed. 



* N. H. Winchell in "Geology of Minnesota," vol. i of "Final Re- 

 port," p. 363. 



