GEOLOGY. 257 



There is no subject in geology of wider popular and scientific interest 

 than that of the relation between human chronology and geologic his- 

 tory, and for two generations geologists have sought to determine tlie 

 remoteness of the latest episode of geology — the last ice invasion — in 

 terms of written history. Up to about a dozen years ago the current 

 estimates of the duration of the post-glacial epoch generally ran from 

 100,000 to 150,000 or 200,000 years; but about that time a number of 

 American geologists began to make shorter estimates, based on new 

 data, generally ranging from five or six thousand to ten or fifteen 

 thousand years ; and since that time the one hundred thousand year 

 and tbe ten thousand year estimates for the post-glacial period have 

 both had their advocates, though of late the ten thousand year men 

 have been in the ascendant. The chronometers were various, and many 

 of them manifestly unreliable. Probably the most reliable of them all 

 is that aflbrded by the gorge of the Mississippi between Fort Snelliug 

 and St. Anthony's Falls, in Minnesota, to which attention was directed 

 several years ago by N. H. Wiuchell. The gorge is walled by the fria- 

 ble St. Peter sandstone and a thin cap of firm Trenton limestone by 

 which the sandstone is protected above, and the relations of the gorge 

 to the adjacent glacial dei^osit are such as to jirove that it was exca- 

 vated by the retrogression of the falls from the present confluence of 

 the Mississippi and Minnesota Rivers at Fort Snelling to their present 

 position. This distance is about 8 miles. Now the gorge was first vis- 

 ited and mapped by Hennepin in 1680 ; the position was again recorded 

 by Carver in 1756, by Pike in 1805, by Long in 1817, by Keating in 

 1823, by Featherstonehaugh in 1835, and by engineers of the U. S. Army 

 in 1856; while the present position is known and comparison of the 

 several maps indicate the rate of retrogression of the falls during the 

 two hundred and seven years between 1680 and the time of the latest 

 survey. If this rate be assumed to have been constant from the close 

 of the glacial period to the present, the entire period covered by the 

 retrogression was about eight thousand years.* This agrees with Gil- 

 bert's primary estimate of the period of excavation of the Niagara 

 gorge. 



One of the most interesting and important lines of investigation in 

 glacial geology undertaken within recent years is that so successfully 

 pursued byChamberlin upon tlie terminal moraines built by the Pleis- 

 tocene ice sheet during certain stages of its existence. The same phe- 

 nomena have also been studied in this country by Upham, Cook, 

 Wright, Lewis, Salisbury, Todd, Leverett, and others, and the number, 

 extent, and significance in glacial geology of the American uioraines 

 have been brought out in an eminently satisfactory manner by these 

 students. 



* Final Roporta Geology of Mitmeaota, vol. 8| pp. 313-41. 



H. Mis. 1^ 17 



