1894.] Quaternary Time Divisible in Three Periods. 981 
dred miles northward from the Missouri River and boundary 
of the drift, and gravels believed by him to be probably of 
the same formation occur in the Wisconsin and Minnesota 
driftless area, while northeastward he has observed the Lafay- 
ette gravels in the Ohio Valley in southern Indiana about 150 
miles from the Mississippi. McGee states that the Lafayette 
beds attain their maximum thickness, which is 200 feet or 
more, in the region about the mouth of the Mississippi, and that 
they vary thence toa thin veneer, the thickness being propor- 
tional directly with the volume of neighboring rivers and 
inversely with the extension inland. 
Previous to the maximum advance of the ice-sheet, the 
Mississippi River and all its large tributaries eroded deep and 
broad valleys through the Lafayette formation and underlying 
strata, cutting at New Orleans to a depth at least 760 feet 
below the present sea level. Along the central valley, from 
Cairo to the Gulf, this erosion averages probably 200 feet in 
depth upon a belt 500 miles long, with a width of 50 to 100 
miles, excepting isolated plateau remnants of the Lafayette 
and older beds, of which the largest are Crowley's and Bloom- 
field ridges, in Arkansas and Missouri. The land during the 
valley erosion was certainly 760 feet higher than now, but 
this I think to be only a small fraction of its uplift. From 
the transportation of northern Archean pebbles and cobbles 
of erystalline rocks to the Lafayette beds of the lower Missis- 
sippi and of Petite Anse Island, on the Gulf shore, in the direct 
line of the axis of the Mississippi Valley, Hilgard believes 
that during the deposition of these beds the valley had a 
greater descent and stronger currents of its river floods. He 
suggests that the increased altitude of the interior of the con- 
tinent needed to give these formerly more powerful currents 
may have been 4000 to 5000 feet, being sufficient, probably, to 
bring the cold climate and ice accumulation of the Glacial 
period. : 
Marine submergence of thelow coastal and Mississippi Val- 
ley areas oceupied by the Lafayette formation is supposed by 
McGee and Spencer to have been requisite for the deposition 
of its sand and gravel beds, but they see that immediately 
