246 COMPTE-RENDU. — QUATRIEME PARTIE 
thc mouth of the Mississippi, and that they vary thence to a thia 
vcneer, the thickncss being proportional directly with the vol- 
ume of neighliouring- rivcrs and inversely with the extension 
inland. 
Previous^to the maximum advance of the ice-slieet, the Mis- 
sissipi river and ail its large tribntaries eroded deep and broad 
valleys through the Lafayette formation and underlying strata, 
cutting at New Orléans to a depth of at least 760 feet below the 
présent sea level. Along the central valley, from Gairo to thc 
Gulf, this érosion averages probably 200 feet in depth upon a 
belt 500 miles long, with a width of 50 to 100 miles, excepting 
isolated plateau remuants of the I.afayette and older beds, oi 
which the largest are Crowley’s and Bloomfield ridges, in Ar- 
kansas and Missouri. The land during the valley érosion was 
certainly 760 feet higher than now, but this I think to be only 
a small fraction of its uplift. From the transportation of nor- 
thern Archean pebbles and cobbles of crystalline rocks to the 
Lafayette beds of the lower Mississippi and of Petite Anse is- 
land, on the Gulf shore, in the direct line of the axis of the 
Mississippi valley, Ililgard believes that during the déposition of 
thèse beds the valley had a greater descent and stronger cur- 
rents of its river floods. He suggests that the increased altitude 
of the interior of the continent needed to give these formerly 
more powcrful currents may hâve been 4000 to 5000 feet, being 
siifficient, probably, to bring the cold climate and ice accumu- 
lation of the Glacial period. 
Marine submergence of the low Coastal and Mississippi valley 
areas occupied by thc Lafayette formation is supposcd by Mac 
Gee and Spencer to bave been reqiiisite for the déposition of 
its sand and gravel beds, but they see that imniediately after- 
ward the land was much higher than now to permit the exten- 
sive and deep river érosion of that time. A simpler view of the | 
epeirogenic movements, closing the Tertiary era and inaugurât- | 
ing the Quaternary, seems to me to be foiind in ascribing these 
beds to déposition on land areas by Ilooded rivcrs descendiug 
from the Appalachian mountain région and from the Mississipp’ 
basin, spreading gravel, sand and loam over the Coastal plau> 
and along the great valley during the carly part of a time of 
continental élévation. The land had lain during the long Ter- 
tiary periods at lower altitudes, and its surface was largely en- 
veloped by residual clays and by alluvial sand and gravel. 
