247 
CO.MMl’N'ICATION DE M. WARREN UPIIAM 
clevalioR of tlie continent, increased rainfall and snowfall 
âud resulliiig river floods swept away these supcrficial materials 
froni tlie highcr lands and spread thein on the coastal plain and 
along the Mississippi valley, where the streains expanded ovei 
*^road areas wilh shallow and slackened currents. As the éléva- 
tion increased, however, the rivers would attain steeper slopes 
and finally erode inuch of the dcposits which they had prcviously 
«>adc. During the cidmination of the uplift, bringing the ko âge, 
Wiesapeake and Delaware bays were excavated and érosion was 
progress at a far more rapid rate than with the présent low 
altitude of this région. 
The Lafayette formation seems to me more closely related to 
*^he Glacial period and the conditions producing the ice-sheets 
dian to the preceding very long Tcrtiary era, and for the same 
reasons which hâve been well stated by Hilgard and Spencer, 
namciy, their dcpendcnce alike on the cpeirogenic élévation. 
^^^HirUie Ice âge we should unité this probably much longer 
P^nglacial time of graduai uplilt of the continent, ami the 1 ost- 
§lacial or Recent period in which we livc, to lorm together the 
three successive parts of the Quaternary era. IIow long the early 
part comprising the epeirogenic uplift, represented by the depo- 
sition and érosion of the Lafayette formation may hâve been, 
''^e can only vaguely or perhaps approximately estimate. During 
llie beginning of the uplift its clfect would be probably to in- 
^t'ease the transportation and déposition of gravel and sand by 
lhe rivers many times beyond their présent action. The rate of 
^verage land érosion now prevailing throughout the drainage 
®rea of the Mississipi is supposed by Mac Gee to be competent 
to suppiy aijoui 120 000 years a volume of river gravel, sand 
®^pd silt equal to the original Lafayette formation in the Missis- 
®*Ppi valley. With the greater altitude and increasing slopes of 
land during the déposition of the Lafayette beds it may bave 
'■oquired a third or a sixth of the time herc mentioned, that is, 
‘■^ome 40 000 or 20 000 years. As the élévation coutinued, how- 
rapid fluvial érosion of those deposits and of the underly- 
strata ensued, which was extended over so long and broad 
f^rea of the lower Mississippi valley, and to such depth, that, 
^'"en vvith the high continental élévation of 2000 to 3000 feet 
'^o\vn from submerged valleys off both the Atlantic and Paci- 
^ coasts it must hâve recpiired a long cpoch. Perhaps it may 
^ rcasonably estimated twice as long as the time of the depo- 
