CO.MMUXICATIOX UK M. WARREN UPIIAM 
249 
*‘^3 its departure up to tlie lime of formation of the gréai mar- 
ginal moraines. The loess liais testifies thaï previous to lhe 
nrthest glacial advance the land sank to its présent altitude, 
nd probably somewhat lower on the area of the earlj drift, 
ut not to the sea level. The vast weight of the continental gla- 
seems to hâve been the chief or only cause of this subsi- 
^nnce, as shown by Jamieson for tlie similar dépréssion of lhe 
^l'ilish Isles and Scandinavia at the time of final melting of the 
j^^^^Punn ice-sheet. The explanation of this continnance of the 
ncciimnlation and advance after the dépréssion of the land 
and until the maxinia both of the land subsidence and 
de wcre attained, with a low altitude and even Icss 
scent of tlie lower Mississippi than now, has been wcll given 
n Conte C The subsidence was doiibtless slow, even though, 
n lably many times faster than the prcccding iiplift. It may 
occupied only 5000 years, bcing at a yearly rate of a half 
nne foot ; but possibly it was two or three times as 
g ^ ^^hile the slow sinking of the land was taking place, the 
^ nmiilation of the ice by snowfall may hâve proceeded at a 
more rapid rate, so that the thickness of lhe ice-sheet 
De nitiliide of its surface were incrcasing up to a maximum 
y uoincident with that of lhe subsidence. Finally, however, 
n subsidence brought a warmer climate on the Southern bor- 
uer of fl • • • 
ipc, caiising it to retreat, and giving to it in the re- 
an l' marginal moraines a mainly steeper frontal gradient 
and 
uation 
more vigorous currents than during its growth and culmi- 
Tl 
general retreat of the ice-sheet in North America, 
CQg^t altitude of the land and marine submergence of the 
^Os^t ^ ^urders of iiortheastern New England, northward from 
und of the eastern provinces of Canada, with ingress of 
arjd^M^ ulong the valleys of the St. Lawrence and Ottawa rivers 
Cijg liasin of lake Ghamplain, has been named by Dana the 
epoch. It was the final stage of the Glacial period, 
UiodT^*^ uharacterized by the rapid déposition of the glacial and 
tlrift, whose materials had been contained in the lower 
the ice-sheet. 
ino- Recent, or Présent period. Closely follow- 
* le déposition of the modified drift as wide and deep tlood- 
Soc. of America, vol. II, 1891, p. 329, 330. Eléments of 
sy, tliird édition, 1891, p. 589. 
