COMMl'NICATION DE M. WARREN UPIIAM 
243 
*■ objection, it must be acknowledged that tlie preglacial higb 
^^evation whicb I think tliese areas experienced was geologically 
short. With the steep gradients of tlie Hudson, of lhe 
^'•reanis whicb formed the now submerged channels of tlie Cal- 
ornian coast, and of the Congo, these rivers, if allowed a 
lime for érosion, must hâve formed even longer and broader 
^^^leys than the still very impressive troughs which are now 
^^d on these submarine continental slopes. But the duration 
® the epeirogenic uplift of these areas on the border of the gla- 
'^'^tion for the Hudson, beyond il for the Califoriiian rivers, and 
the equator in western Africa, can scarcely be compared 
Ils brevity with lhe prolonged high altitude lield during late 
*^rtiary and early Ouaternary time by the Scandinavian penin- 
and by ail the northern coasts of North America, from 
j^^ine and Puget sound to the great Arctic archipelago and 
^^eiiland. The abondant long and branchiug fjords of these 
'^Oîthern régions, and the wide and deep channels dividing the 
ftiany large and small islands north of this continent, attest a 
long lime of preglacial high élévation there. At the time 
jift^'^'ï^ination of the long continued and slowly increasing up- 
lhe north, they seem to hâve extended during a short 
®Poch far to the south, coiiicident with the formation of ice- 
®ls in liigh latitudes. But when these lands became depress- 
Ui burden of the glacialed counlries was removed, 
m some instances, as in Créât Britain and New England, 
tli nearly to their original levels,beautifullyillustrating 
1^*^ '^^liiral condition of equilibrium of the earth’s crust, which 
lias named isostasi/, that when not subjected to spécial 
lie stresses, it acts as if Hoating on a heavier plas- 
'^^and mobile interior. 
q arther, it has been objected that, with an epeirogenic subsi- 
llie^ tlie glacially burdened land, the still high surface of 
i 
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llie ice-sheet disappeared. In North America, when the pro- 
J AJ t/ > AJ 
.J.J §reater part of the ice-sheet would rctain an arctic climate. 
**Uin readily granted, yet it is évident that the warm 
jjj . *^ars along the ice-border would cause it to be rapidly 
tl '• ^^**^*' process must bave extended inward unlil 
'dv tlie marginal melting upon the Mississipi basin had 
fül &*^^erally steep gradients of the ice-front, its more power- 
^urrents formed the retreatal moraines and the many lake 
