18 The American Geologist. July, 1894 
The abundant long and branching fjords of these northern 
regions, and the wide and deep channels dividing the nian} T 
large and small islands north of this continent, attest a very 
long time of high elevation there. At the time of culmina- 
tion of the long continued and slowly increasing uplifts at 
the north, they seem to have extended during a short epoch 
far to the south, coincident with the formation of ice-sheets 
in high latitudes. But when these lands became depressed 
and the ice burden of the glaciated countries was removed, 
they in some instances, as in Great Britain and New England, 
returned very nearly to their original levels, beautifully illus- 
trating the natural condition of equilibrium of the earth's 
crust, which Button has named isostasy, that when not sub- 
jected to special and exceptional stresses it acts as if floating 
on a heavier plastic and mobile interior. 
In the great erosion of the Lafayette formation on the At- 
lantic coastal plain and Gulf border of the United States, 
and in the lower Mississippi basin, which formation is re- 
garded by Hilgard, Spencer, E. A. Smith, and the present 
writer, as of early Quaternary age, formed during the initial 
stages of the high uplift that culminated in the Ice age, we 
have an impressive record of that epoch of great altitude of 
the northern and principal part of this continent. Similar 
rapidity of erosion must also have ensued when any moderate 
retreat of the ice-sheet during the time of high uplift and ice 
accumulation permitted much of its drift gravel and sand to 
be deposited, as in the valleys of the Ohio river and its trib- 
utaries, described by Chamberlin and Leverett in the April 
number of the Am. Journal of Science (also seethe Am. Geol- 
ogist for March, pages 217-219). With an elevation of that 
area probably 8.000 feet higher than now, giving its streams 
far more rapid descent, and with the aid of a dam formed by 
the ice-sheet crossing the Ohio valley at Cincinnati while 
most of the valley drift was being laid down, and the subse- 
quent removal of this barrier while the stream erosion took 
place, we may well suppose that an interval of a few thou- 
sand years between the stages of ice advance would suffice for 
an amount of stream channelling many times greater than has 
been accomplished in the less abundant later deposits of val- 
ley drift during all the Postglacial epoch of low altitude and 
