360 J. D. Dana—Flood of the Connecticut River Valley. 
metric discussion and reserve the remainder for a new paper. 
In performing the experiment for the first time, confusion is 
generally experienced ; after a few moments the form of the 
binocular resultant is clearly perceived, and in subsequent 
trials the perception is attained much more quickly, all parts 
of the image being sensibly equally distinct at the same mo- 
ment, if the inclination of the cards be not great. It is only 
when this inclination is considerable that play of the eyes be- 
comes necessary, and the associated exercise of the rectus 
muscles thus furnishes suggestions that are complementary to 
the modification of retinal impressions just discussed. ‘There 
are further experiments in regard to this mode of stereoscopy 
by momentary illumination, which I hope to make and pub-, 
lish at some future time. 
New York, 40 W. 40th St., April 3d, 1882. 
Art. XXXVIII.—On the Flood of the Connecticut River Valley 
from the Quarternary Glacier; by J. D. DANA. 
{Continued from page 202.] 
Tux facts, presented in the preceding part of this paper, on 
the dimensions and velocity of the flooded Connecticut appear 
to make it certain that, during the era of the great flood, the 
pitch of the Connecticut valley was very much less than it Is 
now, and that a change equivalent to 1 foot a mile from the 
Sound to Springfield, and 24 feet a mile to Haverhill (and an 
undetermined distance beyond)—but corresponding to some 
curving plane between the extreme points—would not be too 
great to meet the requirements.* ‘ 
But I have not said that the land over the interior of New 
England was depressed to the amount stated, or to any amount. 
It has been left an open question whether the change was a 
change in the sea-level or in the land-level. ; 
Leaving this point for discussion in another number of this 
Journal, I here take up the subject of the disappearance of the 
ice from the Connecticut valley. 
5. The Retreat of the Glacier. 
the amount of ice to melt. But there are various unknown 
* This curving plane would make right the heights for Middletown and Spring- 
field on page 199, where the former is, by the assumed rate of pitch, the higher. 
