INTERPRETATION OF PHENOMENA AS TO VOLUME. 67 
modern river. The latter of these hypotheses has appeared in several 
modified forms.* The first one seems to be the simpler. Let us apply 
it and note the river history which it suggests. 
INTERPRETATION ON THE BASIS OF VARYING VOLUME 
Cove section.—The origin of the gorge characters may be considered 
best in the order in which they were made. First, then, as to the Cove 
section. What volume is indicated by its characters, and how does it 
compare with the Upper Great gorge? Its prevailing top width is close 
to 1,300 feet, and it is a deep, placid pool. This corresponds almost ex- 
actly with the lower two-thirds of the Upper Great gorge. The geolog- 
ical conditions in the two sections are so closely identical that there is 
no reason to expect appreciable differences of gorge characters from their 
influence. Hence it seems a plain inference that, since all the other con- 
ditions are substantially the same, and since the gorges produced are of 
the same magnitude, the volumes of the two cataracts must have been 
substantially the same in the two cases. The widest and narrowest parts 
in the two sections are also closely alike—1,600 and 900 feet in the lower 
section and 1,700 and 1,000 feet in the upper. The width at the water 
surface is noticeably less in the lower section, but there is good reason 
for this difference. ‘The hard bed of the gray quartzose sandstone (Me- 
dina), more than 20 feet thick, forms a projecting shelf on which the 
talus of the beds above rests, and its edge forms a low cliff. In the lower 
section this layer is a few feet above the water, while in the Upper Great 
gorge it is 40 to 50 feet below the surface. If the lower section were filled 
with water up to the same height above the quartzose sandstone, the width 
at the water surface would be about like that in the Upper Great gorge. 
In fact, the parity of characters in these two widely separated sections is 
as perfect as one could expect if it were certainly known that they were 
-made by cataracts of exactly the same volume. 
Whirlpool basin.— As the falls cut their way back from Wintergreen 
flat there finally came a time when there remained only a thin high wall 
of rock between the drift-filled Saint Davids gorge and the upper end of 
the newly made Cove section. This frail partition finally began to break 
down, and there was, of course, a coincident lowering of the water sur- 
face over the Whirlpool ; but even before the partition began to crumble 
the water above probably took on a rapid, agitated motion and began to 
wash out the drift. The falling of the river and the washing out of the 
drift must have followed the lowering of the wall nearly simultaneously, 
and, considering the softness of the drift-filling, this process must have 
*TJn one form or another this view is held by Pohlman, Wright, Upham, Winchell, Claypole, Tarr, 
Spencer, and others. 
X—Butu. Grou. Soc. Am., Von. 9, 1897 
