74 The American Geologist. Foi)iuaiy, itioi. 
While there is no way to measure the erosion that has 
taken place at this point since the supposed capture, nor the 
relative rapidity of the lowering of the land here, as compared 
with the lowering of the bed of the Tugaloo, yet as it is now 
the divide between the two creeks and is undergoing consid- 
erable erosion, it is not unreasonable to suppose that at the 
time of capture it was as much as 500 feet above the present 
level of the Tugaloo at the jucture of the three streams, and 
that the grade of the Chattooga at that point, and therefore 
necessarily at the mouth of the Tallulah, was before the cap- 
ture that much above its present level at the mouth of the 
Tallulah. 
From an examination of the profile of the Tallulah, as 
shown on the map (Plate XI), it will be seen that a lipe of 
level from the river bed at the head of the gorge to the Grand 
Chasm is 525 feet above the river at the latter point, or ap- 
proximately coincides with the brink of the cliff at its highest 
point. In other words, if the Chattooga was once 500 feet 
above its present level where it is joined by the Tallulah the 
latter river could have reached it by an easy grade without 
having had to cut a gorge. On the capture, however, of the 
Chattooga at a point a short distance below the mouth of 
the Tallulah by the Tugaloo, the grade of the first named 
river would have been lowered sufficiently to necessitate the 
cutting of the gorge through which the Tallulah now flows. 
Careful search in the region across which the Chattooga 
would have had to flow failed to reveal anything like river 
gravel, but subsequent erosion may have entirely obliterated 
the old stream bed. 
The theory of capture has lately received corroborative 
evidence from a comparative biological study showing simi- 
larity in a portion of the unione fauna of the Chattahoochee 
and Savannah rivers.* There is also some evidence of forms 
in the Savannah system that point to a round-about derivation 
from forms of the Tennessee river through former connection 
between it and the Coosa river in Alabama and between the 
Coosa and the Chattahoochee by way of the Etowah in Geor- 
gia, and from the Chattahoochee to the Savannah system by 
•Chas. T. SrMPSON. On the evidence of the Unionidae regarding the former 
courses of the Tennessee and other Southern rivers. Scieace, vol. xii, No. 1^91, 
p. 133. 
