HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 167 
tirely of altitude, but also of age and structure. This plateau has a 
foundation of Cretaceous (and in Guatemala earlier sedimentaries) no- 
where as yot positively found in the Panama Isthmus. Upon these, in 
places, are Tertiary sediments similar to those of the Isthmus, These in 
turn have been buried beneath the thousands of feet of débris from the 
volcanoes which here have continued active from the Tertiary period to 
the present, during which time the Isthmian land has been undergoing 
a general lowering through erosion. 
TOPOGRAPHIC EVIDENCE BEARING UPON THE ANTIQUITY OF THE 
ISTHMIAN REGION. 
I have represented two parallel adjacent profiles of the Isthmus. 
(See Plate IV.) One of these is along the line of the canal and railway 
which follows the lowest passes of the drainage, and conveys an erro- 
neous impression that the country consists of a simple anticlinal slope 
separated by a continental divide. 
The other (Plate IV. Fig. 2) is run in a straight line across the 
same section, commencing on the west side of Limon Bay, and pass- 
ing through the Cerro Angon at the city of Panama and across the Gulf. 
This profile shows the topography of the country in its natural aspect 
and relations. It will be seen that, instead of presenting the two well 
defined coastal slopes and a single continental divide, the country is 
one of great irregularity, showing no such features. Its hills, called 
by courtesy mountains, rise precipitously almost directly from the level 
of each ocean, and occur completely across the Isthmus into the Gulf of 
Panama, regardless of a continental comb. The whole topography of 
the region indicates that it is an area of irregular summits arranged 
without the presence of a single central ridge. The Culebra summit, 
so called, instead of being the highest point of the mountainous topog- 
raphy representing a continental backbone divide, is excelled in height 
by many eminences. At Colon the hills on the west side of Limon 
Bay of the Atlantic shore are almost as lofty as the Zulebra summit, 
while at Gorgona some of the volcanic hills not over a mile south of the 
road are over 1,000 feet in height. Other eminences a few miles away 
from the railroad, eastward toward Puerto Bello and in the region of 
Chorrera, rise to a height of 1,000 and 1,500 feet. In the light of this 
profile the Culebra summit, as far as it alludes to the continental divide, 
is a misnomer and misleading. The pass is nothing more or less than 
au oroded drainage saddle, which has been and is constantly being 
lowered by the headwaters of the Obispo and Rio Grande. Even the 
