162 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
The most conspicuous eminences are the grand volcanic peaks of 
Guatemala, San Salvador, and Costa Rica. These rise to an average 
height of 10,000 feet in irregular masses, standing nearer the Pacific 
coast than the Atlantic until reaching the Republic of Costa Rica, when 
they trend diagonally towards the Caribbean side, again assuming in the 
southern portion of that Republic a central continental position, These 
great eminences are built up of accumulations of volcanic débris which 
have buried and largely concealed a most interesting antecedent geologic 
structure. This older structure must be interpreted before the complete 
history of the region can be written. 
The western termini of the east and west Antillean trends of the 
Caribbean half of Central America are buried in Guatemala, Honduras, 
and Costa Rica by the overlying volcanic masses. They are not so lim- 
ited on the Pacific side of Panama, however, but continue across it. 
Upon entering the State of Panama from the west (Costa Rica), signs 
of recent volcanic activity cease, and the chain of high Central American 
summits is succeeded by the more broken and apparently intangible 
lower Isthmian topography. 
The Isthmus of Panama can now be accurately defined as the stretch 
of land lying east of the southern end of the Central American active 
voleanic region, commonly called the Costa Rican Volcanic Plateau, and 
extending to the northern termination of the Andes. Its limit on the 
east is the Rio Atrato, which flows northward from the Equator along 
the valley marking the western flank of the Andes, and on the west the 
southern boundary of Costa Rica, stretching from Burica Point to the 
island of Veraguas, and extending between the meridians of 79? 15' and 
82°, a distance of 180 miles. The axial trend of the Isthmian region is 
east and west, or in a direction contrary to the north and south conti- 
nental trends, and conformable with the Antillean axes. 
'The outline of the Isthmian region is that of a gentle arc extending 
in an east and west direction, the northern or Caribbean shore being 
concave in outline, while that of the Pacific is deeply convex and in- 
dented by many bays and estuaries. 
It has been the custom of later writers to restrict the use of the 
term “Isthmus of Panama" to the low pass lying between the cities 
of Colon (formerly Aspinwall) and Panama ; but this usage is improper, 
for the region is a geographic and political unit, and the name should 
not be restricted to amy particular pass across it. 
Topographically, the surface of Panama consists of exceedingly irregu- 
larly rounded, low pointed mountains and hills covered by dense forests. 
