162 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The most conspicuous eminences are the grand volcanic peaks of 

 Guatemala, San Salvador, and Costa Eica. 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 Eica, when 

 they trend diagonally towards the Caribbean side, again assuming in the 

 southern portiou of that Republic a central continental position. These 

 great eminences are built up of accumulations of volcanic debris 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 Eica 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 Eica), 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 

 volcanic region, commonly called the Costa Eicau 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 Eica, 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 geutle 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 Aspimvall) and Panama ; but this usage is improper, 

 for the region is a geographic and political unit, and the name should 

 not be restricted to any particular pass across it. 



Topographically, the surface of Panama consists of exceedingly irregu- 

 larly rounded, low pointed mountains and hills covered by dense forests. 



