158 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



of the region would require not only the labor of years, but a long resi- 

 dence. The exploration of the Isthmus is as yet very incomplete, and 

 much has been made hastily in order to escape malarial influences ; 

 besides, the Isthmus lying between Colon and the Atrato is inhabited 

 by inhospitable Indians, who do not permit the invasion of their country. 

 So that the most elementary features of large areas are still unknown. 



I shall endeavor, however, to give a complete geological section across 

 the Isthmus from ocean to ocean, along the line of the Panama Railroad 

 and Canal route, together with such other observations as may have 

 come under my notice. After having presented this section across the 

 narrowest portion of our hemisphere, I shall by way of contrast present 

 a section across the Republic of Costa Rica to the northward, showing a 

 type of the Central American volcanic region. 



PART I. 

 The Geographic Position of the Isthmian Region. 



The Humboldtian geography has taught that the two American con- 

 tinents are practically dominated by a continuous Cordilleran system, 

 running like a backbone through South, Central, and North America, 

 connecting the whole Avestern border of the hemisphere into one great 

 mountain system. 



The Andean Cordilleran trend, which dominates the western coast 

 of South America, after crossing to the north of the Equator, trifurcates, 

 bends slightly eastward, and abruptly terminates in Northern Colombia. 

 Only one doubtful spur of the Andes touches the coast of the American 

 Mediterranean, and this is the Sierra del Marta, lying between the Gulf 

 of Maracaibo and the River Magdalena. The northern end of the 

 Andean system lies entirely west of the Isthmian region, and is sepa- 

 rated from it by the Rio Atrato, and has no genetic connection with 

 that of the north coast of South America, much less with the moun- 

 tains of Central America, or the great Rocky Mountain region of Mexico 

 and the" United States. In fact, the deeply eroded drainage valley of 

 this stream nearly severs the Isthmian region and the Pacific coast of 

 the Republic of Colombia from the South American continent. 



Many geographers, especially Felix and Lenk, 1 have shown that the 



1 Debet die tektonischen Verluiltnisse der Republik Mexiko, von Johannes 

 Felix und Hans Lenk. Berlin, 1892. Zeitschrift d. Deutsch. geolog. Gesellschaft, 

 Jahrgang 1892. 



