HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 163 



Whether composed of igneous or sedimentary rocks, these all have the 

 uniform topographic aspect above described, and their geologic compo- 

 sition cannot be predicated from their appearance, as is the case in 

 most regions of the earth's surface. 1 These hills are irregularly dis- 

 tributed over the surface, and generally without any arrangement into 

 systematic chains or ridges. This is an important point, for most of 

 the maps of the region represent a culminating continental divide or 

 backbone extending along the length of this narrow strip of land, when 

 in fact the summits extend abruptly to either coast, regardless of any 

 axial arrangement. Even upon the islands of Panama Bay these sum- 

 mits exceed in height the continental drainage divides. 



It is difficult to convey an idea of the irregularity of distribution and 

 arrangement of these low mountains and hills. Whatever may have been 

 their original configuration, their present shape, distribution, and rela- 

 tive arrangement are largely the result of the erosion of the intensely de- 

 veloped drainage system, and the marine erosion of the bordering seas. 



Out of this chaotic condition there are two exceptional indications 

 of systematic arrangement. The southern termination of the high 

 central divide of the Costa Rican region continues due eastward for a 

 short distance into the province of Panama, giving an east and west 

 trend to the topographic crests of the western end of the Isthmus. 

 Southward of this line, and parallel to it in echelon arrangement, is the 

 Cordillera of San Bias, following the Caribbean coast from Puerto Bello 

 a few miles east of Colon to Caledonia Bay, and separating the drainage 

 of the headwaters of the Bayano and Tuyra systems flowing into the 

 Pacific from the waters of the Caribbean. This range of mountains 

 is far from being central, however, but is a marginal accompaniment of 

 the Atlantic border, and its east and west trend can in no mauner be 

 made to harmonize with that of the Andes, for if their respective termini 

 could be projected they would intersect at right angles. The trend of 

 the San Bias system is pre-eminently Antillean, and not Andean. 



None of the writers on the Andean system connect the Isthmian 

 summits with it. Karsten, Sievers, Reclus, and others, all agree on 

 the character of the termination of the Andes in a threefold range east 

 of the Rio Atrato. Maack alone of the geologists has connected the 

 Isthmian hills with the Andes. 2 He connects the mountains east of 



1 This fact is no doubt due to the deep under surface decay of the rocks, which 

 here renders the surface of both igneous and sedimentary rocks into a homogeneous 

 mantle of red clay. 



2 Report on Geology and Natural History of the Isthmus of Darien and Panama. 

 Selfridge, Naval Reports, Washington, D. C, 1874, p. 156. 



