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.’ 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 Blas, 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 manner 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 Blas 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 cast 
of the Rio Atrato. Maack alone of the geologists has connected the 
Isthmian hills with the Andes. 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. 
