182 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
This consists of greensand lithologically similar to that of the Mindi 
Cut, and also rich in fossils, which according to Dr. Dall's report are 
similar to those obtained from the cut through the Mindi Hills on the 
canal. The strata are almost horizontal. 
Bujio to the Pacific. — From Bujio, 15.45 miles from Colon, to the 
Pacific, at Panama, the railway and canal enter and extend through 
a peculiar hilly region, composed largely but not entirely of igneous 
rocks, — massive conglomerates and tuffs confusedly mixed with sed- 
imentaries and igneous débris. It is difficult to convey to one who has 
never seen this region an idea of its topography and geology. It is 
marked on every side by conical 
hills with steeply sloping sides 
and pointed summits (see Fig. 
5), not over 500 feet in height 
immediately along the line of 
this section, but rising to 1,000 
feet or more a few miles away 
from the railroad. Erosion has 
long since stripped away all the original surfaces, and the present sur- 
faces are the rocks which formerly constituted the deep substructure of 
the original topography, whatever that may have been. Тһе canal and 
railway follow up the valley of the Chagres a8 far as Mata Chin, from 
which point they continue up the Rio Obispo, not only to the Culebra 
Pass, but through it to the south of the summit. 
The hills of igneous rocks do not differ materially in configuration 
from those elsewhere seen composed entirely of sedimentary rocks, and it 
is only in the artificial cuttings of the railway and canal into their bases 
that some insight into their geologic composition can be determined. 
This region as a whole, lying between the high tides of the Chagres on 
the north and the Rio Grande on the south, may be spoken of as the 
central region, but it is arbitrarily divided, for geologio discussion at 
least, into four distinct subsections : — 
1. The Barbacoas Subsection. — That portion lying between Bujio 
Salado and the Station of Baila Monos. 
9, The Mata Chin Subsection. — From Baila Monos to Las Cascadas 
near the upper falla of the Obispo. 
3. The Culebra Subsection. — From Cascadas through the Culebra 
Pass to Paraiso. 
4, The Pacific Subsection, — From Paraiso to the Pacific coast at 
Panama. 
Ficure 5. Hill near Mamei, showing 
typical Isthmian Eminence. 
