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 de'bris. 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 



j?£z0^.^ 0"?*K au d pointed summits (see Fig. 



^~^^i;., %|& f . ':•" '€?%. 5 )> llot over 500 feet in height 



«=^1 .^^^^fei^j^r^^.^i?'*^^ 5 ^"' immediately along the line of 



this section, but rising to 1,000 



Figure 5. Hill near Mamei, showing c . r ., 



4. ■ it.i •'„ T?™;„,J,„> f ee t or more a few miles away 



typical Isthmian Eminence. J 



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. The canal and 

 railway follow up the valley of the Chagres as far as Mata Chin, from 

 which point they continue up the Eio 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 Eio Grande on the south, may be spoken of as the 

 central region, but it is arbitrarily divided, for geologic discussion at 

 least, into four distinct subsections : — 



1. The Barbacoas Subsection. — That portion lying between Bujio 

 Salado and the Station of Baila Monos. 



2. The Mata Chin Subsection. — From Baila Monos to Las Cascadas 

 near the upper falls 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. 



