176 bulletin: museum of compakattve zoology. 



the former eminences represent the interior upland margin of the old 

 Monkey Hill base level, where the sloping plain was originally about 

 one hundred feet higher than at the station of Monkey Hill. 



The Monkey Hill Formation. — Like all the Isthmus these hills are or 

 were once covered by dense vegetation growing from a thick residual 

 soil of red clay. In fact were it not for the artificial cuts of the railway 

 and canal, the true underlying structure would never be seen. 



Both the canal and railroad companies have made deep cuttings 

 through the sides of Monkey Hill and reveal the substructure of fossilif- 

 erous greensand marl of which it is composed. This marl is dark bluish 

 green in color, sometimes with white calcareous efflorescence, and carries 

 many fossils. The new addition to Colon Island, built by the Canal 

 Company, known as Cristofo Colon, is made of the scrapings of this 

 formation. Collections were made from the cuts at Monkey Hill dm*- 

 ing the construction of the canal, which, with those made by the writer, 

 as shown in the appended report of Dr. Dall, prove the deposits to be 

 of the age of the Middle or Upper Oligocene Tertiary, and synchronous 

 with deposits in the Antilles, Florida, Trinidad, Curacoa, and other points, 

 and which throw interesting light upon the history of the whole of the 

 Caribbean region as will be shown later. Most of the railway between 

 Monkey Hill and Gatuu is through a low, wide swamp. 



Passing only a few yards through the west base of Monkey Hill, the 

 railroad from there to Gatun mostly follows the low swamp level, bor- 

 dered by occasional hills on the south. Keeping these in view, the 

 remnants of the Monkey Hill level can be seen to back up against the 

 Sierra Quebrancha, a line of higher hills which run parallel to the Carib- 

 bean coast back of Gatun Station. These hills are of irregular height, 

 from 300 to 600 feet, and are probably similar to a smaller group cut 

 by the Chagres at Vamos a Vamos. 



The structure of the few eminences between Monkey Hill and Gatun 

 could not be ascertained owing to the thick covering of red clay. Suf- 

 ficient observations in connection with the section to be described along 

 the canal and river were taken to show that these hills were composed 

 of Tertiary sedimentaries. 



At Mindi Station (Colon 4.56 miles). The black looking clays of 

 the Tertiary outcrop in the ravine opposite the station, at the end of a 

 new bridge across a ditch. Another outcrop of these beds is shown in 

 the cut through the north end of the hills near Mindi Station, and 

 there can be little doubt that these hills are composed entirely of 

 marine sedimentary material. 



