176 BULLETIN: MUSEUM OF GOMPARATIVE 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. Іп 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 serapings of this 
formation. Collections were made from the cuts at Monkey Hill dur- 
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, Curagoa, 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 Gatun is through à 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 4 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). Тһе black looking clays of 
the Tertiary outerop in tho 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. 
