234 BULLETIN: MUSEUM OF GOMPARATIVE ZOOLOGY. 
by Professor Wolff’s determination of granitic débris in the Tertiary 
sediments. (See his Report on Specimens, Appendix.) At Pacaurito 
the road is well in the lower country, but the whole formation still con- 
tinues to be boulder débris. At Madre de Dios, altitude 340 feet, a 
little stream flows over a bed of rounded igneous pebbles. 
Two miles east of Madre de Dios the foot of the volcanic plateau is 
passed and we enter an entirely different region. Numerous small flat- 
topped hills appear, identical in topographic aspect and composition with 
the Monkey Hill level seen at Colon, surrounded on all sides by a simi- 
lar swamp as that seen upon the Isthmus. This topography continues 
to Salsipuedes. From Salsipuedes to Limon, 27 miles distant, the 
road follows a typical coastal swamp similar to those seen on the Isthmus 
of Panama and identical in geological character. There is observed 
occasionally a low remnantal hill of the Monkey Hill level. 
The geology in the vicinity of Port Limon is most interesting. There 
are a few peculiar hills standing about 40 feet above the adjacent 
) PLIOCENE 
Figure 24. Detailed Section at Port Limon, Costa Rica. 
swamp level. These hills are 4 monadnocks," or remnants of an ex- 
teusive higher level which has been destroyed by base levelling around 
them. "They are seen back of the town and on the outlying Grape Key. 
They are composed of unconsolidated sands and clays, the stratification 
of which is greatly erossbedded. These beds contain fossils which Gabb 
has referred to the Pleistocene or recent. The fringing coral reefs are 
later and unconformable with these beds. Back of the town, in tho rail- 
way cuts, this sandy formation rests upon lower lying beds consisting of 
elays containing great numbers of fossils and roef-making corals. Тһе 
numerous fossils collected by the writer from this bed, referred to 
Dr. Dall, were pronounced to be of Pliocene age. These are the type 
localities of the many Pliocene fossils which have been described by 
Mr. Gabb from Costa Rica.! 
Тһе arenaceous formation clearly rests upon these older beds, and is 
of an entirely different lithologie character. The corals from the sup- 
posed Pliocene bed were kindly determined for me by Mr. T. Wayland 
1 Jour. Асай, Nat, Sci. Phila., 2d ser, Vol. VIII. No. 4, p. 849. 
