234 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



by Professor "Wolff's determination of granitic debris 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 debris. 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. Xumerous 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 viciuity of Port Limon is most interesting. There 

 are a few peculiar hills standing about 40 feet above the adjacent 



Figure 24. Detailed Section at Port Limon, Costa Rica. 



swamp level. These hills are " mouadnocks," or remnants of an ex- 

 tensive 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 crossbedded. 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 the rail- 

 way cuts, this sandy formation rests upon lower lying beds consisting of 

 clays containing great numbers of fossils and reef-making corals. The 

 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. 1 



The arenaceous formation clearly rests upon these older beds, ami is 

 of an entirely different lithologic character. The corals from the sup- 

 posed Pliocene bed were kindly determined fi>r me by Mr. T. Wayland 

 1 Jour. Acad. Nat. Sci. Phila., 2d ser , Vol. VIII. No. 4, p. 349. 



