248 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



while others have shown that they are Lower Tertiary to the eastward. 

 Collectively this testimony strongly points to the existence of such a 

 littoral in early Tertiary time and to a corresponding land which this 

 littoral bordered. 



Pliocene. — The Pliocene formations have not been clearly dis- 

 tinguished in the Tropical American region from the Pleistocene. There 

 is an intermittent fringe of alleged Pliocene deposits around the Carib- 

 bean coast unconformably deposited against the older continental mass, 

 which nowhere attains great development except in the northern half of 

 the great peninsula of Yucatan, where it is most extensively developed, 

 occurring as a white limestone formation previously alluded to. 



At Port Limou, Costa Rica, the supposed Pliocene, as described by 

 Gabb and seen by the writer, appears as impure clay marls near the sea 

 margin. It is missing in the Isthmian section and sparsely represented, 

 if at all, on the Colombian coast, where it has not been differentiated. 



We may infer from the relatively slight area of the marginal develop- 

 ment of rocks of this period, and their absence in the elevated or folded 

 regions away from or much above the coast line, that it was just prior to 

 the Pliocene period or during its earlier days that the Caribbean coast 

 line, as a result of the tremendous orogenic processes by which the earlier 

 Tertiary rocks were deformed, practically assumed the shape as we now 

 know it. No sedimentary rocks of Pliocene age have been reported from 

 the Pacific coast of the Central American region. Gabb's studies of the 

 Pliocene of the Costa Rican coast, and Heilprin's of Yucatan, are the only 

 definite determinations of the rocks of this age in the Central American 

 region. 1 



Pleistocene. — The marine Pleistocene rocks of Tropical America are 

 not clearly separable from the Pliocene on the one hand, or the recent 

 formations on the other. Undoubtedly, however, there is a fringe of 

 deposits which can be very safely referred to this epoch which borders 

 more or less intermittently both the Atlantic and Pacific coasts, occurring 

 at or only a few feet above sea level south of the Honduras peninsula. 



In Yucatan, according to Heilprin, the Pleistocene has the white 

 limestone facies, and is indistinguishable from the Pliocene. In Costa 

 Rica, at Port Limon, a more sandy formation, which may be Pleistocene, 

 rests upon the underlying Pliocene clays, described by Gabb. These 



1 While reading these proofs another list of Pliocene fossils comes to hand. This, 

 as determined by Ball, and published by Spencer, is from the Caribbean coast of 

 Tehuantepec. See Bull. Geol. Soc. of America, Vol. IX. pp. 24, 25. Rochester, 



1H'J7. 



