THE PLIOCENE PERIOD. 309 



relations of these marine Pliocene beds to the Lafayette are undeter- 

 mined. 



The marine fossiliferous Pliocene beds of the Atlantic coast con- 

 sist of shell marls, sand, and thin beds of limestone. In Florida, the 

 marine beds have a thickness of but a few feet. The gradual changes 

 in the character of the marine fossils from below upwards in the beds 

 show that a gradual shoaling of the water took place, until the species 

 proper to a moderate depth were replaced by those characteristic 

 of muddy shallows and tidal flats, and .finally by an exclusively fresh- 

 water fauna. 1 



In addition to the marine Pliocene of Florida, there seem to have 

 been coastal lagoons and ponds in which fresh-water mollusks abounded. 

 Occasionally, however, the sea had access to the lagoons, either as 

 a result of slight changes of level of land or sea, or of severe storms, 

 so that marine fossils are sometimes associated with those of fresh- 

 water species. In addition to the coastal lakes and lagoons,, there 

 were lakes in the low interior syncline of the peninsula. 2 



The Gulf coast. — Pliocene beds of marine origin have not been 

 certainly identified on the Gulf coast of the United States, 3 west of 

 Florida, but they cover considerable areas farther south. Yucatan 

 is generally covered with marine Pliocene, and corresponding deposits 

 are known both to the north and south of that peninsula. 4 In general, 

 the Pliocene beds of the tropical portion of the continent have not 

 been clearly separated from the younger Pleistocene beds, with which 

 their relations are said to be close. According to Hill, the great inter- 

 ruption in the Tertiary history of this region was in the later part of 

 the Miocene, or at its close. 5 In the Antilles also, Pliocene beds are 

 known on the borders of some of the islands. 6 



The Pacific coast. — On the Pacific coast, the post -Miocene emer- 

 gence left little of the present land-area submerged; but a little later, 

 coastal depression allowed the sea to encroach upon the land to a 

 slight extent, and Pliocene beds were deposited unconformably on 



1 W. H. Dall and G. D. Harris, Bull. U. S. Geol. Surv., No. 84, p. 191. 



2 Ibid., De Soto beds, pp. 133, 324. 



3 The upper part of the Grand Gulf series is referred to the Pliocene by Smith 

 and Aldrich, Science, New Series, Vol. XVI, p. 836. 



4 Gabb, Lumon clays. Jour. Acad. Nat. Sci. Phil., Vol. VIII, 1881, p. 349. 



5 The Geol. History of the Isthmus of Panama and Portions of Costa Rica. 



6 Hill, Geology and Physical Geography of Jamaica, 1889. 



