798 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



ward to the hills of the Sierra de Yucatan. Heilprin *^^ describes the plain and 



says: 



The rock formation over the greater part of the plains is that of a gray or white shell lime- 

 stone, highly indurated or subcrystalline in local areas, but rarely to the extent of obliterating 

 its fossiliferous character. Secondary depositions of calcite, in the form of veins, crystals, and 

 nodular masses, are abundant. Where less compact the rock may be said to be a mass of loosely 

 united shells, a condition that is best shown in the superficial layers. Good sections of the rock 

 are seen only in the walls of the aguadas and cenotes and in a number of railway cuts which 

 traverse it both in a north-south and east-west line; the rock surface is, however, visible over a 

 very large part of its extent, being but scantily covered with soil and supporting only an indiffer- 

 ent vegetation. Its decomposition has liberated large quantities of red earth, similar to that 

 which is found in our own northern region {terra rosso) and on coral islands (for example, the 

 Bermudas, Bahamas), and which is seemingly a residual product representing impurities of one 

 form or another which Were introduced into the limestone at the time of its formation. * * * 



The paleontological evidence of the fossils contained in the limestone is to the effect that 

 the latter belongs to two periods of geological time, the Pliocene and the post-Pliocene, but 

 stratigraphically it is not easy to draw a line of demarcation between the two formations. It, 

 indeed, appears as though the post-Pliocene, except in the coastal area, were present only in 

 patches, having been removed through atmospheric decay and denudation. It is in most places 

 easily distinguished by the large numbers of Venus canceUata which fill the rock, making a 

 true Venus cancellata bed, such as I observed capping the PHocene beds on the Caloosahatchie, 

 Florida, just below Fort Thompson. The beds occupy similar positions and- hold equivalent 

 relations to the construction of the land and may, therefore, be considered as counterparts of an 

 identical formation. 



For lists of living and extinct species, see the work cited. Heilprin continues : 



The^xact position in the Pliocene series which these Yucatan rocks hold can not, perhaps, 

 be stated, but they with little doubt correspond at least in part .with the series occurring in 

 Florida which I have designated the "Floridian." It is true that the number of extinct species 

 of the mollusks is seemingly less in the Yucatan rock than in that of Florida, but it should be 

 said that in addition to the forms above enumerated, there are a considerable number, occurring 

 mainly in the condition of unrecognizable casts, which may largely represent extinct species. 



Dall and Vaughan agree with Heilprin on the Pliocene age of the fossils cited 

 by him. Sapper ®®*'' says : 



The Tertiary beds are in general but slightly inclined. In the vicinity of Istapa, of San 

 Antonio, Tenejapa, and Tumbala there are horizontal beds. They are more modem than the 

 andesitic eruptions, for they include the andesites in the conglomerates and the horizontal 

 strata even rest directly upon the andesite. 



In the peninsula of Yucatan the Tertiary terranes predominate and it would seem that from 

 south to north there is a sequence of more recent beds until one reaches the post-Pliocene 

 (Quaternary) deposits of the northern coast. 



The southern region of Yucatan is composed of calcareous formations which in many places 

 abound in cherts. Between the beds of limestone there are also some beds of marl and others 

 of alabaster, and in the marls there may be found in the Cerro de Ixconconcal, near Icaiche, 

 certain fossils which may serve to determine the relative age of the deposits. 



These fossils are apparently not cited, but the beds are supposed to be those 

 which are mapped by Sapper as limestones with marl and alabaster and are regarded 

 by Dall as Oligocene. All the other formations distinguished by Sapper, comprising 

 the white and red limestones, as well as his Pliocene, are here placed in the Pliocene. 



