242 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



to the vast accumulations of more modern igneous and sedimentary 

 rocks of Tertiary and Post-Tertiary age, a foundation of granitic rocks 

 occurring in east and west arrangement existed in the South Isthmian 

 and Central American region, extending in echelon arrangement from 

 the longitude of Trinidad through forty degrees to near that of Aca- 

 pulco, Mexico, directly across the path of the main continental trends. 



Hie Paleozoic Sedimentaries. — Except a few outcrops in northern 

 Sonora fossiliferous Paleozoic rocks are unknown in the Mexican Plateau 

 region south of the Rio Grande of Texas. The Paleozoic foundation of 

 the Mexican Plateau was completely buried beneath Cretaceous lime- 

 stone sediments, and since the elevation of the latter above the sea 

 erosion has not yet been sufficient to re-expose them. In fact, between 

 the southern boundary of the United States and the Empire of Brazil 

 I know of but one well defined region of authenticated fossiliferous 

 Paleozoic rocks, and this is in the Republic of Guatemala and the adja- 

 cent Mexican border region of Chiapas, where Dr. Carl Sapper l has 

 shown the undoubted occurrence of a large series lying above the so 

 called Azui granites. He describes the existence of an extensive system 

 of Pre-Carboniferous rocks, probably Silurian, above which are Carbo- 

 niferous limestones containing 36 enumerated species of characteristic 

 fossils. Above this is a series of strata between the Carboniferous and 

 the Cretaceous, then undoubted Cretaceous, and finally the Tertiary. 

 This Guatemalan section, including its continuation into Chiapas, is the 

 only one in the whole Central American region where undoubtedly 

 fossiliferous Paleozoic rocks are exposed. All positive evidence of the 

 existence of the Paleozoic rocks elsewhere south of the United States is 

 thoroughly concealed by overlap, — in Mexico by the overlying strata 

 of Mesozoic, and in Central America by the later igneous and Creta- 

 ceous and Tertiary rocks. 



The Older Mesozoic. — In Ti'opical America as in the United States, 

 owing to the absence of fossil remains, the old Pre-Jurassic Mesozoic 

 is a problematic formation, and is represented by three small outcrops 

 only. Two of these occur in Mexico. One of them is at Miquehuana 

 in the State of Tamaulipas, as discovered by the writer. 2 In these 

 two localities, not only by occurrence beneath the Jurassic, but by their 

 characteristic composition and red colors, they are analogous to the Red 

 Beds formation of the Western United States. The third locality, or 

 group of localities, is in the State of Guatemala and Chiapas, where Dr. 



1 Op. cit. (see footnote 1, p. 239). 



2 American Journal of Science, Vol. XLV. p. 311, 1893. 



