250 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



geologic history of this region. By examining the results of these pro- 

 cesses we may possibly obtain some light upon the architecture of the 

 Isthmian country, while the paleontologic data will give us the histori- 

 cal facts. 



Predominance of East and West Otogenic Trends. — In the introduc- 

 tory chapter of this paper we have outlined the general arrangement of 

 the mountain systems influencing the topography of Tropical America. 

 From the publications of Felix and Lenk upon the southern termina- 

 tion of the Mexican Cordilleras, 1 those of Dr. Carl Sapper on the Gua- 

 temala-Chiapas province, 2 of Sievers's map and resume of the Caribbean 

 coastal region of South America, 3 and of my own, 4 we find a description 

 in widely separated localities of the prevalent east and west trend of the 

 orogenic systems of the Great Antilles and the peripheral Caribbean lands, 

 and the demonstration further that in this region there has been an area 

 of late Tertiary mountain making, whereby a new system of mountains 

 has been made directly tangential to that of the prevalent trends of the 

 main Xorth and South American Cordilleras. 



Excepting the gigantic eminences piled up by volcanic extrusion, 

 beside which all other surface features seem trivial, this late Tertiary 

 mountain folding is the most conspicuous and far reaching event of all 

 geologic history in Tropical America. Before it, however, other orogenic 

 periods and results are manifest, concerning which we shall endeavor 

 to present what little is known. 



Evidence Indicating the Existence of a Pre- Tertiary Orogenic Complex. 

 — The granitic mountains of Oaxaca, Chiapas, and Guatemala, with their 

 laterals of Paleozoic and Mesozoic rocks, apparently belong to one or 

 more orogenic complexes of great antiquity in origin. It is immaterial 

 here whether they are Archaean or not, but they may in part antedate 

 the Tertiary period, and seem to be of Pre-Cretaceous age, and give us 

 a faint insight into the existence of an ancient land in this region. 

 According to Sapper 5 the Archaean foundation along which is arranged 

 the Paleozoic and Pre-Carboniferous complex of strata plainly shows in 

 two chief chains, lying to the north and south of Rio Motagua, while in 

 the southeastern portion of the country they show in the spurs of a 

 third chain. From these foundations stretching through the north are 

 Carboniferous, Cretaceous, and finally the Tertiary strata. Besides, one 



i "Ueber die tektonischen Verhiiltnisse der Kepublik Mexico," Berlin, 1892. 

 2 Op. at. z Op. at. 



4 National Geographic Magazine, May, 1806. 



5 Op. cit. 



