606 T. W. STANTON MESOZOIC OF MEXICO AND CENTRAL AMERICA 



by a land barrier between the Atlantic and Pacific throughout North 

 America. 



Comparatively little is known abont the Cretaceous rocks of Central 

 America which have been mapped by Willis as Lower Cretaceous in 

 Guatemala, Honduras, Costa Rica, and Panama. Some geologists have 

 described them as Upper Cretaceous, bu.t from the brief statements about 

 the fauna found in them it is inferred that they are probably Comanche.- 

 No Lower Cretaceous rocks are known in the West Indies, with the pos- 

 sible exception of some imperfectly known beds on Trinidad. 

 ■ The Upper Cretaceous faunas of the Pacific border, while complex and 

 showing many local and temporal variations, are bound together by com- 

 mon species, so that in a broad sense they form a unit which is distributed 

 from Alaska to the peninsula of Lower California. These faunas are 

 remarkably distinct from the Upper Cretaceous faunas in and east of the 

 Rocky Mountains, which also show considerable regional differentiation 

 among themselves and are yet more or less bound together by relation- 

 ships of various kinds. In Mexico the Pacific, or Chico, fauna is known 

 only in Lower California, while the Gulf and Interior types of faunas do 

 not extend much west of the meridian of El Paso, on the northern border, 

 but are widely distributed east of that line in the northern half of Mexico. 

 Farther south, in Puebla and Guerrero, isolated areas have been referred 

 to the Upper Cretaceous, apparently of Atlantic type, but in general the 

 Upper Cretaceous seems to be absent from southern Mexico. Near Car- 

 denas, in southeastern San Luis Potosi, there is a Lower Senonian fauna, 

 described by Bose, which differs markedly in facies from all the contem- 

 poraneous faunas of the Gulf Coastal Plain of the United States on 

 account of the abundance of Actseonella, Rudistffi, corals, etcetera. It 

 shows, however, close relationship with the West Indian Cretaceous fauna 

 as developed in Cuba, Jamaica, and Santo Domingo, and also resembles 

 the Gosau fauna of Europe. 



All the known facts of faunal relationships and distribution seem to 

 call for a continuous land during Upper Cretaceous time extending par- 

 allel with the Pacific border from British Columbia and farther north to 

 South America. The history of the Mesozoic changes in land and sea 

 which probably involved land connections between North and South 

 America, or marine connections between the Gulf of Mexico and the 

 Pacific, is epitomized in the tabular summary submitted by Doctor 

 Vaughan. 



2 A small collection of fossils from the Cretaceous limestones of Honduras received 

 from Mr. R. W. Pack since these lines were written seem to belong to the Comanche 

 fauna. 



