HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 253 



their debris, that relatively the vulcauism has been more quiescent 

 and the movements have become more of the epeirogenic than of the 

 orogenic character, and the land as a whole has undergone oscillations 

 of upward and downward movements, as recorded in the base levelling 

 described in this paper. 



Unfortunately the only bench mark by which the igneous disturb- 

 ances can be gauged are the older Tertiary and later sediments. From 

 the co-deposition of the volcanic debris in these sediments, we know 

 that the volcanic activity was in progress during their formation. 

 Whether this vulcanism, which we know was in operation at the close 

 of Cretaceous time, with its accompanying disfiguration of topography, 

 was continuous, to the present, or alternated with long periods of qui- 

 escence, cannot be answered. 



The Orogenic Revolution of Later Tertiary Time. — We know that 

 earlier Tertiary sediments laid down in alternating layers have since 

 their deposition been elevated above the sea to great heights by folding 

 on the Caribbean side of the old Isthmian protaxis until they stand 

 3,000 feet in Guatemala, 5,000 in Talamanca, 300 near Colon, 500 at 

 Cartagena, 9,000 in the Great Antilles, and several hundred feet at 

 various places along the Lesser Antilles. 



The older Tertiary strata of Chiapas and Guatemala, according to 

 Sapper, occur entirely on the Atlantic side of the Cordilleran masses. 

 The summits of these masses composed of Paleozoic and granitic ma- 

 terial reach an altitude of 12,500 feet. The Cretaceous on their 

 northern flank lies below altitudes of 6,560 feet, while the Tertiary 

 lie below 3,281 feet. 



According to Mr. Gabb's researches in Talamanca and my own obser- 

 vations in Costa Rica, the upturned Miocene strata produced by these 

 movements occur at great altitudes of from 3,000 to 5,000 feet. 



The admirable geologic maps and profiles of Northern Venezuela by 

 both Sievers and Karsten show the remarkable manner in which the 

 Tertiary rocks have been folded in the east and west mountains ex- 

 tending along the north coast of South America. The almost vertical 

 folded Tertiaries on the peninsula of Paraguana now stand at an 

 altitude of nearly 1,600 feet above sea level. South of Coro, between 

 longitude 69° and 70°, similar folds are seen on the mainland. 



In Hayti, Cuba, and Jamaica these plicated Cretaceous and early 

 Tertiary rocks are found at altitudes exceeding 10,300, 8,000, and 7,250 

 feet respectively above the ocean. 



The east and west strike of both the Tertiaries and of the basic igue- 



