HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 259 
their débris, that relatively the vulcanism 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 débris in these sediments, we know 
that the volcanic activity was in progress during their formation. 
Whether this vuleanism, 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 fect, 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 igne- 
