Limits of the Chalk- Formation. 143 
great abundance that it may be considered as characteristic 
in regard to the whole of this formation. One is surprised, he 
says, to meet with such great masses of fossil shells, so many 
fragments of ammonites of many feet in diameter, or stalks of 
gigantic corals in these localities to such a degree, that there 
is not perhaps, in the whole surface of the earth, a point 
where, over so many leagues, we find such a mass of organic 
remains. This trigonia reappears in South America, in the 
mountains of Santa Fe de Bogota, whence M. de Humboldt 
brought it to Europe for the first time. These mountains of 
Santa Fe present, in the most evident manner, judging from 
the products found in the formations, the middle stage of the 
chalk. This I have endeavoured to shew in the description 
of M. de Humboldt’s Collection of American Fossils (Berlin, 
1849) ; and the proof has been given at still greater length by 
M. Alcide d’Orbigny, in his learned and well-considered 
work on M. Boussingault’s collections. As all the forma- 
tions of chalk in New Granada reach a thickness of 5000 
feet, it ought not to appear surprising that we there meet 
with organic remains of the inferior stage of the chalk, 
the neocomian formation. M. d’Orbigny has described an 
Exogyra from Socorra, which differs in nothing from the 
Exogyra Couloni of the neocomian formation. The same 
exogyra has been collected in abundance by the late Meyen, 
on the declivity of the voleanoes of Maypo, in Chili, at a 
height of 13,000 feet, but it has been only imperfectly figured 
(Acta der Leopold Acad. xvii., p. ii., 649, t. 27. f. 5). Dar- 
win (Geolog. Observ. on South America, 1846) has likewise 
found it not only at a little distance from Maypo, in the pass 
leading to Portillo, in the Penquene chain, but also 60 English 
miles beyond, towards the north, in the pass of Uspaletta. 
The Exogyra Couloni or aquila is nevertheless a shell, truly 
characteristic of the neocomian formation. 
All that Darwin has collected among the mountains above 
Copiapo and Coquimbo, in the north of Chili,—all that 
M. Domeyko, professor of mineralogy at Coquimbo, has sent 
to Paris, belong to the recent chalk-formations, and are met 
with even at a great distance from these localities, beyond 
