Limits of the Chalk-Formation. 145 
rict (d’Orb., Pl. 216), Conoidea Glifs, Holaster dilatatus and 
H. complanatus or Spatangus retusus, both recognised by M. 
Agassiz, Diadema Bourgeti, likewise met with at Neufchatel, 
Pecten cretosus, Brong., and Pecten quinquecostatus. 
It would appear, therefore, that the chalk-formation in 
South America is developed in a different manner ; that it is 
thicker and more varied than to the north of the Gulf of Mexico, 
and that its resemblance to the chalk-formations of Europe 
is much more complete among the Andes. But it is a very 
remarkable circumstance that, in North America, the chalk 
strata, altogether horizontal, occupy a considerable surface, 
and consist in a great measure of clay and sand, and other 
masses by no means compact. In South America, on the con- 
trary, we only meet with black limestones or compact sand- 
stones, the cohesion of which is such that this may be often 
taken for true quartz, such as those found between the 
Maranon and Lima, where these beds, far from affecting a 
horizontal position, are all, more or less, inclined ; a violent 
situation which can only be the result of the action of power- 
ful forces. Throughout the whole of Brazil, in the vast ex- 
tent of La Plata, Paraguay, and Bolivia, we no longer find 
the chalk, and none exists. 
Darwin has followed the chalk-formations to the extreme 
point of the continent. He has seen chalk-shells in quantity 
at the summit of Mount Tarn, 2000 feet in height; at Port 
Famine, in the Straits of Magellan, and at 53° of south lati- 
tude, consequently three degrees of latitude higher than at 
Missouri. The Ancyloceras simplex, d’Orb., and the Hamites 
elatior, Low, leave no doubt as to the nature of this chalk. 
The Hamites is even, says M. E. Forbes, one of the largest 
shells I have ever seen; it is 2} inches at its greatest dia- 
meter. Darwin’s discovery probably indicates the southern 
limits of the chalk-formation ; and the polar influences, con- 
sequently, seem thus to be opposed, at this point, to a great 
development towards the pole.* 
* Monatsbericht der Konig]. Preuss. Academie der Wissenschaften zu 
terlin, March 1849; also L’ Institute, No. 821, p. 306. 
VOL. XLVI. NO. xcvy.—vJANn. 1850. K 
