VON BUCK ON THE LIMITS OF THE CHALK FORMATION. 25 



Arqueros, and in like manner above Guasco and near Las Amolanos 

 in the principal valley of Copiapo. This Pleurotomaria is always con- 

 joined with the Pecten, occurring even in the northern regions be- 

 tween Montan and Guancavelica in such incredible numbers, that it 

 forms fields, nay, mountains of petrifactions, long and very generally 

 known to the natives under the name of " Choropampas.'^ {Pecten 

 alatus, Dufresnoyi, d'Orb.) It was this shell also that, in 1761, 

 excited so great astonishment in Ulloa at the great elevation above 

 the level of the sea, at which mountains composed of shells were seen, 

 and this astonishment was repeated in all text-books, till it was dis- 

 covered that the shells had not necessarily lived at this elevation, but 

 might have been raised up from the depths of the sea. ^mc^Hippu- 

 rites organisans (D'Orb. p. 107, t. 22) occurs with the pecten-strata, 

 it is evident that all these beds in Peru, as at Coquimbo and Copiapo, 

 must be conjoined at least with the gault ; a result which is most 

 strikingly confirmed by an Exogyra which M. Domeyko has sent to 

 Paris. This is indeed perfectly identical with the Gryphcea {JExo- 

 gyra)Pitscheri from Texas, already described and figured by Morton, 

 and the position of which above the gault at Friedrichsberg has been 

 very accurately ascertained by Ferdinand Romer. 



Lower cretaceous strata, similar to those of Aconcagua, are never- 

 theless not altogether unknown in the Andes mountains near Lima. 

 The celebrated zoologist Von Tschudi has found, on the eastern de- 

 clivity of the mountains, between Oroja and Yauti, near Tarma, along 

 with many others, some perfectly characteristic neocomien shells : — 

 Pterocera Emerici (D'Orb. p. 216), conoidea, Goldfs. ; Holaster 

 dilatatus and Holaster complanatus or Spatangus retusus, both iden- 

 tified by Agassiz ; Diadema Bourgeti, also determined as such in Neuf- 

 chatel ; Pecten cretosus, Brgnt, and Pecten quinquecostatus. 



According to this, the cretaceous formation in South America ap- 

 pears to be developed in an entirely different manner, in much greater 

 thickness and variety than to the north of the Gulf of Mexico, and 

 the agreement with the European cretaceous strata is also much more 

 complete in the Andes. It is, however, highly remarkable, that in 

 North America the cretaceous strata are spread out quite horizontally 

 over immense spaces, and that they consist chiefly of clay and sand, 

 and other slightly coherent masses. In South America we only see 

 black limestones, or compact sandstones, of such consistence, that one 

 often believes them to be pure quartz, as between the Maranon and 

 Lima ; along with this, the strata are never horizontal, but always more 

 or less inclined ; a disturbed position which they evidently can only 

 owe to powerful disturbing forces. There can be less doubt in regard to 

 this, when it is seen, as Meyen informs us, that the precipitous cone 

 of the volcano of Maypo consists for two-thirds of its height of chalk 

 rich in petrifactions, and that throughout the whole of Chili masses of 

 gypsum, many thousand feet thick, surround the volcanos, and the 

 cretaceous strata first appear quite above them. But when we leave 

 this desolation, the chalk also has vanished. It never reaches the 

 plain of the Pampas on the east ; a chain of Devonian strata at the 

 eastern foot of the Andes does not permit it even once to touch on 



