24 GEOLOGICAL MEMOIRS. 



Scabrce of Agassiz, and differs but slightly from Trigonia aliformis. 

 Sow. The latter is however characteristic for the middle chalk, craie 

 chloritee, as also for the gault. In the middle of the chief Cordillera 

 of Anahuac, twelve French miles north-west from Tehuacan, this 

 shell is so universally abundant and large that it may be regarded as 

 the distinctive mark of the entire formation. One is astonished, he 

 says, to find such immense accumulations of fossil shells, so many 

 fragments of ammonites several feet in diameter, or of gigantic coral 

 stems in this place, so much so that there is perhaps no other place 

 on the surface of the globe where such an enormous mass of organic 

 remains lies scattered over many square miles. Now this Trigonia 

 appears again in South America in the mountains of Santa Fe de 

 Bogota, from which M. von Humboldt first brought it to us. The 

 organic remains enclosed in the strata of these mountains of Santa Fe 

 de Bogota prove most decidedly the occurrence of the middle chalk, 

 as I have endeavoured to show in the description of Humboldt's col- 

 lection of American petrifactions (Berlin, 1839), and as Alcide d'Or- 

 bigny has still more fully proved in his no less instructive than 

 masterly work on M. Boussingault's collections. But as the creta- 

 ceous formations in New Granada attain a thickness of more than 

 5000 feet, it is not surprising that the organic remains of the lower 

 cretaceous strata, or the neocomien, should also be found in this place. 

 D'Orbigny has described an Exogyra from Socorra, which is not di- 

 stinct from the Eocogyra Couloni of the neocomien. Many specimens 

 of this same Exogyra were collected by the late Dr. Meyen on the 

 declivities of the volcano of Maypo in Chili at the height of 13,000 

 feet, and (badly) figured*. Darwin alsof found it on the Portillo 

 Pass in thePeukenes chain, not far fromMaypo,but also sixty English 

 miles further north on the Uspallata Pass. The Exogyra Couloni 

 or aquila is, however, a true and very decided characteristic shell for 

 the neocomien. 



The fossils collected by Darwin in the mountains above Copiapo, 

 and Coquimbo in northern Chili, and those which Domeyko, Pro- 

 fessor of Mineralogy in Coquimbo, has sent to Paris, again belong to 

 newer cretaceous strata, and are found in part also at a distance from 

 it beyond the great knot of transition beds which, along with older 

 rocks, have intruded into the chain of the Andes ^. The most re- 

 markable of these forms is the beautiful univalve, which Humboldt 

 first brought from San Felipe in the south of Quito, near the Amazon 

 river, and which was figured and described by me asPLEUROTOMARiA 

 HuMBOLDTii in the ' Petrifactions,' v. f. 26, first published in 1839. 

 D'Orbigny, followed by Darwin, names it ^' Turritella Andii^'"' but 

 whether correctly is still very doubtful. It appears peculiarly charac- 

 teristic of the whole southern districts of America. Darwin has found 

 it in abundance in the strata of Coquimbo, on the Rio Claro and near 



'^ Acta der Leopold. Academie, vol. xvii. pt. ii. p. 649. t. 27. f. 5. 



t Geol. Observations on South America, 1846. 



X " Beyond and north of the great, shut in, mountain-hasin of Titicaca, com- 

 posed of older rocks, the trias and carboniferous limestone." (Bet. iiber die Ver. 

 der Kreide-Bild. p. 27.) 



