282 PLATEAU OP CAXAMARCA. 



the point where, between Micuipampa and Caxamarca (in 

 6 57' S. lat. and 78 34' W. long, from Greenwich), they 

 are intersected, according to my observations, by the magnetic 

 equator. Ascending to a still higher elevation among the 

 mountains, the celebrated silver mines of Chota are reached, 

 and from thence with a few interruptions the route descends 

 until the low grounds of Peru are gained; passing inter- 

 mediately over the ancient Caxamarca, > where -316 years ago 

 the most sanguinary drama in the annals of the Spanish 

 Conquista took place, and also over Aroma and Gangamarca. 

 Here, as almost everywhere in the Chain of the Andes and 

 in the Mexican Mountains, the most elevated parts are 

 picturesquely marked by tower-like outbreaks of porphyry 

 (often columnar), and trachyte. Masses of this kind give to 

 the crest of the mountains sometimes a cliff-like and preci- 

 pitous, and sometimes a dome-shaped character. They have 

 here broken through calcareous rocks, which, both on this 

 and on the northern side of the equator, are largely deve- 

 loped ; and which, according to Leopold von Buch's researches, 

 belong to the cretaceous group. Between Guambos and 

 Montan, 12000 French (12790 English) feet above the sea, 

 we found marine fossils ( n ) (Ammonites nearly fifteen English 

 inches in diameter, the large Pecten alatus, oyster shells, 

 Echini, Isocardias, and Exogyra polygona). A species of 

 Cidaris, which, according to Leopold von Buch, cannot be 

 distinguished from that which Brongniart found in the 

 lower part of the chalk series at the Perte du Rhone, was 

 collected by us, both at Tomependa in the basin of the 

 Amazons and at Micuipampa, stations of which the ele- 



