the point where, between Micuipampa and Caxamarca (in 6° 57´ S. lat. and 78° 34´ W. long. from Greenwich), they are intersected, according o 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 intermediately 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 precipitous, 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 developed; 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[11] (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-
