416 TIEWS OP NATURE. 



physiognomy totally different from that of the Swiss Alps, 

 the Pyrenees, and the Siberian Altai. 



From Cunturcaga and Aroma we descended, by a zigzag 

 route, a steep declivity of 6400 feet into the cleft-like 

 valley of the Magdalena, the lowest part of which is 4260 

 feet above the sea level. Here there is an Indian village 

 consisting of a few miserable huts, surrounded by the same 

 species of cotton-trees (Bombax discolor), which we first 

 observed on the banks of the Amazon. The scanty vege- 

 tation of the valley of Magdalena somewhat resembles that of 

 the province of Jaen de Bracamoros, but we missed, with 

 regret, the red groves of Bougainvillaea. Magdalena is one 

 of the deepest valleys I have seen in the chain of the Andes. 

 It is a decided cleft, running transversely from east to west, 

 and bounded on each side by the Altos of Aroma and Guan- 

 gamarca. Here recommences the same quartz formation 

 which was so long enigmatical to me. We had previously 

 observed it in the Paramo de Yanaguanga, between Micui- 

 pampa and Caxamarca, at an elevation of 11,722 feet, 

 and on the western declivity of the Cordillera it attains 

 the thickness of many thousand feet. Since Leopold von 

 Buch has proved that the cretaceous formation is widely ex-- 

 tended, even in the highest chains of the Andes, and on both 

 sides of the isthmus of Panama, it may be concluded that the 

 quartz formation, of which I have just made mention (perhaps 

 transformed in its texture by the action of volcanic power), 

 belongs to the free sandstone intervening between the inner 

 chalk and the gault and greensand. From the genial valley 

 of the Magdalena we again proceeded westward, and, foi 

 the space of two hours and a half, we ascended a steep wall 

 of rock 5116 feet high, which rises opposite to the porphyri- 

 tic groups of the Alto de Aroma. In this ascent we felt the 

 change of temperature the more sensibly, as the rocky accli- 

 vity was frequently overhung with cold mist. 



After having travelled for eighteen months without inter 



