TRUE VOLCANOES. 313 



Analogous but more complicated phenomena are presented 

 by another also band-like mass of rocks. On the eastern 

 declivity of the Antisana, probably about 1280 feet perpen- 

 dicularly below the plain of the hacienda, in the direction 

 of Pinantura and Pintac, there lie two small round lakes, 

 of which the more northern is called Ansango, and the 

 southern Lecheyacu. The former has an insular rock, and 

 is surrounded by rolled pumice-stone, a very important point. 

 Each of these lakes marks the commencement of a valley ; 

 the two valleys unite, and their enlarged continuation bears 

 the name of Yolcan de Ansango, because from the margins of 

 the two lakes narrow lines of rock debris, exactly like the 

 two dikes of the plateau which we have described above, do 

 not, indeed, fill up the valley, but rise in its midst like dams 

 to a height of 213 and 266 feet. A glance at the local plan 

 which I published in the *' Geographical and Physical At- 

 las " of my American travels (pi. 26), will illustrate these 

 conditions. The blocks are again partly sharp-edged, and 

 partly scorified and even burned like coke at the edges. It is 

 a basaltic, black, fundamental mass, with sparingly scattered 

 glassy feldspar ; some fragments are blackish-brown, and of 

 a dull, pitch-stone-like lustre. Basaltic as the fundamental 

 mass appears, however, it is entirely destitute of the olivin 

 which occurs so abundantly on the Rio Pisque and near 

 Guallabamba, where I saw basaltic columns of 72 feet in 

 height and 3 feet thick, which contained both olivin and 

 hornblende scattered in them. In the dike of Ans'ango nu- 

 merous tablets, cleft by weathering, indicate porphyritic 

 slates. All the blocks have a yellowish-gray crust from 

 weathering. As the detritus ridge (called los derrumbami- 

 entos, la reventazon, by the natives, who speak Spanish) may 

 be traced from the Rio del Molina, not far from the farm 

 of Pintac, up to the small crater-lakes surrounded by pum- 

 ice-stone (chasms filled with water), the opinion has grown 

 up naturally, and, as it were, of itself, that the lakes are the 

 openings from which the blocks of stone came to the surface. 

 A few years before my visiting the district, the ridge of frag- 

 ments was in motion for weeks upon the inclined surface, 

 without any perceptible previous earthquake, and some 

 houses near Pintac were destroyed by the pressure and shock 

 of the blocks of stone. The detritus ridge of Ansango is 

 still without any trace of vegetation, which is found, al- 

 though very sparingly, upon the two more weathered and 

 certainly older eruptions of the plateau of Antisana. 



Vol. v.— O 



